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Biomass and production of heterotrophic bacterioplankton in the oceanic subarctic Pacific
Abstract
As part of the Subarctic Pacific Ecosystem Research (SUPER) program, we measured the abundance and biomass production of heterotrophic bacterioplankton in the subarctic Pacific and compared these parameters with those of phytoplankton during four cruises in 1987 and 1988. Bacterial biomass was about equal to phytoplankton biomass during all cruises. Based on rates of bacterial biomass production and assuming a growth efficiency of 50%, we estimate that heterotrophic bacteria consumed 10% (June 1987) to 24% (August 1988) of primary production in the euphotic zone. These percentages are low compared with other aquatic ecosystems, apparently due to low bacterial growth rates (<0.1 day−1) iin the subarctic Pacific. In contrast, phytoplankton growth rates were much higher (0.1–8.8 day−1). Bacterial growth rates were limited by the supply of dissolved organic matter and temperature. Even with these low growth rates, however, bacterial biomass and rates of biomass production increased by 2–5-fold in May and August 1988, changes that were not obviously related to corresponding changes in phytoplankton biomass nor primary production. Heterotrophic bacterioplankton constitutes a large reservoir of carbon and nitrogen that needs to be considered in modelling ecosystem dynamics of the subarctic Pacific.
... Heterotrophic bacteria and archaea, referred to here as bacterioplankton, are key to the recycling and retention of carbon within the surface ocean at OSP (Kirchman et al., 1993;Boyd et al., 1995;Tortell et al., 1996;Sherry et al., 1999), contributing to its relatively low sinking export efficiency compared with other nutrient-rich and physically dynamic open ocean environments (Buesseler and Boyd, 2009;Siegel et al., 2016;Buesseler et al., 2020). Primary production at OSP is dominated by small-sized (e.g., <5 mm) phytoplankton cells Boyd and Harrison, 1999) that can be consumed by either migratory zooplankton or microzooplankton grazers (e.g., McNair et al., 2021), resulting in a vertical flux out of the surface ocean via sinking pellets or zooplankton migration to depth. ...
... Thus, BCD can be used as a proxy for the flux of DOM derived from all processes (Carlson and Hansell, 2015). Historically, BP (or more accurately as BCD) has been compared directly with rates of photosynthetic carbon fixation to estimate the amount of carbon that enters the microbial food web and fluxes through bacteria in a given system (Ducklow, 1992;Kirchman et al., 1993;Carlson and Ducklow, 1996;Ducklow, 1999Ducklow, , 2000. Given that BCD has exceeded rates of photosynthetic carbon fixation for a range of systems (e.g., Ross Sea, BATS, Equatorial Pacific; Carlson et al., 2007), comparisons of BCD to all forms of new carbon entering the food web (dissolved þ particulate) should be considered. ...
... Based on samples collected at OSP throughout the year, DOC can accumulate in the late summer by up to 15-20 mM C over winter values in the surface mixed layer (ML; Bif and Hansell, 2019), suggesting that there could be a shift in DOM bioavailability to bacterioplankton thereby allowing DOM to accumulate. Additionally, an early study at OSP comparing bacterioplankton responses to DOM amendments of varying quality found that dissolved amino acids resulted in the most rapid bacterioplankton growth (Kirchman et al., 1993). However, there are currently no studies that compare the natural variability of the composition of DOM at OSP over time scales of weeks. ...
Bacterioplankton play a central role in carbon cycling, yet their relative contributions to carbon production and removal can be difficult to constrain. As part of the Export Processes in the Ocean from RemoTe Sensing (EXPORTS) program, this study identifies potential influences of bacterioplankton community and dissolved organic matter (DOM) composition on carbon cycling at Ocean Station Papa in August 2018. Surface (5–35 m) bacterioplankton production rates and stocks spanned a 2- to 3-fold range over the 3-week cruise and correlated positively with the DOM degradation state, estimated using the mole proportion of total dissolved amino acids. When the DOM was more degraded, 16S rRNA gene amplicon data revealed a less diverse bacterioplankton community with a significant contribution from members of the Flavobacteriaceae family. Over the course of 7–10 d, as the DOM quality improved (became less degraded) and bacterioplankton productivity increased, the responding bacterioplankton community became more diverse, with increased relative contributions from members of the SAR86, SAR11 and AEGEAN-169 clades. The cruise mean for mixed layer, depth-integrated bacterioplankton carbon demand (gross bacterioplankton production) was 5.2 mmol C m−2 d−1, representing 60% of net primary production, where the difference between net primary production and bacterioplankton carbon demand was less than sinking flux at 50 m. The concentrations of dissolved organic carbon (cruise average of 58.5 µM C) did not exhibit a systematic change over the cruise period. Therefore, we hypothesize that carbon supplied from gross carbon production, values that were 2- to 3-fold greater than net primary production, provided the carbon necessary to account for the sinking flux and bacterioplankton carbon demand that were in excess of net primary production. These findings highlight the central contributions of bacterioplankton to carbon cycling at Ocean Station Papa, a site of high carbon recycling.
... Keywords: dissolved organic matter, 16S rDNA, Ocean Station Papa, bacterial growth efficiency, total hydrolyzable amino acids, LC-MS/MS, organic matter remineralization, alpha diversity INTRODUCTION Ocean Station Papa (OSP), located in the subarctic NE Pacific, experiences seasonal cycles in bacterioplankton biomass and productivity (Kirchman et al., 1993;Boyd et al., 1995;Sherry et al., 1999). Both net primary and bacterioplankton production (BP) nearly double at OSP in spring and summer relative to winter (Sherry et al., 1999) despite iron limitation that leads to high-nutrient low-chlorophyll conditions (Martin and Fitzwater, 1988;Boyd and Harrison, 1999;Harrison et al., 1999). ...
... BP can represent up to ∼25% of primary production (Sherry et al., 1999), exhibiting the greatest partitioning of primary production into BP in summer months. Such rate comparisons demonstrate that bacterioplankton can comprise a sizable portion of the carbon (C) demand at OSP, though there still remains uncertainty as to the contributions of top-down (predation) vs. bottom-up (organic matter supply) controls on BP (Kirchman et al., 1993;Doherty, 1995;Sherry et al., 1999). ...
... Bacterioplankton are limited to utilizing low molecular weight (LMW) (<600 Da; Weiss et al., 1991) dissolved organic matter (DOM) and so must hydrolyze high molecular weight or particulate organic matter to LMW compounds to consume it (Amon and Benner, 1994;Arnosti et al., 2005;Arnosti, 2011). At OSP, short-term radiotracer-based experiments (conducted over hours) demonstrated that bacteria were primarily limited by the supply of DOM, particularly as dissolved amino acids (Kirchman et al., 1989(Kirchman et al., , 1993Kirchman, 1990;Sherry et al., 1999). Bacteria found in other domains also exhibit enhanced growth and growth efficiencies when grown on amino acids compared with other substrates like sugars (Russell and Cook, 1995), illustrating the importance of this LMW DOM substrate for bacterioplankton growth. ...
The bioavailability of organic matter (OM) to marine heterotrophic bacterioplankton is determined by both the chemical composition of OM and the microbial community composition. In the current study, changes in OM bioavailability were identified at Ocean Station Papa as part of the 2018 Export Processes in the Ocean from Remote Sensing (EXPORTS) field study. Removal rates of carbon (C) in controlled experiments were significantly correlated with the initial composition of total hydrolyzable amino acids, and C removal rates were high when the amino acid degradation index suggested a more labile composition. Carbon remineralization rates averaged 0.19 ± 0.08 μmol C L–1 d–1 over 6–10 days while bacterial growth efficiencies averaged 31 ± 7%. Amino acid composition and tandem mass spectrometry analysis of compound classes also revealed transformations to a more degraded OM composition during experiments. There was a log2-fold increase in the relative abundances of 16S rDNA-resolved bacterioplankton taxa in most experiments by members of the Methylophilaceae family (OM43 genus) and KI89A order. Additionally, when OM was more bioavailable, relative abundances increased by at least threefold for the classes Bacteroidetes (Flavobacteriaceae NS2b genus), Alphaproteobacteria (Rhodobacteraceae Sulfitobacter genus), and Gammaproteobacteria (Alteromonadales and Ectothiorhodospiraceae orders). Our data suggest that a diverse group of bacterioplankton was responsible for removing organic carbon and altering the OM composition to a more degraded state. Elevated community diversity, as inferred from the Shannon–Wiener H index, may have contributed to relatively high growth efficiencies by the bacterioplankton. The data presented here shed light on the interconnections between OM bioavailability and key bacterioplankton taxa for the degradation of marine OM.
... However, investigating microbial growth and substrate dynamics, nutrients and biogeochemical variables systematically across latitudes and biogeographic provinces in such a large ocean is a considerable challenge. This may be a reason why most studies have focused on one particular region, such as the subarctic Pacific, or the north or south Pacific subtropical gyres (e.g., Kirchman et al., 1992;Church, 2014, 2017;Reintjes et al., 2019b), but only a handful of studies have assessed these properties concurrently across large-scale latitudinal gradients. For example, Nagata et al. (2000) and Yokokawa et al. (2013) investigated numbers and biomass production of prokaryotes between the subarctic and Antarctic regions, but with an emphasis on the bathypelagic zone. ...
... Hydrography, nutrient availability, primary production and related biogeochemical features undergo large seasonal fluctuations in different latitudinal regions of the Pacific Ocean (Kirchman et al., 1992;Polovina et al., 2001;Church, 2014, 2017). We are aware of the fact that sampling along a latitudinal transect cannot capture such seasonal variations. ...
The Pacific Ocean constitutes about half of the global oceans and thus microbial processes in this ocean have a large impact on global elemental cycles. Despite several intensely studied regions large areas are still greatly understudied regarding microbial activities, organic matter cycling and biogeography. Refined information about these features is most important to better understand the significance of this ocean for global biogeochemical and elemental cycles. Therefore we investigated a suite of microbial and geochemical variables along a transect from the subantarctic to the subarctic Pacific in the upper 200 m of the water column. The aim was to quantify rates of organic matter processing, identify potential controlling factors and prokaryotic key players. The assessed variables included abundance of heterotrophic prokaryotes and cyanobacteria, heterotrophic prokaryotic production (HPP), turnover rate constants of amino acids, glucose, and acetate, leucine aminopeptidase and β-glucosidase activities, and the composition of the bacterial community by fluorescence in situ hybridization (FISH). The additional quantification of nitrate, dissolved amino acids and carbohydrates, chlorophyll a , particulate organic carbon and nitrogen (POC, PON) provided a rich environmental context. The oligotrophic gyres exhibited the lowest prokaryotic abundances, rates of HPP and substrate turnover. Low nucleic acid prokaryotes dominated in these gyres, whereas in temperate and subpolar regions further north and south, high nucleic acid prokaryotes dominated. Turnover rate constants of glucose and acetate, as well as leucine aminopeptidase activity, increased from (sub)tropical toward the subpolar regions. In contrast, HPP and bulk growth rates were highest near the equatorial upwelling and lowest in the central gyres and subpolar regions. The SAR11 clade, the Roseobacter group and Flavobacteria constituted the majority of the prokaryotic communities. Vertical profiles of the biogeochemical and microbial variables markedly differed among the different regions and showed close covariations of the microbial variables and chlorophyll a , POC and PON. The results show that hydrographic, microbial, and biogeochemical properties exhibited distinct patterns reflecting the biogeographic provinces along the transect. The microbial variables assessed contribute to a better and refined understanding of the scales of microbial organic matter processing in large areas of the epipelagic Pacific beyond its well-studied regions.
... These studies have estimated bacterial biomass based on the assumption that one marine bacterial cell contains 20 fg of carbon. This value was determined with coastal bacterial assemblages grown in filtered seawater (29) and has been commonly applied to oceanic bacterial assemblages without much critical confirmation (11,16,25,31). Recently, Christian and Karl (12) examined the biomass distribution of microbial communities in the subtropical Pacific Ocean by using biomass indicators of microorganisms and a leastsquares inverse method. ...
... Phytoplankton carbon was estimated from the chlorophyll a concentration by assuming carbon-to-chlorophyll a weight ratios of 20 to 100. Estimates derived by using a fixed carbon-to-chlorophyll a weight ratio of 50 will be used in the following discussion to facilitate comparison with other studies (11,16,25). Note that, for estimating bacterial biomass, we use the carbon content (5.9 to 47.9 fg of C cell Ϫ1 ) which was directly determined for natural populations in each region, whereas previous studies set the value at 20 fg of C cell Ϫ1 . ...
In order to better estimate bacterial biomass in marine environments, we developed a novel technique for direct measurement of carbon and nitrogen contents of natural bacterial assemblages. Bacterial cells were separated from phytoplankton and detritus with glass fiber and membrane filters (pore size, 0.8 μm) and then concentrated by tangential flow filtration. The concentrate was used for the determination of amounts of organic carbon and nitrogen by a high-temperature catalytic oxidation method, and after it was stained with 4′,6-diamidino-2-phenylindole, cell abundance was determined by epifluorescence microscopy. We found that the average contents of carbon and nitrogen for oceanic bacterial assemblages were 12.4 ± 6.3 and 2.1 ± 1.1 fg cell ⁻¹ (mean ± standard deviation; n = 6), respectively. Corresponding values for coastal bacterial assemblages were 30.2 ± 12.3 fg of C cell ⁻¹ and 5.8 ± 1.5 fg of N cell ⁻¹ ( n = 5), significantly higher than those for oceanic bacteria (two-tailed Student’s t test; P < 0.03). There was no significant difference ( P > 0.2) in the bacterial C:N ratio (atom atom ⁻¹ ) between oceanic (6.8 ± 1.2) and coastal (5.9 ± 1.1) assemblages. Our estimates support the previous proposition that bacteria contribute substantially to total biomass in marine environments, but they also suggest that the use of a single conversion factor for diverse marine environments can lead to large errors in assessing the role of bacteria in food webs and biogeochemical cycles. The use of a factor, 20 fg of C cell ⁻¹ , which has been widely adopted in recent studies may result in the overestimation (by as much as 330%) of bacterial biomass in open oceans and in the underestimation (by as much as 40%) of bacterial biomass in coastal environments.
... Heterotrophic prokaryotic production (HPP) was estimated using the 3 H-leucine incorporation method [Kirchman et al., 1993], adapted with the centrifuge method [Smith and Azam, 1992]. Triplicate aliquots of 1.5 mL were taken for each depth plus one trichloacetic acid (TCA, 5% final) control. ...
... After three runs of centrifugation/aspiration of the supernatant (once with the fixed sea water sample, once with a 5 % TCA rinse, once with an 80 % ethanol rinse), the pellet was resuspended in scintillation liquid. We used the standard conversion factors of 1.5 kg C mol Leu -1 [Kirchman et al., 1993], assuming no isotopic dilution as checked occasionally from concentration kinetics. ...
Linking atmospheric deposition to marine carbon and nutrient cycle is hampered by the lack of data on atmospheric fluxes of organic matter. To fill this gap, this study reports the first quantification of atmospheric fluxes of soluble organic carbon (SOC), nitrogen (SON) and phosphate (SOP) to the NW Mediterranean Sea. Simultaneous measurements of dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and dissolved organic phosphate (DOP) in the surface mixed layer (SML) allowed estimating the potential contribution of atmospheric fluxes to marine DOC, DON and DOP inventories. We found an annual atmospheric flux of 59 mmol C m⁻² year⁻¹ for SOC, 16.4 mmol N m⁻² year⁻¹ for SON and 23.6 µmol P m⁻² year⁻¹ for SOP, with proportions of SON and SOP to total soluble nitrogen and phosphate of 40% and 25%, respectively. Assuming these annual fluxes valid for the entire western Mediterranean basin, atmospheric fluxes would be higher than DOC, DON and DOP fluxes from the Rhône River by a factor of 6, 17 and 2, for C, N and P, respectively. Inventories of DOC, DON and DOP in the surface mixed layer displayed similar trends over the study period with maximum values at the end of the stratification period. DOP contributed by 85 ± 11% to total dissolved phosphate (TDP) pool and exhibited a labile fraction (LDOP) of 27 ± 19%. The contribution of atmospheric deposition to the DOC, DON and DOP pools in the SML, estimated for the stratification period, was low for C (3%) and P (4.5%) and moderate for N (12%). The labile fraction of atmospheric SOP (LSOP) was quantified throughout the sampling period and showed a high variability ranging from 0 to 97%. Atmospheric fluxes of LSOP contributed by 7% to marine LDOP pool and could sustain up to 8% of the heterotrophic prokaryotic phosphate demand in the SML of the NW Mediterranean Sea during the stratification period. The results obtained in this study stress the need to include atmospheric fluxes of organic matter in marine biogeochemical models to achieve a more complete picture of carbon and nutrient cycle in the Mediterranean Sea.
... They prefer to consume polymers rather than monomers [41]. Bacteroidetes are abundant especially during and following algal blooms [42]. In this study, we found that the changes in bacterial functions of the bacterioplankton communities in the nearshore and offshore groups were lower than those at various taxa classification levels. ...
Bacterioplankton communities are critical components of varied ecosystems in the oceans. Their occurrences represent a variety of connections between environmental and ecological elements. However, our current knowledge about the shaping factors of surface bacterioplankton communities in the eastern East China Sea (ECS) is still limited. In this study, we reveal the spatial patterns of the taxonomic and functional profiles of the surface bacterioplankton communitiesies in the nearshore and offshore areas in the eastern ECS, based on 16S rRNA gene pyrosequencing and functional annotation analysis. The obtained results show that the surface bacterioplankton communities in the nearshore areas are mainly dominated by the firmicutes (85.9%), actinobacteria (8.1%), and proteobacteria (5.4%), which are mainly involved in organic compound metabolism. Meanwhile, different bacteria predominate the composition of the offshore group, namely proteobacteria (71.1%) and bacteroidetes (22.0%) responsible for nitrogen and sulfur metabolism. Furthermore, their distribution pattern is shown to be spatially determined, along with a modest finding of functional diversity when comparing the bacterial species. The primary two shaping factors of bacterioplankton diversity are found to be the offshore distance and temperature. Overall, these findings add to those previously published on bacterial species and offer up functional information on the surface bacterioplankton communities in the eastern ECS. To extend our research, we propose that, in the future, it may be beneficial to monitor the dynamics of the ecosystem in this sea area.
... These phytoplankton data were converted to carbon biomass using a C:Chl a ratio of 50 g g −1 (Welschmeyer et al., 1991;Booth et al., 1993;Kirchman et al., 1993;Boyd et al., 1995). Although recent data show that the C:Chl a ratio varies seasonally (i.e. ...
When attempting to model an ecosystem, compromises must be made to retain the essence of the system and its functioning. Here, we explore the implications of these aggregations on a modelled ecosystem by using Ecopath to model the plankton food web in the Strait of Georgia (SoG), a temperate coastal system on the west coast of Canada, in three seasons. The food web is defined by fifteen functional groups representing autotrophic phytoplankton, the microbial loop, and mesozooplankton in three seasons, differentiated by node biomass, productivity, and diet. Variations in the community composition lead to shifts in trophic behaviour and food-web structure seasonally. In spring, autotrophic phytoplankton (i.e. diatoms) are the main source of carbon for the system. In summer and winter, the microbial loop becomes more important due to limited primary production. These structural changes within the plankton food web throughout a year have implications for higher trophic levels, including the seasonal availability and quality of food for planktivorous fish. Finally, we compared this detailed seasonal approach to plankton modelling with a more common simplified approach and examined its impacts on the entire ecosystem. The microbial loop is often excluded from coastal ecosystem models but is an important component, influencing trophic positions and transfer efficiencies. However, aggregating plankton groups appears to be an adequate approach to plankton modelling in the SoG, but modelling decisions should be driven by the research question.
... Accordingly, a strong relationship between BP and BB suggests bottom-up control of the bacteria, while the lack of a relationship suggests top-down control of the bacterial population. The biomass of heterotrophic bacteria was calculated using the following conversion factor of 20 fgC cell −1 [61,62]. ...
Artificial neural network analysis (ANN) is used to study the seasonal distribution of viruses and microbial food web (MFW) components in the open Adriatic Sea. The effect of viruses within the MFW is often overlooked, although viruses play an important role in microbial community dynamics. The results showed that the strongest influence is found in the nonlinear relationship between viruses and temperature. In addition, the algorithm showed that the number of viral populations in the P-limited open sea varies by season and according to the abundance of their main hosts, HB. A strong positive relationship between viruses and HB was found in more than 50% of the observed data. Moreover, this algorithm confirmed the association of the virus with the autotrophic part of the picoplankton and with heterotrophic nanoflagellates. The dynamics of the four resulting clusters, characterized by biological and environmental parameters, is described as a cyclic pattern in the water layer above the thermocline. Neural gas network analysis has been shown to be an excellent tool for describing changes in MFW components in the open Adriatic.
... A definite amount of samples stained with Acridine Orange (final concentration of 0.01%), which was passed through a 0.2 µm black polycarbonate Nuclepore membrane filter and taken back to the laboratory for counting using an electron fluorescence microscope (Leica, Germany). BP was determined using the 3 H-Leucine tracer method (Kirchman et al., 1993). The water sample was collected using a Niskin bottle (HYDRO-BIOS, Kiel, Germany) and 20 ml of water was added to each of the three 50 ml sterile culture tubes, with 1 ml of formaldehyde added to one tube as a control sample and 3 H-Leucine added to the other two tubes to achieve a final concentration of 20 nmol/L in the sample. ...
Photosynthetically produced dissolved organic carbon (PDOC) released by marine phytoplankton has an important significance on the marine carbon cycle and on the growth of heterotopic bacteria (HB). However, PDOC is often neglected in the estimation of global marine primary productivity and carbon sequestration capacity. This work studied the issue of PDOC and its environmental regulation mechanism through field investigation and lab experiments in an estuarine bay during southwest and northeast monsoon. We used the percentage of extracellular release (PER) as a key indicator to evaluate the contribution of PDOC to total primary productivity (TPP). We also compared PER among different seasons and sectors and then analyzed the inter-relationship between PDOC and bacterial carbon demand (BCD), size-fractionated phytoplankton, bacterial production (BP), and TPP. We finally discussed the impact factors of PDOC productivity. The results showed that the average contribution of PDOC to TPP in Qinzhou bay could reach 15% during two seasons, which satisfies about 25% of the carbon requirement by HB in the bay. Multiple factors contribute to the seasonal (SW monsoon 13% < NE monsoon 18%) and sectoral variation (outer bay 26% > middle bay 17% > inner bay 7%) in PER, the most significant of which are salinity and nitrogen to phosphorus ratios. PER is also related to phytoplankton community structure and nutrient limitation, the higher PER in the outer bay is attributed to the dual effect of picophytoplankton being the predominant species and a severe imbalance in the nitrogen to phosphorus ratio, both of which lead to increased phytoplankton PDOC release. On the other hand, the lower PER in the inner bay was mainly due to the dominance of microphytoplankton and nitrogen to phosphorus ratio which is close to the redfield ratio. There is still a non-uniform conclusion on the relationship between PER and TPP, suggesting that this relationship needs to be evaluated not only in a horizontal-independent way but also through seasonal and vertical dimensions. The non-linear relationship between PDOC and BCD found in this study potentially indicated weak coupling between phytoplankton exudation and bacterial metabolism. In addition to phytoplankton exudate, HB is still dependent on other sources of DOC to meet their carbon requirements.
... The biomasses of studied picoplankton groups were calculated using the following cell-to-carbon conversion factors: 20 fgC cell −1 for HB (Lee and Fuhrman, 1987;Kirchman et al., 1993), 36 fgC cell −1 for PROC (Buitenhuis et al., 2012), 255 fgC cell −1 for SYN (Buitenhuis et al., 2012), 2,590 fgC cell −1 for PE (Buitenhuis et al., 2012) and 0.22 pgC µm −3 for HNF (Borsheim and Bratbak, 1987). The size of HNF cells was measured at about 500 specimens per environment type, and their biovolume was estimated by the geometric method, i.e., by comparing the body shape of each organism with the geometric body. ...
The mechanisms responsible for the development of various structural and functional features of the microbial food web (MFW) and their dynamics at spatial and temporal scales, which are important for predicting their responses to future environmental changes, are largely unknown. More than 3000 datasets of environmental and microbial variables collected over a decade on a seasonal and large spatial scale in the Adriatic Sea were analyzed. The sets of environmental variables were classified into four clusters (representing different environmental states) using Neural Gas analysis and the differences in MFW structure between the clusters were analyzed. Different variants of MFW evolve in the different clusters in terms of the abundance of MFW components, their ratios, growth and grazing rates, predator preference in prey selection, the strength of predator-prey interaction, and the relative importance of top-down and bottom-up control. However, these clusters are neither spatially nor temporally fixed; rather, the area studied represents a mosaic of different environmental conditions that alternate from one state to another on a time scale. In each of the environmental states, a distinct structure of MFW develops that shows consistent and repeatable changes that strictly follow the switching in environmental conditions from one state to another.
... La différence entre les temps d'incubation optima pourrait s'expliquer par un turnover plus rapide du fer en océan Austral, simplement parce que les concentrations en fer in situ sont nettement plus faibles. Dans le cas des mesures de la production bactérienne employant la thymidine ou la leucine, les incubations sont de préférence courtes (de 30 minutes à quelques heures) car ces composés peuvent être incorporés à d'autres molécules que l'ADN ou les protéines, ce qui peut biaiser l'analyse(Kirchman et al. 1993). Le fer, au contraire, ne peut pas être synthétisé de novo par la cellule. ...
Les réponses métaboliques à la limitation de deux souches bactériennes d’Alteromonas macleodii issues d’environnements contrastés (côtier et océanique) ont été étudiées. Chez A. Macleodii, la limitation en fer entraine une réduction de la respiration et induit des variations significatives dans l’expression d’enzymes clé du catabolisme du carbone, en particuliers celles impliquées dans le cycle de Krebs et la glycolyse. Par ailleurs, la souche océanique est moins affectée dans sa croissance que la souche côtière, suggérant une adaptation de ces microorganismes à leur environnement. La combinaison des techniques de microautoradiographie et de CARD-FISH a été développée avec le radioisotope 55Fe, et a démontré le potentiel de cette méthode pour tracer le devenir du fer au sein d’une communauté bactérienne. Les premières applications ont révélé une contribution importante des Gammaprotéobactéries, incluant le genre Alteromonas, dans l’acquisition du fer en mer Méditerranée et en océan Austral. En océan Austral, la demande en fer des bactéries hétérotrophes a aussi été évaluée pendant la campagne KEOPS2. En début de bloom printanier, les bactéries sont en concurrence avec le pico-nanoplancton pour l’accès au fer. L’incorporation du fer par les bactéries est près de 20 fois supérieure lorsque le pico-nanoplancton est retiré des expériences. Une forte corrélation entre incorporation du fer par les bactéries et production primaire a aussi été trouvée. Les résultats suggèrent fortement que la disponibilité en carbone organique dissous conditionne la demande en fer des bactéries hétérotrophes et influe indirectement sur les relations trophiques.
... After resuspension of the pellet in 1.5 mL scintillation liquid (Ultima Gold MV), radioactivity was determined by a liquid scintillation counter. Leucine incorporation rates were converted into carbon production using the conversion factors of 1.5 kg C per mole of leucine incorporated (Kirchman, 1993). ...
The surface ocean receives important amounts of organic carbon from atmospheric deposition. The degree of bioavailability of this source of organic carbon will determine its impact on the marine carbon cycle. In this study, the potential availability of dissolved organic carbon (DOC) leached from both desert dust and anthropogenic aerosols to marine heterotrophic bacteria was investigated. The experimental design was based on 16 d incubations, in the dark, of a marine bacterial inoculum into artificial seawater amended with water-soluble Saharan dust (D treatment) and anthropogenic (A treatment) aerosols, so that the initial DOC concentration was similar between treatments. Glucose-amended (G) and non-amended (control) treatments were run in parallel. Over the incubation period, an increase in bacterial abundance (BA) and bacterial production (BP) was observed first in the G treatment, followed then by the D and finally A treatments, with bacterial growth rates significantly higher in the G and D treatments than the A treatment. Following this growth, maxima of BP reached were similar in the D (879 +/- 64 ngC L-1 h-1; n = 3) and G (648 +/- 156 ngC L-1 h-1; n = 3) treatments and were significantly higher than in the A treatment (124 ng C L-1 h-1; n = 2). The DOC consumed over the incubation period was similar in the A (9 μM; n = 2) and D (9 +/- 2 μM; n = 3) treatments and was significantly lower than in the G treatment (22 +/- 3 μM; n = 3). Nevertheless, the bacterial growth efficiency (BGE) in the D treatment (14.2 +/- 5.5 %; n = 3) compared well with the G treatment (7.6 +/- 2 %; n = 3), suggesting that the metabolic use of the labile DOC fraction in both conditions was energetically equivalent. In contrast, the BGE in the A treatment was lower (1.7 %; n = 2), suggesting that most of the used labile DOC was catabolized. The results obtained in this study highlight the potential of aerosol organic matter to sustain the metabolism of marine heterotrophs and stress the need to include this external source of organic carbon in biogeochemical models for a better constraining of the carbon budget.
... Our results reveal a strong coupling of bacterial biomass production to primary production and phytoplankton standing stocks, suggesting that the availability of labile organic matter was an important constraint on heterotrophic bacterial activity. The average ratio of bacterial production to primary production was 0.11 in the Eurasian Basin at the time of our study and substantially higher than ratios in the western Arctic Ocean and other polar marine systems but in the range reported for the Equatorial Pacific and the subarctic North Pacific (Kirchman et al. 1993;Ducklow et al. 1995). Further evidence for a strong coupling between bacterial growth and phytoplankton dynamics is provided by the steep slope of the log-log regression between bacterial production and Chl a. ...
The Arctic Ocean is highly susceptible to climate change as evidenced by rapid warming and the drastic loss of sea ice during summer. The consequences of these environmental changes for the microbial cycling of organic matter are largely unexplored. Here, we investigated the distribution and composition of dissolved organic matter (DOM) along with heterotrophic bacterial activity in seawater and sea ice of the Eurasian Basin at the time of the record ice minimum in 2012. Bacteria in seawater were highly responsive to fresh organic matter and remineralized on average 55% of primary production in the upper mixed layer. Correlation analysis showed that the accumulation of dissolved combined carbohydrates (DCCHO) and dissolved amino acids (DAA), two major components of fresh organic matter, was related to the drawdown of nitrate. Nitrate‐depleted surface waters at stations adjacent to the Laptev Sea showed about 25% higher concentrations of DAA than stations adjacent to the Barents Sea and in the central Arctic basin. Carbohydrate concentration was the best predictor of heterotrophic bacterial activity in sea ice. In contrast, variability in sea‐ice bacterial biomass was largely driven by differences in ice thickness. This decoupling of bacterial biomass and activity may mitigate the negative effects of biomass loss due to ice melting on heterotrophic bacterial functions. Overall, our results reveal that changes in DOM production and inventories induced by sea‐ice loss have a high potential to enhance the bacterial remineralization of organic matter in seawater and sea ice of the Arctic Ocean.
... Therefore, the 560 increasing abundance of bacteria was probably a hindrance to POC export as they are well 561 known agents of carbon recycling in the layer just below the euphotic zone. In the euphotic zone, 562 bacteria may compete with phytoplankton for nutrients (Kirchman et al., 1993;Ducklow et al., 563 1993). The rapid recycling of organic matter by bacteria is likely also to be associated with a 564 rapid recycling of 210 Po, since microbial degradation of POM involves a rapid loss of proteins 565 (Taylor, 2003). ...
During the North Atlantic Bloom Experiment (NABE) of the Joint Global Ocean Flux Study (JGOFS), water column sampling for particulate and dissolved ²¹⁰Po and ²¹⁰Pb was performed four times (26 April and 4, 20, 30 May 1989) during a month-long Lagrangian time-series occupation of the NABE site, as well as one-time samplings at stations during transit to and from the site. There are few prior studies documenting short-term changes in ²¹⁰Po and ²¹⁰Pb profiles over the course of a phytoplankton bloom, and we interpret the profiles in terms of the classical “steady-state” (SS) approach used in most studies, as well as by using a non-steady state approach suggested by the temporal evolution of the profiles. Changes in ²¹⁰Po profiles during a bloom are expectable as this radionuclide is scavenged and exported. During NABE, ²¹⁰Pb profiles also displayed non-steady state, with significant increases in upper water column inventory occurring midway through the experiment. Export of ²¹⁰Po from the upper 150 m using the classic “steady-state” model shows increases from 0.5 ± 8.5 dpm m⁻² d⁻¹ to 68.2 ± 4.2 dpm m⁻² d⁻¹ over the ∼one-month occupation. Application of a non-steady state model, including changes in both ²¹⁰Pb and ²¹⁰Po profiles, gives higher ²¹⁰Po export fluxes. Detailed depth profiles of particulate organic carbon (>0.8 μm) and particulate ²¹⁰Po (>0.4 μm) are available from the 20 and 30 May samplings and show maxima in POC/Po at ∼37 m. Applying the POC/²¹⁰Po ratios at 150 m to the “steady state” ²¹⁰Po fluxes yields POC export from the upper 150 m of 8.2 ± 1.5 mmol m− 2 d⁻¹ on 20 May and 6.0 ± 1.6 mmol C m⁻² d⁻¹ on 30 May. The non-steady state model applied to the interval 20 to 30 May yields POC export of 24.3 mmol C m⁻² d⁻¹. The non-steady state (NSS) ²¹⁰Po-derived POC fluxes are comparable to, but somewhat less than, those estimated previously from ²³⁴Th/²³⁸U disequilibrium for the same time interval (37.3 and 45.0 mmol m⁻² d⁻¹, depending on the POC/Th ratio used). In comparison, POC fluxes measured with a floating sediment trap deployed at 150 m from 20 to 30 May were 11.6 mmol m⁻² d⁻¹. These results suggest that non-steady state Po-derived POC fluxes during the NABE agree well with those derived from ²³⁴Th/²³⁸U disequilibrium and agree with sediment trap fluxes within a factor of ∼2. However, unlike the ²³⁴Th-POC flux proxy, non-steady stage changes in profiles of ²¹⁰Pb, the precursor of ²¹⁰Po, must be considered.
... Biomasses of the studied picoplankton groups were calculated using the following cell-to-carbon conversion factors: 20 fgC cell −1 for heterotrophic bacteria (Lee and Fuhrman, 1987;Kirchman et al., 1993), 36 fgC cell −1 for Prochlorococcus (Buitenhuis et al., 2012), 255 fgC cell −1 for Synechococcus (Buitenhuis et al., 2012), 2590 fgC cell −1 for picoeukaryotes (Buitenhuis et al., 2012) and 0.22 pgC µm −3 for HNF (Borsheim and Bratbak, 1987). ...
Oceanographic measurements carried out in the middle Adriatic during summer 2017 revealed anomalous conditions in both physical and microbial properties. High salinities were observed throughout the entire water column, with an ‘inverse’ salinity profile in August and a maximum in the surface layer, recorded for the first time in the middle Adriatic. Surface salinity of 39.02 recorded in August was 2.5 standard deviations above the long-term average (1961–2016). The observed salinity distributions are the result of both local and remote drivers, whereby the North Ionian cyclonic gyre controlled by the Adriatic-Ionian Bimodal Oscillating System has been responsible for the overall above-average salinities since 2011. Yet, local factors present in 2017, such as strong evaporation caused by extremely high air temperatures, lack of precipitation and low river discharges, combined with a decrease in horizontal transport estimated from the Regional Ocean Modeling System simulations, contributed substantially to the observed surface salinity anomaly. The decrease in horizontal advection was conjoined with high values of repelling barriers in the fields of the finite-time Lyapunov exponent. Documented physical conditions were synchronized with considerably lower bacterial production and abundance of the most studied picoplankton groups in comparison to values during the last decade. The exception was the euryhaline organism Synechococcus, whose abundance was 88% higher than the average in the study area. Nutrient content and chlorophyll-a concentrations followed regular seasonal cycles during 2017, with typical low values pointing to salinity as a possible driver of the observed changes in the microbial food web. Following ongoing climate change and future projections, these documented anomalous physical and microbiological conditions may become more frequent in the Adriatic Sea.
... The extraction procedure and assumptions for the conversion factor are detailed in Van Wambeke et al. (2018). The incorporation of leucine was converted to carbon considering a conversion factor of 1.5 kg C mol -1 leucine (Kirchman, Keil and Simon 1993). BP rates were corrected for unassimilated leucine in the DON mixture added. ...
Mixotrophy, the combination of heterotrophic and autotrophic nutrition modes, is emerging as the rule rather than the exception in marine photosynthetic plankton. Trichodesmium, a prominent diazotroph ubiquitous in the (sub)tropical oceans, is generally considered to obtain energy via autotrophy. While the ability of Trichodesmium to use dissolved organic phosphorus when deprived of inorganic phosphorus sources is well known, the extent to which this important cyanobacterium may benefit from other dissolved organic matter (DOM) sources is unknown. Here we provide evidence of carbon-, nitrogen- and phosphorus-rich DOM molecules enhancing N2 fixation rates and nifH gene expression in natural Trichodesmium colonies collected at two stations in the western tropical South Pacific. Sampling at a third station located in the oligotrophic South Pacific Gyre revealed no Trichodesmium but showed presence of UCYN-B, although no nifH expression was detected. Our results suggest that Trichodesmium may behave mixotrophically in response to certain environmental conditions, providing them with metabolic plasticity and adding up to the view that mixotrophy is widespread among marine microbes.
... Many studies have indicated that temperature, labile DOC (Amon & Benner 1996, Azam & Malfatti 2007, Kirchman et al. 2009b, and nutrient concentrations (Guildford & Hecky 2000, Sala et al. 2002, Matz & Jürgens 2003 are the main bottom-up factors influencing bacterial dynamics in marine environments. In pelagic marine systems, various organic matter sources fulfill the bacterial DOC demand: phytoplankton exudation (Azam & Cho 1987, Kirchman et al. 1993, spontaneous autolysis of phytoplankton (van Boekel et al. 1992), viral lysis (Bratbak et al. 1992), excretion by herbivores (Nagata & Kirchman 1991), sloppy feeding by large zooplankton (Roy et al. 1989), and degradation of fecal material and other detritus (Jumars et al. 1989). Hetero trophic bacteria ac count for a large portion of total up take of both phosphate and ammonium in marine systems (Kirchman 1994). ...
This study was conducted in 4 Labrador fjords (Nachvak, Saglek, Okak, and Anaktalak) during the summers of 2007 and 2013, early fall 2010, and late fall 2009. Our results show that water temperature combined with the availability of nutrients and organic substrates are the main abiotic factors controlling the abundance of heterotrophic bacteria in Labrador fjords. Bacterivory also played a crucial role, with heterotrophic bacteria exerting a significant bottom-up control on the abundance of heterotrophic nanoflagellates (r = 0.35, p < 0.05) and ciliates (r = 0.70, p < 0.01). During summer 2013, the intrinsic phytoplankton growth rate varied between <0 and 0.64 d ⁻¹ , with a mean value of 0.36 d ⁻¹ . The herbivory rate was highly variable, ranging from 0.01 to 0.86 d ⁻¹ , with a mean value of 0.31 d ⁻¹ . Grazing mortality was 6-fold higher than phytoplankton growth rate. Mean phytoplankton growth and herbivory rates in Labrador fjords were comparable to the Barents and Bering seas. The intrinsic growth rate of total heterotrophic bacteria ranged between <0 and 0.68 d ⁻¹ , with a mean value of 0.30 d ⁻¹ . Bacterivory varied from 0.01 to 0.95 d ⁻¹ , with a mean of 0.30 d ⁻¹ . Mortality due to grazing was up to 2.3 times higher than total bacterial growth rate. This study improves our understanding of the factors influencing the dynamics of heterotrophic bacteria and indicates that herbivory and bacterivory exert substantial control on microbial communities in Labrador fjords.
... The biomasses of studied picoplankton groups was calculated using the following cell-to-carbon conversion factors: 20 fgC cell −1 for heterotrophic bacteria (Lee and Fuhrman, 1987;Kirchman et al., 1993), 36 fgC cell −1 for Prochlorococcus (Buitenhuis et al., 2012), 255 fgC cell −1 for Synechococcus (Buitenhuis et al., 2012), 2590 fgC cell −1 for picoeukaryotes (Buitenhuis et al., 2012) and 0.22 pgC μm −3 for HNF (Borsheim and Bratbak, 1987). The bio-volumes of HNF and autotrophic nano-eukaryotes were estimated using the geometrical method, i.e. comparing the body shape of each organism to the geometrical body. ...
Vertical mixing and stratification are among the most important physical processes controlling nutrient dynamics, the dominant category of primary producers and consequently the dominant types of food web, and are therefore important for the assessment of the marine ecosystem's response to global climate change. This study showed consistent short-term cyclic successions of the plankton food web types, governed by the dynamics of water column stability changes, occurring in generally oligotrophic, phosphate deficient surface waters of the open middle Adriatic Sea. The biogeochemical nitrogen cycle appeared as a key driving force responsible for the food web structure changes. The ‘herbivorous food web’ dominated during the nitrate-rich mixed water column period (winter) and gradually changed to ‘multivorous food web’ where large phytoplankton still constitute a significant fraction of phytoplankton. This intermediate type of food web lasted for a short time and quickly changed to the typical ‘microbial food web’, which then dominated during the stratified water column period (summer) and was characterised by a large proportion of picoplankton size-fraction organisms in total plankton biomass and production. Furthermore, at the very end of summer, the high bacterial carbon flux through the ‘microbial loop’ was established. The succession of food web types affects the mechanisms of bacterial control in a way that ‘bottom up’ control dominated during the mixed water column period and ‘top-down’ control prevailed during the stratified period. Since the ongoing global warming is expected to change water column stability dynamics and thereby significantly affect the supply of nutrients in surface waters, this study helps to understand the possible direction of changes in the plankton food webs of the Adriatic Sea, and consequent changes in marine nitrogen and carbon biogeochemical cycles.
... The radioactivity incorporated into bacterial cells was counted in Hidex 300SL Liquid Scintillation Counter. A factor of 1.55 kg C mol leucine −1 was used to convert the incorporation of leucine to carbon equivalents, assuming no isotope dilution (Kirchman et al., 1993). Isotopic dilution ranged from 1.0 to 1.3 as determined on three occasions using a kinetic approach. ...
Iron (Fe) is a paradox in the modern ocean – it is central to many life-critical enzymes but is scarce across most surface waters. The high cellular demand and low bioavailability of Fe likely puts selective pressure on marine microorganisms. Previous observations suggest that heterotrophic bacteria are outcompeted by small diatoms for Fe supply in the subantarctic zone of Southern Ocean, thereby challenging the idea of heterotrophic bacteria being more competitive than phytoplankton in the access to this trace metal. To test this hypothesis, incubation experiments were carried out at the Southern Ocean Time Series site (March–April 2016). We investigated (a) whether dissolved organic carbon (DOC), dissolved Fe, or both limit the growth of heterotrophic bacteria and, (b) if the presence of potential competitors has consequences on the bacterial Fe acquisition. We observed a pronounced increase in both bulk and cell-specific bacterial production in response to single (+C) and combined (+Fe+C) additions, but no changes in these rates when only Fe was added (+Fe). Moreover, we found that +Fe+C additions promoted increases in cell-specific bacterial Fe uptake rates, and these increases were particularly pronounced (by 13-fold) when phytoplankton were excluded from the incubations. These results suggest that auto- and heterotrophs could compete for Fe when DOC limitation of bacterial growth is alleviated. Such interactions between primary producers and nutrient-recyclers are unexpected drivers for the duration and magnitude of phytoplankton blooms in the Southern Ocean.
... Biovolumes of AAP cells were calculated by approximating the most similar geometric shapes. Biomasses of the studied picoplankton community were calculated from cell abundance by using the following conversion factors from cell abundance to carbon biomass from the literature: 20 fg C cell −1 for heterotrophic bacteria (Lee and Fuhrman, 1987;Kirchman et al., 1993), 36 fg C cell −1 for Prochlorococcus (Buitenhuis et al., 2012), 255 fg C Fig. 1. Study area with sampling stations. ...
Although Aerobic Anoxygenic Phototrophs (AAPs) are widespread in aquatic environments, relatively little is known about their dynamics in the Adriatic Sea. Therefore, we investigated their distribution along the trophic gradient from coast to the open sea over a year-round period, for the first time, as well as the factors that influenced the spatio-temporal distribution. AAP abundances varied between 0.26 × 10⁴ at the open sea, and 23.45 × 10⁴ cell mL⁻¹ in the estuarine area, while their relative abundances ranged between 1.13% and 23.88% respectively. The biomass of AAPs ranged between 0.07 and 6.24 μg C L⁻¹; thus, we observed a trend of decreasing abundances and biomass of the studied groups from more eutrophic coastal to the oligotrophic open sea area. AAP biomass exhibited the highest values during summer in the estuarine areas and the lowest during winter in the open sea. The main driving factors for the whole picoplankton community variance were transparency (Secchi disc), chlorophyll and salinity, whereas AAPs were grouped with heterotrophic bacteria, picoeukaryotes and heterotrophic nanoflagellates. Our data indicate that phosphorus limitation in the environment could be an important factor for the AAP growth.
... Abundance to biomass conversion. The biomass of studied PP groups was calculated using the following cell-tocarbon conversion factors: 20 fgC cell −1 for BAC (Lee and Fuhrman, 1987;Kirchman et al., 1993), 36 fgC cell −1 for Prochlorococcus (Buitenhuis et al., 2012), 255 fgC cell −1 for Synechococcus (Buitenhuis et al., 2012) and 0.22 pgC μm −3 for HNF (Borsheim and Bratbak, 1987). ...
Temperature and phosphorus positively interacted in controlling picoplankton biomass production and its transfer towards higher trophic levels. Two complementary approaches (experimental and field study) indicated several coherent patterns: (1) the impact of temperature on heterotrophic bacteria was high at temperatures lower than 16oC and levelled off at higher temperatures, whereas this impact on autotrophic picoplankton was linear along the entire range of the investigated temperatures; (2) the addition of phosphorus increased the values of picoplankton production and grazing, but did not change the nature of their relationships with temperature substantially; (3) the picoplankton carbon flux towards higher trophic levels was larger during the warmer months (grazing by HNF dominated during the warmer period and by ciliates during the colder period) and also strengthened in conditions without phosphorus limitation; (4) the hypothesis that the available phosphorus can be better utilized at higher temperatures was confirmed for both autotrophic and heterotrophic picoplankton; (5) the hypothesis that the rise in temperature stimulates growth only in conditions of sufficient phosphorus was confirmed only for heterotrophic bacteria. Therefore, in the global warming scenario, an increase of the picoplankton carbon flux towards higher trophic levels can be expected in the Adriatic Sea, particularly under unlimited phosphorus conditions.
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... The biomass of studied PP groups was calculated using the following cell-to-carbon conversion factors: 20 fgC cell −1 for BAC (Lee and Fuhrman 1987;Kirchman et al. 1993 HNF (Borsheim and Bratbak 1987). The HNF biovolume was estimated using the lengths and widths of flagellate cells. ...
An assessment of the temperature increase effect on processes within the microbial food web provides a better insight into the carbon transfer and energy flow processes in marine environments in the global warming perspective. Modified laboratory dilution experiments that allow simultaneous estimates of protozoan grazing and viral lysis on picoplankton groups (bacteria, Prochlorococcus, Synechococcus and pico-eukaryotic algae) under in situ and 3°C above in situ temperatures were performed at seasonal scale. Picoplankton mortality due to grazing was generally higher than that caused by viral lysis, especially in the cold months. The largest part of HNF carbon demand was satisfied by grazing on bacteria throughout the year. Although ciliates satisfied their carbon demand predominantly through grazing on HNF and bacteria, the role of autotrophic picoplankton (APP) as their prey increased significantly in the cold months. Bacteria constituted the most important host for viruses throughout the year. However, during the warm months, APP groups were also significant hosts for viral infection. Under the warming condition the amount of picoplankton biomass transferred to protozoan grazers exceeded the lysed biomass, suggesting that global warming could further increase picoplankton carbon flow toward higher trophic levels in the Adriatic Sea.
... The DOM released by the algal cells has long been recognized as high quality substrates for bacteria (Cole et al. 1982). The factors such as dissolved organic matter and temperature tend to limit the bacterial growth rates (Kirchman et al. 1993). Moreover, the temperature has a greater effect on bacterial growth when compared to substrate supply (Yokokawa and Nagata 2005;Longnecker et al. 2006). ...
The Northeastern Arabian Sea (NEAS) experiences convective mixing during winter monsoon, which facilitates nutrient enrichment in the surface layers. This region is also known to harbour several sea surface temperature (SST) fronts and filaments, hotspots of primary production and bacterioplankton activity, which play an important role in the microbial loop. Observations on bacterioplankton with an emphasis on their physiological state, i.e., metabolically active (HNA; high nucleic acid content) bacteria and metabolically inactive (LNA; low nucleic acid content) bacteria were carried out in the NEAS region during early and peak winter monsoon (EWM and PWM). HNA bacteria were dominant in frontal zones coupled with high bacterial production (BP) indicating their significant role, whereas, LNA bacteria were abundant in non-frontal regions irrespective of the seasons. The differentiation in bacterial metabolic types points out organic matter enrichment in the fronts. During PWM, the nutrient concentration increased resulting in a further increase in HNA bacteria and BP in frontal regions. The transparent exopolysaccharides (TEP) were also higher in frontal zones, and inversely related to total bacterial abundance indicating a fast turnover of the organic matter. The age of the front and background condition prior to the formation of the front influences the relative contribution by bacterioplankton food web dynamics and these attributes would probably also help predict bacterial activities.
... The following cell-to-carbon conversion factors were used for picoplankton biomass estimation: 1.665 fmol C/cell for heterotrophic bacteria (Kirchman et al., 1993;Lee and Fuhrman, 1987), 2.998 fmol C/cell for Prochlorococcus, 21.23 fmol C/cell for Synechococcus, and 215.7 fmol C/cell for picoeukaryotes (Buitenhuis et al., 2012). Determinations of microbial species were not performed at station CJ008. ...
Microbial transformations of toxic monomethylmercury (MMHg) and dissolved gaseous mercury (DGM) at the lower levels of the marine food web are not well understood, especially in oligotrophic and phosphorus-limited seas. To examine the effects of probable phosphorus limitation (PP-limitation) on relations between mercury (Hg) fractions and microorganisms, we determined the total mercury (THg), total methylated mercury (MeHg), DGM, and microbiological and chemical parameters in the Central Adriatic Sea. Using statistical analysis, we assessed the potential microbial effects on Hg transformations and bioaccumulation. Only in the absence of PP-limitation conditions (NO–PP-limitation) is MeHg significantly related to most chemical and microbial parameters, indicating metabolism-dependent Hg transformations. The heterotrophic activity of low nucleic acid bacteria (abundant in oligotrophic regions) seems responsible for most of Hg methylation under NO–PP-limitation. Under these conditions, DGM is strongly related to microbial fractions and chlorophyll a, indicating biological DGM production, which is probably not metabolically induced, as most of these relations are also observed under PP-limitation. MMHg biomagnification was observed through an increased bioaccumulation factor from microseston to mesozooplankton. Our results indicate that Hg transformations and uptake might be enhanced under NO–PP-limitation conditions, emphasizing their impact on the transfer of Hg to higher trophic levels.
... Picoplankton biomass estimation. Biomasses of studied picoplankton groups were calculated by using the following cell-tocarbon conversion factors: 20 fgC cell −1 for heterotrophic bacteria (Lee and Fuhrman, 1987;Kirchman et al., 1993), 36 fgC cell −1 for Prochlorococcus (Buitenhuis et al., 2012), 255 fgC cell −1 for Synechococcus (Buitenhuis et al., 2012) and 2590 fgC cell −1 for picoeukaryotes (Buitenhuis et al., 2012). ...
Global and atmospheric climate change is altering the thermal conditions in the Adriatic Sea and, consequently, the marine ecosystem. Along the eastern Adriatic coast sea surface temperature (SST) increased by an average of 1.03 °C during the period from 1979 to 2015, while in the recent period, starting from 2008, a strong upward almost linear trend of 0.013 °C/month was noted. Being mainly oligotrophic, the middle Adriatic Sea is characterized by the important role played by the microbial food web in the production and transfer of biomass and energy towards higher trophic levels. It is very important to understand the effect of warming on microbial communities, since small temperature increases in surface seawater can greatly modify the microbial role in the global carbon cycle. In this study, the Self-Organizing Map (SOM) procedure was used to analyse the time series of a number of microbial parameters at two stations with different trophic status in the central Adriatic Sea. The results show that responses of the microbial food web (MFW) structure to temperature changes are reproducible in time. Furthermore, qualitatively similar changes in the structure of the MFW occurred regardless of the trophic status. The rise in temperature was associated with: (1) the increasing importance of microbial heterotrophic activities (increase bacterial growth and bacterial predator abundance, particularly heterotrophic nanoflagellates) and (2) the increasing importance of autotrophic picoplankton (APP) in the MFW.
... According to the metabolic theory of ecology, the rate of chemical reactions usually increases with temperature (Stanier et al., 1977;Gillooly et al., 2001, Brown et al., 2004. At low temperature, bacterial growth rate is generally considered to be limited (Pomeroy et al., 1991;White et al., 1991;Wiebe et al., 1992;Kirchman et al., 1993;Russell, 1993;Vrede, 2005). ...
Il s’agit de l’étude des successions écologiques du phytoplancton dans la lagune de Ghar El Melh au niveau de 5 stations pendant (i) un suivi bimensuel allant du janvier 2011 à janvier 2012 et (ii) un suivi saisonnier à partir du novembre 2012 à mars 2013. L’étude écologique a été consacrée au microphytoplancton en relation avec les facteurs abiotiques (température, salinité et nutriments) et biotiques (microalgues épiphytes, kystes des dinoflagellés, ultraplancton et ciliés). L’analyse hydrologique du milieu a montré une variation principalement saisonnière de la plupart des paramètres avec des valeurs élevées de la température et de la salinité en été et des concentrations exceptionnelles des nutriments en automne et en hiver. Le phytoplancton de la lagune de Ghar El Melh a été principalement dominé par les dinoflagellés et les diatomées avec une manifestation alarmante des espèces nuisibles atteignant plus que 70 % du peuplement. Ces proliférations ont été remarquées durant toute l’année avec une manifestation plus marquante dans les régions internes de la lagune ce qui est principalement tributaire à la température de l’eau, l’azote total, le nitrogène et l’orthophosphate. Une importante contribution des espèces épiphytes au phytoplancton a été signalée notamment celle du dinoflagellé toxique Prorocentrum lima. L’inventaire spécifique du phytoplancton dominé par l’espèce Prorocentrum micans (> 28 %) et d’autres espèces ne produisant pas de kyste a été en concordance avec les faibles valeurs enregistrées pour les kystes de dinoflagellés dans le sédiment où le maximum enregistré a été de l’ordre de 229 kystes g-1. Une part importante de l’ultraplancton a été représentée par le nanophytoplancton et les hétérotrophes procaryotes, dépassant largement les concentrations microphytoplanctoniques, ce qui est un indice de l’eutrophisation du milieu. Des corrélations hautement significatives ont été trouvées entre le microphytoplancton et les organismes microzooplanctoniques évoquant un probable rôle de ces derniers dans le contrôle des proliférations microalguales nuisibles.
... Briefly, rates of 3 H-leucine (20 nmol/L final concentration at 69 Ci mmol −1 , Amersham) incorporation into the cold trichloroacetic acid insoluble fraction were measured in a scintillation counter in four 1.7 ml subsamples incubated on a rotator at in situ temperatures for 1 hr. Rates of 3 H-leucine uptake were converted to rates of carbon production, assuming a conversion factor of 3.09 kg C mol leu −1 (Kirchman, Keil, Simon, & Welschmeyer, 1993). ...
Estuarine turbidity maxima (ETM) function as hotspots of microbial activity and diversity in estuaries, yet, little is known about the temporal and spatial variability in ETM bacterial community composition. To determine which environmental factors affect ETM bacterial populations in the Columbia River estuary, we analyzed ETM bacterial community composition (Sanger sequencing and amplicon pyrosequencing of 16S rRNA gene) and bulk heterotrophic production (3H-leucine incorporation rates). We collected water 20 times to cover five ETM events and obtained 42 samples characterized by different salinities, turbidities, seasons, coastal regimes (upwelling vs. downwelling), locations, and particle size. Spring and summer populations were distinct. All May samples had similar bacterial community composition despite having different salinities (1–24 PSU), but summer non-ETM bacteria separated into marine, freshwater, and brackish assemblages. Summer ETM bacterial communities varied depending on coastal upwelling or downwelling conditions and on the sampling site location with respect to tidal intrusion during the previous neap tide. In contrast to ETM, whole (>0.2 μm) and free-living (0.2–3 μm) assemblages of non-ETM waters were similar to each other, indicating that particle-attached (>3 μm) non-ETM bacteria do not develop a distinct community. Brackish water type (ETM or non-ETM) is thus a major factor affecting particle-attached bacterial communities. Heterotrophic production was higher in particle-attached than free-living fractions in all brackish waters collected throughout the water column during the rise to decline of turbidity through an ETM event (i.e., ETM-impacted waters). However, free-living communities showed higher productivity prior to or after an ETM event (i.e., non-ETM-impacted waters). This study has thus found that Columbia River ETM bacterial communities vary based on seasons, salinity, sampling location, and particle size, with the existence of three particle types characterized by different bacterial communities in ETM, ETM-impacted, and non-ETM-impacted brackish waters. Taxonomic analysis suggests that ETM key biological function is to remineralize organic matter.
... The radioactivity incorporated into the pellet was counted using a Packard Tri-Carb 4000TR scintillation counter. BP was calculated according to Kirchman et al. (1993), from the 3 H-leucine incorporation rates. ...
The effects of atmospheric deposition on plankton community structure were examined during a mesocosm experiment using water from the Cretan Sea (Eastern Mediterranean), an area with a high frequency of atmospheric aerosol deposition events. The experiment was carried out under spring-summer conditions (May 2012). The main objective was to study the changes induced from a single deposition event, on the autotrophic and heterotrophic surface microbial populations, from viruses to zooplankton. To this end, the effects of Saharan dust addition were compared to the effects of mixed aerosol deposition on the plankton community over 9 days. The effects of the dust addition seemed to propagate throughout the food-web, with changes observed in nearly all of the measured parameters up to copepods. The dust input stimulated increased productivity, both bacterial and primary. Picoplankton, both autotrophic and heterotrophic capitalized on the changes in nutrient availability and microzooplankton abundance also increased due to increased availability of prey. Five days after the simulated deposition, copepods also responded, with an increase in egg production. The results suggest that nutrients were transported up the food web through autotrophs, which were favored by the Nitrogen supplied through both treatments. Although, the effects of individual events are generally short lived, increased deposition frequency and magnitude of events is expected in the area, due to predicted reduction in rainfall and increase in temperature, which can lead to more persistent changes in plankton community structure. Here we demonstrate how a single dust deposition event leads to enhancement of phytoplankton and microzooplankton and can eventually, through copepods, transport more nutrients up the food web in the Eastern Mediterranean Sea.
... Heterotrophic bacteria play a central role in the microbial loop, and increasing evidence indicates that not only phytoplankton but also heterotrophic bacteria are limited by inorganic nutrients, mainly P and N, in oligotrophic oceanic systems [1,2]. There are many factors regulating bacterial distribution and growth, such as grazing by micro zooplankton, viral infection, and the supply of these resources like P, N, and effects of temperature. ...
Bacteria, as an essential part of microbial food web, play a significant role in the marine ecosystem. Dust deposits into the surface ocean carrying with vital nutrient such as Inorganic nitrogen and phosphorus etc., which has an important influence on the life activities of heterotrophic bacteria. The microcosm experiments with Asian dust deposition was carried out on board in the station K3 (26.18°N, 136.73°E) in April 2015, aiming to estimate the impact of dust deposition on the oligotrophic Northwestern pacific Sea, the main goal of the present paper was to assess how dust deposition events affect the abundance and activity of heterotrophic bacteria in low nutrient and low chlorophyll (LNLC) sea area. Station K3 located in the central northwestern Pacific Ocean, which has the characteristic of low nutrient and low chlorophyll. The study shows that there was an N–P co-limitation in station K3, and the deposition of Asian dust can increase the abundance, and promote the activity of heterotrophic bacteria in the station K3.
... Later, the samples were returned to the Takuvik Radio-isotope Laboratory at Université Laval where samples were processed using the centrifugation method of Smith and Azam (1992) and then radioassayed in a scintillation counter to measure [ 3 H]leucine incorporation into protein. A conversion factor of 3.1 kg C mol leucine −1 was used to estimate net bacterial carbon production (Iriberri et al. 1990;Kirchman et al. 1993). ...
Permafrost thawing and erosion results in the enrichment of northern lakes by soil organic matter. These allochthonous inputs favour bacterial decomposition and may cause the draw-down of dissolved oxygen to anoxic conditions that promote methanogenesis. Our objective in the present study was to determine the seasonal variations in dissolved oxygen in a set of permafrost peatland lakes in subarctic Quebec, Canada, and to relate these changes to metabolic rates, ice cover, and mixing. The lakes had high dissolved organic carbon concentrations, and their surface waters in summer had greenhouse gas concentrations that were up to one (CO2) to three (CH4) orders of magnitude above air-equilibrium values, indicating their strongly heterotrophic character. Consistent with these observations, the peatland lakes had elevated rates of bacterial production and oxygen consumption. Continuous measurements of oxygen by in situ sensors and of ice cover by automated field cameras showed that the lakes became fully anoxic shortly after freeze-up. The waters were partially re-oxygenated by mixing events in spring and fall, but in one lake, the bottom waters remained anoxic throughout the year. These observations provide a foundation for subsequent biogeochemical and modelling studies of peatland thaw lakes as an abundant class of Arctic freshwater ecosystems.
... Abundance was converted to carbon biomass assuming 20 fgC cell −1 for heterotrophic bacteria (Lee and Fuhrman, 1987;Kirchman et al., 1993) and 0.22 pgC μm −3 for HNF (Borsheim and Bratbak, 1987). For HNF, the estimation of biovolume was performed using the lengths and widths of flagellate cells (300 cells were measured for each experiment). ...
Mediterranean Sea (including Adriatic Sea) has been identified as a ‘hotspot’ for climate change, with prediction of the increase in water temperature of 2-4 °C over next few decades. Being mainly oligotrophic, and strongly phosphorus limited, Adriatic Sea is characterised by important role of microbial food web in production and transfer of biomass and energy towards higher trophic levels. We hypothesized that predicted rise in temperature of 3 °C in the near future may be resulted with increase of bacterial production and bacterial losses to grazers, which could significantly enlarged trophic base for metazoans. This empirical study, based on combined ‘space-for-time-substitution’ analysis (performed on 3583 data sets) and experimental approach (36 in situ grazing experiments performed at different temperatures), showed that predicted temperature increase of 3 °C in the near future, as a result of global warming, could cause a significant increase in bacterial growth at temperatures lower than 16o C (during the colder winter-spring period, as well as in the deeper layers). The effect of temperature on bacterial growth could be additionally doubled in conditions without phosphorus limitation. Furthermore, temperature increase of 3 °C, could double the grazing on bacteria by HNF and ciliate predators, and could increase the proportion of bacterial production transferred to metazoan food web by 42%. Therefore, it is expected that global warming may further strengthen the role of microbial food web in a carbon cycle in the Adriatic Sea.
... Samples were processed using the centrifugation method of Smith and Azam (1992) and then radio-assayed in a scintillation counter to measure the 3 H-leucine uptake. Net bacterial C production was estimated using a conversion factor of 3.1 kg C per mole of leucine (Kirchman et al., 1993;Iriberri et al., 1990). Bacterial community specific growth rates (BG, d −1 ) were determined (as described in Kirchman, 2001) via BG = BP/B, where BP are bacterial production rates (µg C L −1 h −1 ), and B is the bacterial biomass (fg C mL −1 ) that was converted from bacterial cellular counts (cells mL −1 ) to units of carbon using an average cellular carbon content of 18 fg C cell −1 , as measured in the nearby KWK lakes (Roiha et al., 2015). ...
Peatlands extend over vast areas of the northern landscape. Within some of these areas, lakes and ponds are changing in size as a result of permafrost thawing and erosion, resulting in mobilization of the carbon-rich peatland soils. Our aims in the present study were to characterize the particle, carbon and nutrient regime of a set of thermokarst (thaw) lakes and their adjacent peatland permafrost soils in a rapidly degrading landscape in subarctic Québec, Canada, and by way of fluorescence microscopy, flow cytometry, production measurements and an in situ enrichment experiment, determine the bacterial characteristics of these waters relative to other thaw lakes and rock-basin lakes in the region. The soil active layer in a degrading palsa (peatland permafrost mound) adjacent to one of the lakes contained an elevated carbon content (51 % of dry weight), high C : N ratios (17 : 1 by mass), and large stocks of other elements including N (3 % of dry weight), Fe (0.6 %), S (0.5 %), Ca (0.5 %) and P (0.05 %). Two permafrost cores were obtained to a depth of 2.77 m in the palsa, and computerized tomography scans of the cores confirmed that they contained high concentrations (> 80 %) of ice. Upon thawing, the cores released nitrate and dissolved organic carbon (from all core depths sampled), and soluble reactive phosphorus (from bottom depths), at concentrations well above those in the adjacent lake waters. The active layer soil showed a range of particle sizes with a peak at 229 µm, and this was similar to the distribution of particles in the upper permafrost cores. The particle spectrum for the lake water overlapped with those for the soil, but extended to larger (surface water) or finer (bottom water) particles. On average, more than 50 % of the bacterial cells and bacterial production was associated with particles > 3 µm. This relatively low contribution of free-living cells (operationally defined as the
... Samples were processed using the centrifugation 244 method of Smith and Azam (1992) and then radio-assayed in a scintillation counter to measure 245 the 3 H-leucine uptake. Net bacterial C production was estimated using a conversion factor of 3.1 246 kg C per mole of leucine (Kirchman et al., 1993;Iriberri et al., 1990). Bacterial community 247 specific growth rates were determined using an average cellular carbon content of 18 fg C cell -1 , 248 as measured in the nearby KWK lakes (Roiha et al., 2015). ...
Peatlands extend over vast areas of the northern landscape. Within some of these areas, lakes and ponds are changing in size as a result of permafrost thawing and erosion, resulting in mobilisation of the carbon-rich peatland soils. Our aims in the present study were to characterize the particle, carbon and nutrient regime of a set of thermokarst (thaw) lakes and their adjacent peatland permafrost soils in a rapidly degrading landscape in subarctic Québec, Canada, and by way of fluorescence microscopy, flow cytometry, production measurements and an in situ enrichment experiment, determine the bacterial characteristics of these waters relative to other thaw lakes and rock-basin lakes in the region. The soil active layer in a degrading palsa (peatland permafrost mound) adjacent to one of the lakes contained an elevated carbon content (51 % of dry weight), low C:N ratios (17:1 by mass), and large stocks of other elements including N (3 % of dry weight), Fe (0.6 %), S (0.5 %), Ca (0.5 %) and P (0.05 %). Two permafrost cores were obtained to a depth of 2.78 m in the palsa, and CT scans of the cores confirmed that they contained high concentrations (> 80 %) of ice. Upon thawing, the cores released nitrate and dissolved organic carbon (from all core depths sampled), and soluble reactive phosphorus (from bottom depths), at concentrations well above those in the adjacent lake waters. The active layer soil showed a range of particle sizes with a peak at 229 µm, and this was similar to the distribution of particles in the upper permafrost cores. The particle spectrum for the lake water overlapped with those for the soil, but extended to larger (surface water) or finer (bottom water) particles. On average, more than 50 % of the bacterial cells and bacterial production was associated with particles > 3 µm. This relatively low contribution of free-living cells (operationally defined as the < 1 µm fraction) to bacterial production was a general feature of all of the northern lakes sampled, including other thaw lakes and shallow rock-basin lakes (average ± SE of 25 ± 6 %). However, a distinguishing feature of the peatland thaw lakes was significantly higher bacterial specific growth rates, which averaged 4 to 7 times higher values than in the other lake types. The in situ enrichment experiment showed no no difference between organic carbon or phosphorus enrichment treatments at day 5 relative to the control, however there was a significant, > 100 % increase in bacterial growth rates between days 1 and 5 in the soil and the carbon plus phosphorus enrichments. Collectively these results indicate that particles, nutrients and carbon are released by degrading permafrost peatland soils into their associated thermokarst lakes, creating favorable conditions for production by particle-based as well as free-living aquatic bacterial communities. The reduced bacterial concentrations despite high cellular growth rates imply that there is strong control of their population size by loss-related factors such as grazing and viral lysis.
... According to the metabolic theory of ecology, chemical reaction rates increase with temperature (Stanier et al., 1977;Gillooly et al., 2001;Brown et al., 2004). At a low temperature, the bacterial growth rate is generally considered to be limited (Pomeroy et al., 1991;White et al., 2010;Wiebe et al., 1992;Kirchman et al., 1993;Russell, 1993;Vrede, 2005). ...
The warming of the Arctic Ocean impacts the dissolved organic matter (DOM) imports into the Arctic region, which affects the local bacterial communities. This review addressed the current status of DOM inputs and their potential influences on bacteria data (e.g., population, production, and metabolic activity of bacteria), as well as the projected changes of DOM inputs and bacterial communities as a result of climate warming. Microbial communities are likely affected by the warming climate and the transport of DOM to the Arctic Ocean. Imported DOM can alter Arctic bacterial abundance, cell size, metabolism, and composition. DOM fluxes from Arctic River runoff and adjacent oceans have been enhanced, with warming increasing the contribution of many emerging DOM sources, such as phytoplankton production, melted sea ice, thawed permafrost soil, thawed subsea permafrost, melted glaciers/ice sheets, atmospheric deposition, groundwater discharge, and sediment efflux. Imported DOM contains both allochthonous and autochthonous components; a large quantity of labile DOM comes from emerging sources. As a result, the Arctic sea water DOM composition is transformed to include a wider range of various organic constituents such as carbohydrates (i.e., glucose), proteinaceous compounds (i.e., amino acid and protein-like components) and those with terrigenous origins (i.e., humic-like components). Changes to DOM imports can alter Arctic bacterial abundance, cell size, metabolism, and composition. Under current global warming projections, increased inflow of DOM and more diverse DOM composition would eventually lead to enhanced CO2 emissions and frequent emergence of replacement bacterial communities in the Arctic Ocean. Understanding the changes in DOM fluxes and responses of bacteria in the Arctic broadens our current knowledge of the Arctic Ocean’s responses to global warming.
Meromictic Lake Cadagno, an ancient ocean analogue, is known for its permanent stratification and persistent anoxygenic microbial bloom within the chemocline. Although the anaerobic microbial ecology of the lake has been extensively studied for at least 25 years, a comprehensive picture of the microbial food web linking the bacterial layer to phytoplankton and viruses, with explicit measures of primary and secondary production, is still missing. This study sought to understand better the abundances and productivity of microbes in the context of nutrient biogeochemical cycling across the stratified zones of Lake Cadagno. Photosynthetic pigments and chloroplast 16S rRNA gene phylogenies suggested the presence of eukaryotic phytoplankton through the water column. Evidence supported high abundances of Ankyra judayi , a high-alpine adapted chlorophyte, in the oxic mixolimnion where oxygenic-primary production peaked. Through the low- and no-oxygen chemocline and monimolimnion, chlorophytes related to Closteriopsis acicularis , a known genus of meromictic lakes, and Parachlorella kessleri were observed. Chromatium , anoxygenic phototrophic sulfur bacteria, dominated the chemocline along with Lentimicrobium , a genus of known fermenters whose abundance was newly reported in Lake Cadagno. Secondary production peaked in the chemocline suggesting primary producers depend on heterotrophs for nutrient remineralization. As previously observed, sulfur-reducing bacteria (SRBs), especially Desulfocapsa and Desulfobulbus , were present in the chemocline and anoxic monimolimnion. Virus-to-microbe ratios (VMR) peaked in the zone of phytoplankton yet were at a minimum at the peak of Chromatium . These dynamic trends suggest viruses may play a role in the modulation of oxygenic and anoxygenic photo- and chemosynthesis in Lake Cadagno and other permanently stratified systems.
Importance
As a window to the past, the study offers insights into the role of microbial guilds of Proterozoic ocean chemoclines in the production and recycling of organic matter of sulfur- and ammonia-containing ancient oceans. The new observations described here suggest that eukaryotic algae were persistent in the low oxygen upper-chemocline in association with purple and green sulfur bacteria in the lower half of the chemocline. Further, this study provides the first insights into Lake Cadagno viral ecology. High viral abundances suggested viruses may be essential components of the chemocline where their activity may result in the release and recycling of organic matter. The framework developed in this study through the integration of diverse geochemical and biological data types lays the foundation for future studies to quantitatively resolve the processes performed by discrete populations comprising the microbial loop in this early anoxic ocean analogue.
Abstract This contribution describes the ocean biogeochemical component of the Geophysical Fluid Dynamics Laboratory's Earth System Model 4.1 (GFDL‐ESM4.1), assesses GFDL‐ESM4.1's capacity to capture observed ocean biogeochemical patterns, and documents its response to increasing atmospheric CO2. Notable differences relative to the previous generation of GFDL ESM's include enhanced resolution of plankton food web dynamics, refined particle remineralization, and a larger number of exchanges of nutrients across Earth system components. During model spin‐up, the carbon drift rapidly fell below the 10 Pg C per century equilibration criterion established by the Coupled Climate‐Carbon Cycle Model Intercomparison Project (C4MIP). Simulations robustly captured large‐scale observed nutrient distributions, plankton dynamics, and characteristics of the biological pump. The model overexpressed phosphate limitation and open ocean hypoxia in some areas but still yielded realistic surface and deep carbon system properties, including cumulative carbon uptake since preindustrial times and over the last decades that is consistent with observation‐based estimates. The model's response to the direct and radiative effects of a 200% atmospheric CO2 increase from preindustrial conditions (i.e., years 101–120 of a 1% CO2 yr−1 simulation) included (a) a weakened, shoaling organic carbon pump leading to a 38% reduction in the sinking flux at 2,000 m; (b) a two‐thirds reduction in the calcium carbonate pump that nonetheless generated only weak calcite compensation on century time‐scales; and, in contrast to previous GFDL ESMs, (c) a moderate reduction in global net primary production that was amplified at higher trophic levels. We conclude with a discussion of model limitations and priority developments.
A recent analysis of the Mediterranean Sea surface temperature showed significant annual warming. Since small picoplankton microorganisms play an important role in all major biogeochemical cycles, fluxes and processes occurring in marine systems (the changes at the base of the food web) as a response to human-induced temperature increase, could be amplified through the trophic chains and could also significantly affect different aspects of the structure and functioning of marine ecosystems. In this study, manipulative laboratory growth/grazing experiments were performed under in situ simulated conditions to study the structural and functional changes within the microbial food web after a 3 °C increase in temperature. The results show that a rise in temperature affects the changes in: (1) the growth and grazing rates of picoplankton, (2) their growth efficiency, (3) carrying capacities, (4) sensitivity of their production and grazing mortality to temperature, (5) satisfying protistan grazer carbon demands, (6) their preference in the selection of prey, (7) predator niche breadth and their overlap, (8) apparent uptake rates of nutrients, and (9) carbon biomass flow through the microbial food web. Furthermore, temperature affects the autotrophic and heterotrophic components of picoplankton in different ways.
Growth of heterotrophic bacteria is generally considered to be controlled by temperature and the availability of organic substrates, however there is evidence that bacterial growth can also be limited by the concentrations or supply rate of inorganic nutrients (i.e. nitrogen, phosphorus or iron). We examined spatial and seasonal patterns of organic carbon and inorganic nutrient (N and P) limitation of bacterial growth along each of two meridional transects through the Atlantic Ocean, during contrasting seasons. Here we used nutrient bioassays to demonstrate widespread inorganic nutrient limitation and co-limitation with organic carbon in the oligotrophic temperate, tropical and subtropical ocean. There were distinct seasonal and spatial differences in the inorganic and organic nutrient limitation of bacterial growth, with inorganic nitrogen as the primary limiting nutrient in May/June, and inorganic nitrogen and organic carbon co-limiting growth in October/November. There was no evidence that the availability of inorganic phosphorus limited bacterial growth in the Southern Hemisphere. We propose that the patterns of nutrient-dependent bacterial growth reflect seasonal and spatial differences in aeolian inputs and the quality of dissolved organic matter, and that bacteria directly compete with autotrophs for inorganic nutrients in the oligotrophic regions of the World Ocean. The findings of this study have important implications for understanding the balance between the biological and microbial carbon pumps, and the modelling of the net metabolic balance of the Ocean in response to climate-driven changes in nutrient inputs.
Bacterial abundance, phytoplankton community structure and environmental parameters were investigated to study the relationships between bacteria and phytoplankton during giant jellyfish Nemopilema nomurai blooms in the central Yellow Sea during 2013. N. nomurai appeared in June, increased in August, reached a peak and began to degrade in September 2013. Results showed that phosphate was possible a key nutrient for both phytoplankton and bacteria in June, but it changed to nitrate in August and September. Phytoplankton composition significantly changed that pico-phytoplankton relative biomass significantly increased, whereas other size phytoplankton significantly decreased during jellyfish bloom. In June, a significantly positive correlation was observed between chlorophyll a concentration and bacterial abundance (r=0.67, P<0.001, n=34). During jellyfish outbreak in August, there was no significant correlation between phytoplankton and bacteria (r=0.11, P>0.05, n=25), but the relationship (r=0.71, P<0.001, n=31) was rebuilt with jellyfish degradation in September. In August, small size phytoplankton occupied the mixed layer in offshore stations, while bacteria almost distributed evenly in vertical. Chlorophyll a concentration significantly increased from (0.42±0.056) μg/L in June to (0.74±0.174) μg/L in August, while bacterial abundance just slightly increased. Additionally, the negative net community production indicated that community respiration was not entirely determined by the local primary productivity in August. These results indicated that jellyfish blooms potentially affect coupling of phytoplankton and bacteria in marine ecosystems.
Recent developments in high temperature catalytic oxidation techniques (Sugimura and Suzuki, 1988) have raised a number of questions about dissolved organic carbon (DOC). Several key aspects of the DOC pool are still unknown, including its size (Martin & Fitzwater, 1992; Ogawa & Ogura, 1992) and turnover rates (Kirchman et al., 1991; Kepkay & Wells, 1992; Bauer et al., 1992). Even the low estimates of DOC concentrations, however, imply that DOC is large enough (700 × 109 tons C) to rival the terrestrial biomass and atmospheric CO2. Thus the dissolved carbon pool needs to be included in biogeochemical models. Yet uncertainty about concentrations and many key aspects of DOC production and consumption does limit our ability to include DOC in these models. Furthermore, the lack of data allows a model to evoke DOC with few restrictions in order to explain otherwise unresolved issues. Because of these uncertainties, the DOC working group spent more time discussing what is known about DOC than about modelling it. What emerged from that discussion are some recommendations for future experimental research in order to provide data that would aid in modeling more accurately the role of DOC in biogeochemical processes.
Primary production by phytoplankton is the largest source of organic matter in the sea. Photosynthetically produced organic matter in the photic layer is transferred to higher and lower trophic levels through marine food webs. Organic matter in the surface water is also transported to the intermediate and deep waters, as well as to the sediment. As a consequence, organic matter plays a role in many biogeochemical processes in the water column, for example, the fueling of marine food webs and the control of the dynamics of other chemicals (by, for example, the regeneration of nutrients, the consumption of dissolved oxygen, and the scavenging of metals, radionuclides, pollutants, etc.), on the one hand, while, on the other hand, organic matter itself is transformed to detrital particulate organic matter (POM) and dissolved organic matter (DOM). The resultant detrital POM and DOM are also tightly coupled to the biogeochemical cycle in the marine system.
Marine chemists have been gradually paying increasing attention to strong ligands of low levels involved in dissolved organic matter (DOM) and particulate organic matter (POM) (Dawson and Duursma, 1981; Farrington, 1992). However, studies on the strong organic ligands in seawater have been focused upon only the field of metal speciation. Knowledge about the natural strong organic ligands has been scanty until now. In this report, the strong organic ligands are defined as organics with some functional groups to form complexes with metals under the conditions of seawater. The strong organic ligands, therefore, may be classified as chelators (Buffle, 1988), which can sterically form a claw-like structure with coordination bonds; EDTA (ethlenediaminetetra-acetic acid)chelation is the commonest example given. Such strong organic ligands may be closely related to the biogeochemical behavior of chemical constituents in the marine environments: first, strong ligands in DOM and POM play a significant role in controling chemical forms and levels of trace metals in seawater; second, strong ligands link the interaction between particulate matter and trace metals including radionuclides, which is related to removal processes of “so-called” particle-reactive metals (Hirose et al., 1992); third, strong ligands are related to the bioavailability of trace metals from marine organisms (Barber and Ryther, 1969; Hirose and Sugimura, 1985). On the other hand, if strong ligands are well characterized chemically, they may become an indicator for understanding the behavior of organic matter in marine environments.
Approximately 71% of the earth's surface is covered by oceans, and represent an enormous pool of potential microbial biodiversity and exploitable biotechnology or "blue biotechnology". This unexploited region resulted in an increasing interest to study marine microorganisms, the role of them in marine food webs, and biogeochemical cycling. In addition, to emphasize the study of marine bacteria capable of produced different novel enzymes and metabolites, represent a potential to biotechnological applications. Diverse marine microorganisms are classified into extremophiles groups, which play a main role in the biodegradation of organic matter in different marine ecosystems. One of these groups, the cold-adapted microorganisms, is interesting for processes that need cold-active enzymes at low temperatures, for example food processing and cold-wash laundry detergents. In the last decade, the focus of microbial diversity has changed due to 16S rDNA, housekeeping genes and metagenomic studies. As a result, the characterization of new sequences allowed the discovery of new genera of cultivated and uncultivated microorganisms, from samples of the marine ecosystem. These new species belong to the γ-and α-Proteobacteria divisions. To understand the diversity of marine bacteria within the areas of exploration, there is the sub-Antarctic waters of the Beagle Channel, Tierra del Fuego, Argentina. The Beagle Channel (55°07'18" S 66°25'00" W) is located at the southern end of America and is the international border between Argentina and Chile. Coastal areas are interesting to study because they are easily accessible and the water temperature ranges between 4.5 and 10°C, optimum for cold-adapted microorganisms. Marine bacteria were isolated from benthonic organism's intestines and seawater samples. The samples were taken from different coastal areas of the Beagle Channel; they were enriched in modified LB medium with seawater and cultivated at 4 and 15°C. Of 296 marine bacteria isolated, 55 were characterized by ARDRA analysis, 16S rDNA sequencing, and construction of phylogenetics trees. The gene sequences allowed us to determine their association with the class Proteobacteria, members of the following genera: Shewanella, Pseudomonas, Pseudoalteromonas, Serratia, Halomonas, Alteromonas, Psychrobacter. These microorganisms showed different glycosyl hydrolases activities, within this group are cellulase, endoglucanase, exoglucanasa, xylanase, β-glucosidase, and α-rhamnosidase. In this chapter, we focus on the studies of marine bacteria, genes, enzymes and metabolites for biotechnological applications, as well as new cold-active enzymes applied to products whose manufacturing processes are performed at low temperatures.
Bacterioplankton are not only decomposers of organic material but also secondary procurers in water ecosystems. They are abundant and dynamic members of remobilizalion of organic carbon and regeneration of inorganir nutrients in water environments. This paper presents the seasonal variation in the abundance of bacteri op lank loti and the difference in different zone, the relationship between the environmental fadors and ihe abundance of bacte-rioplankton in ihe North Lake Taihu from January to December in 2003, the results showed that the numher of bacte-rioplankton changes with the seasons. With the increase of water temperature, the number of bacterioplanklon in the water increases, the smallest value in winter,and the largest value in summer. The number of baolerioplanklon was closely correlated with the concentration of chlorophyll a (r²= 0. 8301,n= 12,P <0.01) and waler temperature (r²=0.463,n= 12,P<0.01) respectively, but has no correlation with the nutriment. The results indicate that ihe nutrimental substance is very high in Lake Taihu, and is not be the limiting faclrs for bacterioplankton; the DOC is an important source for bacterioplanklon which comes from the ph^loplanklon.
The structural and functional parameters of bacterioplankton and their dynamics were evaluated in different oceanographic domains of the Eastern Bering Sea in the late spring and summer. The numbers of bacteria varied from 0.5 × 106 to 3.9 × 106 cells/ml, the bacterial biomass ranged from 7 to 110 mg C/m3, and the bacterial production varied from 0.32 to 27.4 mg C/m2 per day. The minimum values of the bacterial abundance and production were observed during the termination of the phytoplankton spring bloom, and the maximum values were found at the end of July, i.e, one and half months later. The intense growth of bacterioplankton in late July was basically defined by the significant concentrations of the substrate rather than by the temperature conditions. The ratio of bacterial carbon to phytoplankton carbon ranged from 6 to 15% during the decline of the phytoplankton bloom and increased up to 250-800% (in specific cases up to more than 2000%) in the period of the bacterial maximum. The respective changes in the ratio of bacterial production to primary production were from 0.5 to 61 (98)%. The share of the primary production involved in the bacterial food web varied from 1-4% in the time of the spring bloom termination to 27-308% during the bacterial peak. In this period, primary production provided from 5 to 50% of the bacterial production and bacterioplankton consumed the organic matter accumulated in the pelagic zone during the time before the bacterial peak.
Callianassid (ghost) shrimp has been claimed as an ecosystem engineer, as it is one of the most powerful bioturbating macrobenthos in intertidal sandflats. However, our knowledge about the relationship between areal distribution of bottom-dwelling ghost shrimps and dynamics of sediment microbial community structure remains obscured. We used automated ribosomal intergenic spacer analysis (ARISA) to reveal the bacterial community dynamics in the sediment of intertidal sandflat of Tomioka Bay, Kyushu, Japan, which is predominantly inhabited by a burrow-dwelling callianassid shrimp Nihonotrypaea harmandi. We found that the bacterial community structures of high and middle shrimp population areas were significantly differentiated from those of low population area (ANOSIM, R=0.10-0.18, p<0.01), while the former two areas were statistically indistinguishable (ANOSIM, R=-0.015, p>0.1). These results illustrated the potential importance of shrimp population density as a key factor in shaping the bacterial community structure and interpreting their dynamics in the sandflat. Furthermore, greater similarity between burrow and non-burrow communities was found in samples taken in autumn through winter than in those in summer (one-way ANOVA, p<0.05), whereas the phylotype richness was not simply differentiated by seasons. These results suggest not only that environmental variables including water temperature and salinity of the water column overlying the sandflat could exert notable impacts on the sediment bacterial community dynamics, but that the bio-irrigation and mixing by the ghost shrimp in permeable sandflat would strongly homogenize sediment particles, enhance solute transport surrounding the burrow and ambient subsurface substrate, and therefore reduce spatial differentiation of the bacterial community structure between the two sites. A comparison between present and previous studies of axiidean (former taxonomic group name, thalassinidean) ghost shrimps provides us with a comprehensive understanding of the shrimps' impacts on bacterial community dynamics, highlighting the importance of sediment permeability, a characteristic determined by the type of sediment, as a key controlling factor to shape spatial heterogeneity of bacterial community structure around burrow.
In order to understand the roles of the bacterial community in brown tide outbreaks, the bacterial community characteristics in the Qinhuangdao coastal area of Bohai Sea were investigated by pyrosequencing-based approach from April to August 2013. In the studied region, the brown tide has broken out recurrently since 2009. The results showed that the bacterial community had the highest abundance and diversity in August and the lowest value in May. There were clear discrepancies in the species composition and their abundances across months. Proteobacteria was the most predominant phyla, accounting for 46.26%, 41.11%, 49.69%, 38.48% and 40.55% in months from April to August, respectively. α-Proteobacteria was the first predominant class, accounting for 42.20%, 34.36%, 44.68%, 32.17% and 28.79% in months from April to August, respectively. Chloroplast_norank was the first predominant genus in April (21.48%), May (16.44%) and July (11.66%), while Roseobacter_clade_DC5-80-3_lineage became the first predominant genus in June (17.85%) and Candidatus_pelagibacter was the first predominant genus in August (15.55%). Canonical correlation analysis (CCA) suggested that nitrate ρ(NO3-N), ρ(Chla) and trophic level index (TLI) were the main factors regulating the bacterial community, indicating that there were close relationships among the bacterial community, algal biomass and eutrophication in the studied region. Details of the regulation of the bacterial community to algal community deserve further study. ©, 2015, Editorial department of Molecular Catalysis. All right reserved.
We analyzed heterotrophic, pelagic bacterial production and specific growth rate data from 57 studies conducted in fresh, marine and estuarine/coastal waters. Strong positive relationships were identified between 1) bacterial production and bacterial abundance and 2) bacterial production and algal biomass. The relationship between bacterial production and bacterial abundance was improved by also considering water temperature. The analysis of covariance model revealed consistent differences between fresh, marine and estuarine/coastal waters, with production consistently high in estuarine/coastal environments. The log-linear regression coefficient of abundance was not significantly different from 1.00, and this linear relationship permitted the use of specific growth rate (SGR in day(-1)) as a dependent variable. A strong relationship was identified between specific growth rate and temperature. This relationship differed slightly across the three habitats. A substantial portion of the residual variation from this relationship was accounted for by algal biomass, including the difference between marine and estuarine/coastal habitats. A small but significant difference between the fresh- and saltwater habitats remained. No significant difference between the chlorophyll effect in different habitats was identified. The model of SGR against temperature and chlorophyll was much weaker for freshwater than for marine environments. For a small subset of the data set, mean cell volume accounted for some of the residual variation in SGR. Pronounced seasonality, fluctuations in nutrient quality, and variation of the grazing environment may contribute to the unexplained variation in specific growth.
To determine if dissolved organic matter (DOM) limits biomass production of heterotrophlc bacterioplankton in the subarctic Pacific, the effect of various DOM and ammonium additions on bacterial production (3 ~-t h y n ~ l d i n e and '4C-leucine incorporation) and bacterial abundance was examined. Addition of dissolved free amlno acids (DFAA) consistently stimulated 3 ~-t h y m i d i n e incorporation from 31 to 393 % con~pared with unamended controls. Addition of glucose or glucose plus ammonium sometimes stimulated bacterial production, but the effect was always less than that due to DFAA additions A mixture of alkylamines either had no effect or stimulated 3 ~-t h y m i d i n e and 14C-leucine incorporahon to a lesser extent than the DFAA adhtion. Bacterial abundance did not vary significantly during incubatlons, nor were there any differences between treatments, indicating that DFAA add~tions stimulated the average growth rate of the bacterial assemblage. Bactenal growth appeared to b e C-hmited, sometimes, since glucose alone stimulated 3H-thynlldine and '4C-leucine in 2 out of 7 expen-ments. The much greater stimulation of bactenal production by DFAA than by glucose plus ammonium indicated that heterotrophic bacteria in the subarctic Pacific were usually energy-limited.
Postulating that microbial metabolism and production in cold waters are limited by the ability of bacteria to transport and/or assimilate substrates at the low concentrations usually present, the authors measured rates of microbial activity in the water column and benthos in Conception Bay, Newfoundland and adjacent coastal waters, during three spring blooms. Bacterioplankton numbers during the spring phytoplankton blooms exceeded 5 × 105 ml-1 only 15% of the time. These numbers rank at the lower end of the range of bacterial numbers of the world's ocean. Bacterial productivity suggested average generation times of 30-86 d. Bacterial production and respiration, averaged over the entire water column, plus benthic aerobic respiration and denitrification, accounted for 3% of primary production during the early bloom and 28% of primary producing during the late bloom. The difference between the early and late bloom is related to changes in primary production, not to increased microbial activity. Unless bacteiral assimilation efficiency was very low, much less than half of the organic production of the spring diatom bloom was used by microbial processes during the early, highly productive phase of the bloom, while over half of organic production was used in the short term during the later, less productive phase of the spring bloom. -from Authors
This study examines the use of incorporation rates of thymidine (TdR) and leucine (Leu) in order to estimate bacterial production in the oceanic subarctic Pacific. Incorporation rates at different concentrations suggest that isotope dilution is low when 5 nM TdR is used, but other data indicate at least 3-fold and 2-fold dilution for TdR and Leu, respectively. The ratio of incorporation into the hot acid insoluble fraction, compared to the cold acid insoluble fraction, is 0.66 for TdR and 0.86 for Leu. Empirical conversion factors were measured in 10 experiments during four 1 mo-long cruises. Conversion factors did not vary systematically and were not affected by additions of organic compounds or ammonium. Average conversion factors were 1.74 X 10(18) cells mol-1 of incorporated TdR (SD = 0.89 X 10(18) cells mol-1) and 0.108 X 10(18) cells mol-1 of incorporated Leu (SD = 0.085 X 10(18) cells mol-1). It was possible to estimate 'bottle-less' conversion factors during May 1988 when bacterial numbers increased greatly; in situ bottle-less conversion factors were 1.55 and 0.185 X 10(18) cells mol-1 for TdR and Leu, respectively. Estimates of bacterial production based on the TdR and Leu methods agreed within 10 and 25 % for 0 to 40 m (ca the mixed layer) and 40 to 80 m (bottom half of the euphotic zone), respectively.
Recent ice-core measurements have revealed that the atmospheric CO2 level increased comparatively rapidly by about 70 ppm at the end of the last ice age. An ocean-atmosphere model is presented in which changes in the productivity of high latitude surface waters (from which deep water is formed and circulated around the world's ocean) and/or in the thermohaline overturning rate can lead to substantial change in atmospheric partial pressure of carbon dioxide (PCO2), over a concentration range 163-425 ppm. A major contribution to the low PCO2 of the last ice age may have been an increase in the net high latitude productivity, possibly coupled with a decrease in the thermohaline overturning.
Measurements of bacterial biomass, production and mortality have been carried out in a large range of aquatic environments, including eutrophic and oligotrophic ones. The general trends of variations of bacterial biomass, size, specific growth rate and mortality rate in all these environments are examined. The overall flux of bacterial production is taken as an index of the flux of organic matter available to bacteria, thus characterizing the richness of the environment. Bacterial biomass is roughly proportional to richness, while mean cell size increases with it. The turnover rate of biomass, as revealed either by growth or by mortality rates, appears to be fairly independent of richness.
These observations are compatible with a simple resource-limited (bottom-up controlled) model of the dynamics of bacterioplankton. On the other hand, they are in contradiction with the predictions of a predator-controlled (top-down controlled) model.
This paper describes a dilution technique for estimating the micro-zooplankton grazing impact on natural communities of marine phytoplankton. Experiments performed in coastal waters off Washington, USA (October, 1980), yield estimates of micro-zooplankton impact equivalent to 6 to 24% of phytoplankton standing biomass and 17 to 52% of production per day. Indirect evidence suggests that most of this impact is due to the feeding of copepod nauplii and tintinnids; in contrast, non-loricate ciliates, comprising 80 to 90% of numerical abundance, appeared to contribute little to phytoplankton mortality.
Polycarbonate Nuclepore filters are better than cellulose filters for the direct counting of bacteria because they have uniform pore size and a flat surface that retains all of the bacteria on top of the filter. Although cellulose filters also retain all of the bacteria, many are trapped inside the filter where they cannot be counted. Before use, the Nuclepore filters must be dyed with irgalan black to eliminate autofluorescence. Direct counts of bacteria in lake and ocean waters are twice as high with Nuclepore filters as with cellulose filters.
Microscopic estimation of bacterial biomass requires determination of both biovolume and biovolume-to-biomass conversion. Both steps have uncertainty when applied to the very small bacteria typically found in natural seawater. In the present study, natural bacterioplankton assemblages were freshly collected, passed through 0.6-μm-pore-size Nuclepore filters to remove larger particulate materials, and diluted for growth in 0.22-μm-pore-size Millipore filter-sterilized unenriched seawater. This provided cells comparable in size and morphology to those in natural seawater, but the cultures were free of the interfering particulate detritus naturally present. Cells were collected on glass-fiber GF/F filters, and biovolumes were corrected for cells passing these filters; C and N were measured with a CHN analyzer. Our criteria for size measurement by epifluorescence photomicrography were confirmed with fluorescent microspheres of known diameters. Surprisingly, in six cultures with average per-cell biovolumes ranging from 0.036 to 0.073 μm ³ , the average per-cell carbon biomass was relatively constant at 20 ± 0.08 fg of C (mean ± standard error of the mean). The biovolume-to-biomass conversion factor averaged 0.38 ± 0.05 g of C cm ⁻³ , which is about three times higher than the value previously estimated from Escherichia coli , and decreased with increasing cell volume. The C:N ratio was 3.7 ± 0.2. We conclude that natural marine bacterial biomass and production may be higher than was previously thought and that variations in bacterial size may not reflect variations in biomass per cell.
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We examined the simultaneous incorporation of [ ³ H]thymidine and [ ¹⁴ C]leucine to obtain two independent indices of bacterial production (DNA and protein syntheses) in a single incubation. Incorporation rates of leucine estimated by the dual-label method were generally higher than those obtained by the single-label method, but the differences were small (dual/single = 1.1 ± 0.2 [mean ± standard deviation]) and were probably due to the presence of labeled leucyl-tRNA in the cold trichloroacetic acid-insoluble fraction. There were no significant differences in thymidine incorporation between dual- and single-label incubations (dual/ single = 1.03 ± 0.13). Addition of the two substrates in relatively large amounts (25 nM) did not apparently increase bacterial activity during short incubations (<5 h). With the dual-label method we found that thymidine and leucine incorporation rates covaried over depth profiles of the Chesapeake Bay. Estimates of bacterial production based on thymidine and leucine differed by less than 25%. Although the need for appropriate conversion factors has not been eliminated, the dual-label approach can be used to examine the variation in bacterial production while ensuring that the observed variation in incorporation rates is due to real changes in bacterial production rather than changes in conversion factors or introduction of other artifacts.
There has been an explosion of research on marine microbial foodweb processes in the past decade. Today it is widely accepted that about 50% of the primary production in marine and fresh water is processed by bacteria each day (Williams, 1981; Cole et al., 1988). This striking finding was stimulated, as others have noted, by the introduction of convenient methods for the estimation of microbial biomass and activities in natural waters. Hobbie et al. (1977) and Watson et al. (1977) demonstrated conclusively that bacterial populations in the sea were large. By 1980, in addition to the pioneering and prescient work by Sorokin (e.g., Sorokin, 1971, 1973), reports of bacterial production measurements had begun to emerge (Sieburth et al., 1977; Karl, 1979; Larsson and Hagstrom, 1979; Fuhrman and Azam, 1980). Brock (1971) and Sieburth (1977) wrote early reviews on the subject, and Pomeroy (1974) introduced the importance of marine microbial processes to a large audience. In this chapter we review recent research on bacterial production in the ocean. The emphasis is on the open sea, but we will also discuss other marine habitats, partly because there are still few comprehensive studies of oceanic bacterial production. There is an equally large and rapidly growing literature on bacterial production in fresh waters (Cole et al., 1988; Currie, 1990) which deserves a review of its own, as well as comparison with the marine findings (Hobbie, 1988). We will not review related work in sediments, nor for the most part, related work on bacteriovores.
The purpose of this study was to test the hypothesis that NH 4 ⁺ uptake by heterotrophic bacteria in the subarctic Pacific is C limited. Addition of glucose (0.5–1.0 µ M) stimulted NH 4 ⁺ uptake in unfractionated water but had no effect on bacterial abundance. Glucose stimulation of NH 4 ⁺ uptake was even greater in the bacterial size fraction (<0.8 µ m). Regeneration of ¹⁵ NH 4 ⁺ from added [ ¹⁵ N] amino acids was measurable in the bacterial size fraction, but glucose additions prevented net NH 4 ⁺ excretion because of increased uptake of NH 4 ⁺ . In the subarctic Pacific, NH 4 ⁺ concentrations range from undetectable (≤0.05 µ M) to 0.4 µ M, whereas the maximal estimate of monosaccharide concentrations was 0.025 M. These results indicate that the supply of C compounds like glucose can limit NH 4 ⁺ uptake by heterotrophic bacteria.
Seasonal and die1 changes in nutrient concentrations and nitrogen assimilation rates were used to assess the effects of NH,+ on NO,- assimilation. Surface-water NO,- concentrations ranged from 6 to 17 ah4 while NH.,+ concentrations ranged from 0 to 0.4 PM. Total N assimilation ranged from 84 to 732 nM d-r but showed no seasonal trend. NH,' and urea concentrations were < 1% of total dissolved inorganic N, but use of this "regenerated" N still accounted for 44-89*/s of total N assimilation. Rates of NO,- assimilation were negatively correlated with ambient NH,+ concen- trations, and concentrations of NH., + between 0.1 and 0.3 PM caused complete inhibition of NOs- assimilation, NO,- was more important as a source of N in spring than in summer. We attribute this pattern to a summer increase in turnover rates for NH;'. Turnover times for the dissolved NH,+ pool were half as long in August as in May. Grazing and recycling in the euphotic zone apparently both play significant roles in preventing depletion of NO,- in the oceanic subarctic Pacific.
A long-standing hypothesis states that low year-round phytoplankton biomass in the open sub- arctic Pacific Ocean is maintained by herbivorous grazing. To evaluate the balance between phy- toplankton growth and microzooplankton grazing, we carried out seawater dilution experiments at two subarctic Pacific locations in June and September 1987. Pigment-specific phytoplankton growth and grazing rates were obtained from dilution experiments by HPLC separation of phy- toplankton pigments. This approach allowed us to look at the relationship between growth and grazing rates for different phytoplankton taxa. A wide range of rates was observed during any given experiment. Pigment-specific growth rates ranged from 0.0 to 0.8 d-l; pigment-specific microzoo- plankton grazing rates ranged from 0.0 to 0.6 d-l. The highest growth rates appeared attributable to large diatoms, as indicated by both pigment and cell-count data. Grazing rates were most closely matched with growth rates for small (< 10 pm) phytoplankton species; large diatom species seemed to be unaffected by microzooplankton grazing. Higher grazing rates were measured on pigments with the lowest standing stocks.
A strong, positive empirical relationship was found between bacterial abundance and chlorophyll concentration in fresh and marine waters. Freshwater and marine linear regression equations are statistically indistinguishable. The overall equation is log AODC = 5.867 + 0.776 log chl a, r2 = 0.88, where AODC (acridine orange direct count) is the number of bacteria per millilitre and chl a is micrograms of chlorophyll a per litre. It is apparent that planktonic bacteria and algae are tightly linked in lakes and the sea. The slope of the regression line, however, shows that bacterial numbers do not increase as rapidly as algal biomass with an increase in nutrient concentration. We suggest that this disproportionately smaller increase in bacterial numbers need not signify a smaller role for bacteria in lake metabolism with increasing nutrient availability, if bacterial productivity per unit bacterial biomass increases as total bacterial biomass increases between systems.
Spatial and temporal patterns in the flux of sinking organic matter are central to the understanding of elemental dynamics and food-web energetics in the global ocean1-3. Heterotrophic bacteria have been shown to play a part in the decomposition of large, rapidly sinking organic particles within and below the euphotic zone4-8. These previous studies suggest that decomposition by attached bacteria can explain only a trivial fraction of the observed decrease in the flux of organic matter with increasing depth. We report here that free-living bacteria, rather than the particle-feeding zooplankton, are the principal mediators of particle decomposition in the central north Pacific gyre and the eutrophic Santa Monica basin. We suggest that bacterial growth in the mesopelagial gives rise to the large-scale production of fine (0.3-0.6 mum), non-sinking particles at the expense of large, rapidly sinking particles. Our results have implications for models of biogeochemical dynamics of organic particles and surface-reactive materials such as radionu-clides in the ocean's interior3,9.
The “seawater culture” approach is evaluated as a means of observing changes in cell numbers over time to estimate specific growth rates and production of heterotrophic bacterioplankton in oligotrophic open ocean waters. We use models of exponential growth limited by grazers, and of logistic growth limited by substrate availability, to estimate the specific growth rates of natural bacterial populations in warm core Gulf Stream rings. The two models yielded somewhat different rate estimates, but in general the results suggest that in situ specific growth rates for bacterial populations in oligotrophic ocean waters are as fast as rates in rich coastal waters and lakes (1–2 d ⁻¹ ). Estimates of hourly bacterial production averaged 10% of hourly primary production. Since bacteria grew at night, up to 40% of the daytime primary production was estimated to pass through the bacteria every 24 h. Estimates of removal rates by grazers are about the same as bacterial growth rates. However we estimate that only 3–7% of primary production was converted to protozoan biomass via ingestion of bacteria.
Our data suggest that simple dilution of natural samples with cell‐free water is a useful, valid method for estimation of bacterial specific growth rates.
Chlorophyll crops in the subarctic Pacific are low and relatively constant throughout the year; however, net growth of phytoplankton occurs when natural water is enclosed in incubation containers and exposed to adequate irradiance. Pigment-based measurements of taxon-specific growth rates and taxon-specific grazing pressure were made in an attempt to understand the dynamic processes leading to the net growth of phytoplankton in bottles. Specific growth rates, determined from ¹â´C labeling of chromatographically separated pigments showed that fucoxanthin-containing cells (diatoms) were the fastest growing microalgae; a rapid net accumulation of fucoxanthin also occurred. Independent measurements of specific growth rates, determined from chromatographic analysis of microzooplankton dilution experiments, also showed that fucoxanthin-containing cells had the highest specific growth rates. Importantly, microzooplankton grazing rates on fucoxanthin-containing cells were only about half the specific growth rate; hence, the resultant bloom of fucoxanthin. The authors speculate that the lag in chlorophyll growth that has been reported previously is an artifact of subculturing; fast-growing, but dilute, fucoxanthin-containing cells do not become a significant portion of the total chlorophyll signal until late in the experiment. Other diagnostic carotenoids, such as 19â²-hexanoyloxyfucoxanthin and 19â²-butanoyloxyfucoxanthin, remained nearly constant through long-term incubations. However, they became actively ¹â´C labeled and showed positive specific rates of growth. Dilution experiments showed that their specific growth rate was compensated by microzooplankton grazing.
Bacterial abundance and 3H-thymidine incorporation were measured throughout the water column during September–October 1986, along transects in the north western Arabian Sea and Gulf of Oman. Bacterial abundances and production estimated from incorporation rates were high (> 1 × 109 cells 1−1 and 30–92% of primary production, respectively) along the oceanic portions of the transects. These elevated levels may indicate a response to the decline of summer phytoplankton blooms stimulated by monsoonal deepening of the mixed layer. Bacterial production and abundance profiles had complex vertical structure with multiple subsurface maxima related to chlorophyll and oxygen distributions. Production and abundance both declined exponentially with depth below 100–200 m. Rates of dissolved organic carbon (DOC) release from the sinking particle flux may have been adequate to support bacterial production in the Gulf of Oman, but in the open Arabian Sea this source appeared to be insufficient to meet the bacterial demand. The bacterial production estimates reported herein are very conservative because very low conversion factors were used. In general these results suggest that the carbon sources usually assumed to support bacterial production (e.g., phytoplankton exudation, particle breakdown) supply only a fraction of the bacterial demand in the northwest Indian Ocean.
3H-Thymidine incorporation rate, bacterial total count and mean bacterial cell volume were measured at eight depths throughout the euphotic zone at 10 stations in Gulf Stream water and front regions in October 1985. Highest rates of bacterial biomass and production were found in the surface layers of entrained shelf water and in the mixing zones between shelf water and Gulf Stream water. High variability in bacterial production and in biomass related to layers of different water masses was observed in the front regions.Thymidine incorporation rate varied between 3 and 300 pmol l−1 day−1 (mean = 30, S.D. = 50, N = 76). Bacterial total count varied between 3.6 × 107 and 2.3 × 109cellsl−1 (mean = 6.8 × 108, S.D. = 4.4 × 10Su8, N =76). Bacterial mean cell volume varied between 0.01 and 0.11 μm3 (mean 0.056, S.D. = 0.024, N = 75).Thymidine incorporation rate was positively correlated with bacterial total count, but not correlated with temperature and mean bacterial cell volume.
We measured bacterial production and estimated the carbon consumption by bacteria in the mesopelagic zone (80–600 m) in the subarctic Pacific during May and August. Bacterial production was measured by leucine and thymidine incorporation. The two methods gave similar results. Bacterial production in the euphotic zone accounted for about 13% of primary production and in the whole water column for 20% (0–600 m). To bracket bacterial carbon consumption we made a lowest and highest estimate of bacterial production. The lowest estimate assumes zero isotope dilution for converting 14C-leucine incorporation rates into bacterial production and a 50% growth efficiency. In the mesopelagic zone, this estimate implies that bacterial account for 52 and 41% of the POC sinking flux as measured by sediment traps in May and August, respectively. The highest estimate, assuming two-fold isotope dilution of 14C-leucine and a 30% growth efficiency, yields bacterial carbon consumption values of 172 and 137% of the POC downward flux in both months. This indicates that bacteria are important, if not the major consumers of organic matter in the mesopelagic zone of the subarctic Pacific.
In order to assess the relative importance of the pico- and nanoplankton fractions, the composition of entire phytoplankton communities at Weathership Station P (50N; 145W) and at 53N; 145W were studied in May and August, 1984, using epifluorescence, scanning electron, and inverted light microscopy. The biomass of major taxa within five size classes was estimated from cell volume and cell concentration. For both months, approximately twothirds of the total phytoplankton carbon were contributed by cellsm. In May, 16% of plant biomass was contributed by cellsm, and in August 39%. (In both months 90% of plant carbonm was contributed by the bluegreen coccoid Synechococcus spp.) Cells 2 to 5 m contributed about 39% to total plant carbon; they were mostly flagellates in May and nonmotile coccoids in August. The remaining one-third of algal carbon was composed of dinoflagellates, cryptomonads, other flagellates and diatoms, all >5 m. Very little difference between taxa was observed with respect to vertical stratification. Small taxonomic changes were observed in the community between May and August, and within each month.
The objectives of the 3 year study were to determine the relationship between bacterial numbers and phytoplankton standing crops (chlorophyll a) in sub-antarctic Marion Island lakes (33) and to determine the relative importance of labile dissolved organic carbon and water temperature as regulators of heterotrophic bacterial activity and production. Bacterial activity (the incorporation and respiration rates of 14C-labelled substrates) and production (the rate of [methyl-3H]thymidine incorporation into DNA) were measured in oligotrophic Lava Lake and Gentoo Lake, an elephant seal wallow. Samples were incubated under ambient conditions as well as at increased temperature and with additions of labile dissolved organic carbon (DOC). Bacterial numbers ranged from 2.13 105 cell ml–1 to 15.17 106 cells ml–1 in the lake survey. The chlorophyll range was 0.18 to >75 g 1–1. Bacterial numbers were not correlated to chlorophyll concentration in waters where the chlorophyll content was 5 g 1–1 but were correlated in waters with larger algal contents. Heterotrophic bacterial activity and production, which were similar to rates recorded for equivalent lower latitude systems, were higher in Gentoo Lake than in Lava Lake. As a result of qualitative and quantitative differences in the DOC pools, DOC was the stronger regulator of bacterial activity and production in Lava Lake, while temperature was the stronger factor in Gentoo Lake.
During the 1989 spring bloom in the western North Atlantic, we estimated the biomass and productivity of bacteria and phytoplankton at two sites (40 and 45°N) representing different water masses. At 40°N, almost all of the phytoplankton carbon could be accounted for by photosynthetic nanoplankton and picoplankton; in contrast, at 45°N, only about half was thus accounted, implying a substantial contribution by photosynthetic microplankton. At both sites, bacterial abundance was quite high (up to 2 × 109 cells l−1), and the rates of bacterial production assessed by incorporation of [3H]thymidine (up to 8 pmol l−1 h−1) and [3H]leucine (up to 240 pmol l−1h−1) were significant. Specific growth rates of bacteria based on [3H]thymidine incorporation were 0.08–0.25 day−1. Taken together, our measurements and assumptions implied a demand for primary production in the order of 16–36% over the euphotic zone or 24–78% over the upper 100 m in the water column. We conclude that ultraphytoplankton and bacteria played significant roles in the flux of carbon during the 1989 North Atlantic spring bloom.
Bacterial biomass increased five-fold in the euphotic zone (from 450 to 2250 mgC m⁻²) in response to the spring phytoplankton bloom in the eastern North Atlantic Ocean (Lat. 47°N, Long. 20°W) in 1989. Bacterial biomass accounted for about 20–30% of the particulate organic carbon (POC) above 50 m and a somewhat larger fraction in the layer below. Bacterial production averaged about 30% of primary production and remained rather constant while the primary production varied from 600 to 1500 mgC m⁻² day⁻¹ in response to event-scale changes in irradiance. Thus bacterial production varied from 15 to 80% of the concurrent primary production, with peaks occurring on overcast days when photosynthesis was low. Bacterial production in both the euphotic zone and the layer immediately below appeared to respond to the meteorologically-driven variations in photosynthesis with a time lag of 3–4 days, consistent with estimates of turnover rates of 0.2 day⁻¹. In the upper layer incorporation of dissolved free amino acids supported about 20% of the production. The bacterial carbon demand at peak production required subsidies of carbon from the bulk POC and/or DOC pools. In the lower layer, decomposition of the vertical flux of sinking POC may have supported about half the mean production. Our bacterial production estimates for the 50–150 m layer are consistent with vertical flux estimates from drifting sediment traps and support other observations in suggesting that very large amounts of primary production pass through the DOC pool on short time-scales.
The distribution of bacterioplankton biomass and productivity in warm-core Gulf Stream ring 82-B generally corresponded to the physical and dynamical structure of the ring. Mean cell volumes were uniform for 4 months, but were larger by a factor of 2–3 in the high velocity (frontal) region (HVR) near the ring edge. As a result of this gradient and higher abundances, water column biomass and production were highest in the front, which appeared to be a local maximum in those properties. In this regard bacterioplankton contrasted strongly to phytoplankton, which exhibited strong local maxima at the center of the ring in June. In April when the water column inside the ring was isothermal to 450 m, bacterial biomass and production were low and uniform to 250 and 50 m, respectively. Bacterioplankton responded dramatically to the vernal restratification of the ring. In June when the surface layer was characterized by a strong pycnocline at 10–40 m, bacterial biomass and production often had strong subsurface maxima, and were 3 and 5 times greater than in April, respectively. Abundance exceeded 1.5 × 10⁹ cells l⁻¹ at ring center and exceeded 3 × 10⁹ l⁻¹ in the HVR. Turnover rates for the euphotic zone bacterioplankton as a whole were 0.24 d⁻¹ in April, 0.56 d⁻¹ in June, and 0.27 d⁻¹ in August at ring center. Bacterial production averaged 12% of hourly primary production (range 1–32%), suggesting that bacteria control a significant and sometimes large portion of the carbon cycling in the euphotic zone.
Sampling with fine mesh (73 μm) nets from the weatherships patrolling at Sta. P (50°N, 145°W) in the northeastern Pacific revealed prolonged population pulses of large diatoms. Density of Corethron criophilum Castracane exceeded 5000 cells l−1 through three weeks in July 1980. Density of a mixture of species including Thalassiothrix longissima, Chaetoceros atlanticus, Chaetoceros concavicornis, and Rhizosolenia alata was sustained at several hundred to over 1000 cells l−1 from late November 1980 to mid-February 1981. While truly massive diatom blooms are not observed in the nutrient-rich, oceanic subarctic Pacific, the observations simply that waters of the region are not utterly unsuitable as a medium to sustain diatom growth.
Heterotrophic bacteria can assimilate both dissolved organic nitrogen (DON) and ammonium, but they also can excrete ammonium during DON catabolism. The relationships among DON and ammonium uptake and ammonium excretion are not clear. In the subarctic Pacific, our experiments show that the supply of dissolved free amino acids partially controls whether heterotrophic bacteria assimilate or excrete ammonium. Bacteria in these waters apparently preferred to assimilate amino acids rather than ammonium because addition of amino acids significantly inhibited uptake of ammonium. In <0.8 μm size fractions dominated by heterotrophic bacteria, ammonium uptake occurred only when amino acid concentrations were too low to support bacterial growth. When the amino acid supply was adequate, ammonium was regenerated by bacterial assemblages that had been separated from grazers. The C:N ratio of the amino acid pool, which was the predominate nitrogen source for bacteria during initial phases of our experiments, was 2.6. Based on mass balance consideration, ammonium excretion would be expected if bacterial carbon growth efficiency is assumed to be about 50% and the C:N of bacterial biomass is 4. These results suggest that high rates of amino acid turnover should be positively correlated with high rates of ammonium excretion by heterotrophic bacteria. Thus, amino acid cycling increases availability of ammonium to phytoplankton and may affect plankton community structure and rates of new production.
Horizontal spatial variations of microbial and bacterial abundance and biomass, and rates of nucleic acid synthesis in surface waters across cold-core Gulf Stream eddy P were investigated in April 1982. The age of eddy P was estimated at 8 to 10 months. Chlorophyll a reached 0.38 μg l−1 in an area of high chlorophyll a fluorescence located in 18 to 19°C waters. Bacteria were mostly (98%) free-floating, and microbial-ATP biomass correlated with chlorophyll a fluorescence across eddy P. Rates of microbial metabolism and growth, and bacterial production were not proportional across areas of high chlorophyll a fluorescence. Estimates of microbial growth rates (δ), were 0.3 to 0.6 h−1 in the mixed surface waters of eddy P and 0.06 to 0.08 h−1 in adjacent waters of the northwestern Sargasso Sea. Bacterial production ranged from 0.12 to 1.41 μgC l−1 d−1, and production: biomass ratios (turnover times) were 0.024 to 0.27 h−1 in the eddy and ca. 0.002 h−1 in the Sargasso Sea. Because microbial metabolism (RNA synthesis) and growth (DNA synthesis) across eddy P were not closely coupled, although intertwined with diurnal variations, we conclude that the microbial communities, based on the experimental time scale, were in a state of unbalanced growth.
The seasonal variation in temperature characteristics of photosynthetic and heterotrophic activities in the microbial plankton of Bedford Basin, Nova Scotia, was investigated. Measurements were made of the photosynthetic uptake of [C]bicarbonate and its incorporation into cellular protein as well as the heterotrophic uptake of H-labeled amino acids and their incorporation into cellular protein. Activity-temperature curves were analyzed objectively by nonlinear estimation of parameters from various mathematical models. Over the seasonal cycle, the cardinal temperatures and a parameter formally equivalent to the thermodynamic enthalpy of activation for most of the four processes measured were positively correlated with the water temperature. The temperature sensitivity of metabolic activity (i.e., change in activity per unit change in temperature) was indexed by the tangent to the activity-temperature curves. When this index was expressed in dimensionless form by normalization to the scaling factor of the activity-temperature curves, the resulting relative temperature sensitivity, evaluated at the prevailing temperature, proved to be statistically invariant throughout the year. During the height of the spring bloom, the water temperature (-0.3 degrees C) was not so low as to inhibit metabolic activity of either the phytoplankton or the bacterioplankton. The evidence suggests that heterotrophic utilization of products is not suppressed during the spring phytoplankton bloom.
Growth responses and biovolume changes for four facultatively psychrophilic bacterial isolates from Conception Bay, Newfoundland, and the Arctic Ocean were examined at temperatures from - 1.5 to 35 degrees C, with substrate concentrations of 0.15, 1.5, and 1,500 mg of proteose peptone-yeast extract per liter. For two cultures, growth in 0.1, 1.0, and 1,000 mg of proline per liter was also examined. At 10 to 15 degrees C and above, growth rates showed no marked effect of substrate concentration, while at - 1.5 and 0 degrees C, there was an increasing requirement for organic nutrients, with generation times in low-nutrient media that were two to three times longer than in high-nutrient media. Biovolume showed a clear dependence on substrate concentration and quality; the largest cells were in the highest-nutrient media. Biovolume was also affected by temperature; the largest cells were found at the lowest temperatures. These data have implications for both food web structure and carbon flow in cold waters and for the effects of global climate change, since the change in growth rate is most dramatic at the lowest temperatures.
Size classes and major taxonomic groups of phytoplankton at two locations in the subarctic Pacific Ocean in
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Limltation ofbactenal growth by dlssolved organlc matter In the subarctic
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