Fereidoun Rassoulzadegan

Sorbonne Université | UPMC · Centre des Sciences de la Mer (Stations marines UPMC de Banyuls/Mer, Roscoff et Villefranche/Mer)

A general model of species diversity predicts that the latter is maximized when productivity and disturbance are balanced. Based on this model, we hypothesized that the response of bacterial diversity to the ratio of viral to bacterial production (VP/BP) would be dome-shaped. In order to test this hypothesis, we obtained data on changes in bacteria...

Black carbon (BC, soot) has anthropogenic and non-anthropogenic origins. While BC can contribute significantly to dissolved and particulate organic carbon in marine systems, data are still sparse for the water column. Due to the association of BC aerosols with organic acids, the deposition of BC aerosols at the surface of the ocean can decrease the...

In order to understand the microbial processes controlling seasonal DOC accumulation in NW Mediterranean Sea surface waters, nutrient addition experiments were conducted. The dynamics of DOC as well as the response of heterotrophic bacteria in terms of abundance, activity, and community composition were investigated in October, January, March, and...

To better understand the seasonal variation in concentrations and accumulation of dissolved organic carbon (DOC) in north-western Mediterranean surface waters, we investigated changes in bacterial abundance, activity (production, BP; respiration, BR; growth efficiency, BGE) and DOC concentration at different depths during different periods: winter...

Black carbon (BC), the product of incomplete combustion of fossil fuels and biomass, constitutes a significant fraction of the marine organic carbon pool. However, little is known about the possible interactions of BC and marine microorganisms. Here, we report the results of experiments using a standard reference BC material in high concentrations...

Bacterial abundance and community composition were investigated along trophic gradients in the barrier reef lagoon of Noumea, New Caledonia. Bacterial abundance and the percentage of high nucleic acid (%HNA) bacteria (a potential indicator for bacterial production) increased from offshore waters towards the head of the bays. 16S rRNA gene PCR and d...

The mesopelagic zone is the oceanic region through which carbon and other elements must pass in order to reach deeper waters or the sea floor. However, the food web interactions that occur in the mesopelagic zone are difficult to measure and so, despite their crucial importance to global elemental cycles, are not very well known. Recent development...

We tested the hypothesis that viruses can control bacterial growth efficiency (BGE) in marine pelagic environments. In the Bay of Villefranche, Northwestern Mediterranean, three experiments were conducted on different months to determine bacterial and viral variables in seawater cultures. In December, phosphorus (P) addition enhanced bacterial grow...

INTRODUCTION Hensen (1887) proposed that food supply controlled variations in adult fish stocks, and therefore quantitative studies of plant and animal production in the sea might permit predictions of annual fish yields. If fish could be ‘harvested’ by man, Hensen (1887) argued that relationships similar to agriculture existed between primary prod...

During May 2001 and May 2002, the structure and function of the microbial community within and outside the Cyprus quasi-stationary warm-core eddy in the Levantine Basin of the eastern Mediterranean was studied down to the depth of the bathypelagic layer. We present here the detailed description of the microbial food web in one of the most oligotrop...

We compared an idealised mathematical model of the lower part of the pelagic food web to experimental data from a mesocosm experiment in which the supplies of mineral nutrients (nitrogen and phosphorous), bioavailable dissolved organic carbon (BDOC, as glucose), and silicate were manipulated. The central hypothesis of the experiment was that bacter...

Microbes, i.e. viruses, prokaryotes, protists and fungi are main players in biogeochemical cycles in aquatic and soil systems. As predators, pathogens, commensals and symbionts they are associated with multicellular life forms. Microbes show a dazzling array of biodiversity and it has been argued that due to their small size and huge numbers, micro...

Phosphate addition to surface waters of the ultraoligotrophic, phosphorus-starved eastern Mediterranean in a Lagrangian experiment caused unexpected ecosystem responses. The system exhibited a decline in chlorophyll and an increase in bacterial production and copepod egg abundance. Although nitrogen and phosphorus colimitation hindered phytoplankto...

The response of the microbial food web to P-addition was studied during a 10-day Lagrangian experiment in the Eastern Mediterranean during which orthophosphate was added to the surface water of the Cyprus anticyclonic eddy. Very low levels of all microbial populations (heterotrophic bacteria, Synechococcus, Prochlorococcus, autotrophic and heterotr...

CYCLOPS was a European Framework 5 program to further our understanding of phosphorus cycling in the Eastern Mediterranean. The core of CYCLOPS was a Lagrangian experiment in which buffered phosphoric acid was added to a <4×4 km patch of water together with SF6 as the inert tracer. The patch was followed for nine days in total. Results obtained pri...

Phosphate (P), nitrate (N) or P+N added in a microcosm experiment to oligotrophic waters of the Eastern Mediterranean influenced by near-shore waters triggered a range of responses in the autotrophic and heterotrophic compartments of the system. Chlorophyll a increased in all treatments, including the no-addition control, implying that nutrients be...

An on-board microcosm experiment was set up to test the hypothesis that the observed lack of phytoplankton biomass increase response to a mesoscale in situ P-enrichment experiment in the P-limited Eastern Mediterranean (Krom et al., 2005a) was a consequence of co-limitation by P and N availability in this ultraoligotrophic environment. Six microcos...

An on-board microcosm experiment was performed during the CYCLOPS May 2002 cruise to track the biogeochemical response of Eastern Mediterranean surface seawater to a gradient addition of fresh and pre-leached Saharan dust, mimicking the potential fertilization effect as opposed to the impact of adding particles alone. Response parameters examined w...

Export Date: 12 April 2011, Source: Scopus

Phosphate addition to surface waters of the ultraoligotrophic, phosphorus-starved eastern Mediterranean in a Lagrangian experiment caused unexpected ecosystem responses. The system exhibited a decline in chlorophyll and an increase in bacterial production and copepod egg abundance. Although nitrogen and phosphorus colimitation hindered phytoplankto...

Phosphate addition to surface waters of the ultraoligotrophic, phosphorus-starved eastern Mediterranean in a Lagrangian experiment caused unexpected ecosystem responses. The system exhibited a decline in chlorophyll and an increase in bacterial production and copepod egg abundance. Although nitrogen and phosphorus colimitation hindered phytoplankto...

The concentration of transparent exopolymeric particles (TEP) was monitored during Phaeocystis globosa blooms that developed in mesocosms under different initial N:P ratios (from N- to P-limited conditions). TEP concentration was measured using the microscopic (TEPmicro, ppm) and the colorimetric (TEPcolor, Xanthanequiv.L−1) methods. TEP concentrat...

Free-living marine bacteria isolated from oligotrophic Mediterranean waters were enriched in culture to characterize their phospholipid fatty acids (PLFAs). Odd chain iso- and anteiso-FAMEs and n–16:0 were the predominant structural PLFAs, together with a homologous series identified as mid-chain methoxy FAMEs. The dominant methoxy fatty acids iden...

It is widely recognized that organic carbon exported to the ocean aphotic layer is significantly consumed by heterotrophic organisms such as bacteria and zooplankton in the mesopelagic layer. However, very little is known for the trophic link between bacteria and zooplankton or the function of the microbial loop in this layer. In the northwestern M...

The structure of the bacterial population (free vs. attached bacteria), variations in bacterial abundance and ciliate group composition were monitored as a function of transparent exopolymeric particle (TEP) concentration during Phaeocystis globosa blooms that developed in mesocosms. Two ciliate groups dominated at different stages of the blooms. T...

We studied the ingestion of picoplankton-size particles by the oligotrich Strombidium sulcatum as a function of transparent exopolymeric particle (TEP) concentration. Fluorescent microspheres of 1 mum (FMS) were used as prey-item analogs. TEP-FMS aggregates were formed by bubbling seawater solutions containing different TEP concentrations in the pr...

t is widely recognized that organic carbon exported to the ocean aphotic layer is significantly consumed by heterotrophic organisms such as bacteria and zooplankton in the mesopelagic layer. However, very little is known for the trophic link between bacteria and zooplankton or the structure of the microbial loop in this layer. In the northwestern M...

Previous studies have suggested that Mediterranean surface water becomes phosphorus limited for both bacteria and phytoplankton during stratified periods and that orthophosphate uptake in these situations was close to diffusion limitation for both cyanobacteria and autotrophic nanoflagellates. In order to better understand vertical and seasonal var...

Bacterial abundance and production were measured in the mesopelagic layer (110-1000m) at the French-JGOFS time-series station DYFAMED (NW Mediterranean) from June 2001 to October 2002. Abundance and production were seasonally variable down to 300 and 500m, respectively. Depth-dependent decreases of abundance and production were well described by th...

Viruses can influence the genetic diversity of prokaryotes in various ways. They can affect the community composition of prokaryotes by 'killing the winner' and keeping in check competitive dominants. This may sustain species richness and the amount of information encoded in genomes. Viruses can also transfer (viral and host) genes between species....

Viruses can influence the genetic diversity of prokaryotes in various ways. They can affect the community composition of prokaryotes by `killing the winner' and keeping in check competitive dominants. This may sustain species richness and the amount of information encoded in genomes. Viruses can also transfer (viral and host) genes between species....

Summary form only given. A few biological oceanographic variables are documented at large horizontal scales. These primary variables include: phytoplankton biomass (PB, i.e. chlorophyll a) and production (PP), as estimated from remotely sensed ocean colour; dissolved organic matter (DOM), and bacterial biomass (BB) and production (BP), which come f...

Using 32 P, uptake and transfer of phosphorus in the microbial food web were studied in surface water from Villefranche Bay (northwestern Mediterranean) from September to December 2001. During the study, the thermo- cline gradually declined and vertical mixing started, leading to a transition from a nutrient-depleted period to a nutrient-replete pe...

We measured abundance and surface area of pico- and nano-sized organic particles that are stained by 4’6-diamidino-2-phenylindole (Dapi), namely Dapi yellow particles. This study was done monthly from April 1999 to March 2000 from surface down to 2000 m at the French-JGOFS time-series station Dyfamed in the NW Mediterranean. Dapi yellow particles w...

Abundance and biomass of free-living bacteria, heterotrophic nanoflagellates (HNF) and ciliates were measured monthly throughout the water column at the French-JGOFS time-series station DYFAMED in the NW Mediterranean Sea from May 1999 to March 2000. These microbial heterotrophs decreased by one, two and three orders of magnitude, respectively, ove...

Changes in picoplankton population abundance and growth under turbulence have been suggested to be the consequence of turbulence affecting larger trophic levels and hence the grazing pressure. We designed a laboratory set-up to assess the effects of turbulence on plankton assemblages, and tested the degree of food-web complexity needed to produce c...

Orthophosphate uptake by bacteria and phytoplankton was measured with 32P for wa- ter from the Bay of Villefranche (NW Mediterranean) from September to December 2001. The 32P release rate was measured for waters with different size of micro- bial community. During this period, the thermocline was gradually declined and the vertical mixing started,...

Although bacteria are an important component in the cycling of organic material in the ocean, the magnitude of bacterial process in mesopelagic and bathypelagic layers are poorly known. We measured bacterial abundance (BA), production (BP) and res- piration (BR) from 5 to 2000 m at the French-JGOFS time-series station DYFAMED (NW Mediterranean Sea)...

A new approach is described to identify the dominant process (physical versus biological) in a pelagic marine ecosystem, from simple biological oceanographic field variables. The approach is based on quantification of the matching (M) between phytoplankton production (P) and losses, from field estimates of chlorophyll a (Chl) and P. Coefficient M i...

A simple steady-state model based on P-Limited bacterial growth rate and predator controlled bacterial biomass predicts bacterial production (BP) to be proportional to the square of ciliate biomass (C). Changes in ciliate biomass will then drive changes in bacterial production and carbon demand. This model was compared to experimental microcosms wh...

Observations are reviewed which indicate that not only phytoplankton, but also heterotrophic bacteria are P-limited during those seasons when there is stratification in the Mediterranean. It is discussed how these observations fit into a general concept of a size-structured food chain where the structure is a result of combined top-down control fro...

We established a budget of organic carbon utilization of a starved heterotrophic nanoflagellate, Pteridomonas danica, incubated in batch cultures with Escherichia coli as model prey. The cultures were sampled periodically for biomass determinations and total organic carbon dynamics: total organic carbon, total organic carbon <1 μm, and dissolved or...

High temperature catalytic oxidation (HTCO) was used for the first time to determine carbon content of heterotrophic protists (the helioflagellate Pteridomonas danica, the dinoflagellate Oxyrrhis marina, and the scuticociliate Uronema sp.) and bacteria (Escherichia coli). This technique has the advantage, over the conventional CHN analysis of glass...

The two most abundant marine autotrophic prokaryotes, Prochlorococcus and Synechococcus, often have different distributions in the ocean. For example, Synechococcus is restricted to the first 100 m, whereas Prochlorococcus extends much deeper in oligotrophic waters. This is in part explained by differences in adaptation to nutrient and light regime...

We examined the ability of the zooxanthellate coral Stylophora pistillata to feed on microheterotrophs (bacteria and oligotrichous ciliates). The effect of light on the feeding rates was also investigated. Grazing experiments were first conducted by exposing coral colonies to known amounts of super(3)H-thymidine-labeled bacteria and ciliates and me...

We examined the ability of the zooxanthellate coral Stylophora pistillata (Esper, 1797) to feed on microhctero- trophs (bacteria and oligotrichous ciliates). The effect of light on the feeding rates was also investigated. Grazing experiments were first conducted by exposing coral colonies to known amounts of "H-thymidine-labeled bacteria and ciliat...

The egestion of particulate material as well as pigment degradation during microzooplankton grazing on phytoplankton are poorly known processes. In an attempt to evaluate these processes, changes in pigment concentrations within various size fractions were monitored in batch cultures of an assemblage of a pelagic ciliate (Strombidium sulcatum) and...

The elemental composition of individual < 10 μm detrital particles from Mediterranean surface waters was analysed using a Transmission Electron Microscope (TEM) equipped with an energy dispersive X-Ray microanalyser. Results show that carbon and phosphorus content per detritus volume are much higher in pico-detrital particles < 2 μm (42 kg C m−3 an...

Ingestion of picoplankton-size particles was studied in two marine ciliates, a typical grazer of pico- and nano-plankton, the oligotrich Strombidium sulcatum, and a bacteriovorous scuticociliate, Uronema sp. In laboratory experiments, both logarithmic- (food-unlimited) and stationary-phase (food-limited) populations were presented with particles of...

Release of amino acids was examined in the laboratory in the form of dissolved primary amine (DPA) by two marine planktonic protozoa (the oligotrichous ciliate, Strombidium sulcatum and the aplastidic flagellate Pseudobodo sp.) grazing on bacteria. DPA release rates were high (19–25 × 10−6 and 1.8–2.3 × 10−6 μmol DPA cell−1 h−1 for flagellates and...

Surface-water microbial populations were investigated in the northwest Mediterranean for possible indicators of phosphate deficiency and limitation. Low phosphorus availability was suggested by short turnover time (min. observed 0.68 h), high-alkaline phosphatase activity (V(max) = 28 nM hydrolyzed h-1), subsaturation of phosphate uptake (2.6-9% of...

Surface-water microbial populations were investigated in the northwest Mediterranean for possible indicators of phosphate deficiency and limitation. Low phosphorus availability was suggested by shore turnover time (min, observed 0.68 h), high-alkaline phosphatase activity (V-max = 28 nM hydrolyzed h(-1)), subsaturation of phosphate uptake (2.6-9% o...

The effect of rain-borne nutrients in spring on the marine microbial community was studied in Villefranche Bay (France), a system in which previous studies have suggested that competition exists within the microbial food web for nutrient salts. In 5 incubation experiments, conducted from May 1994 to March 1996, surface seawater was amended with 2 t...

Recent literature indicates that dissolved organic carbon (DOC) may accumulate in productive surface waters. Such accumulation will allow export of DOC to the aphotic zone by diffusion and downwelling. As an alternative to models based on low degradability, we here propose a mechanism where bacterial carbon consumption is restricted due to food web...

This paper describes an approach to determine, using a small number of food-web or hydrodynamic variables, the partitioning of phytoplankton production among 3 carbon fluxes, i.e, remineralization within the euphotic zone, food-web transfer, and sinking to depth of organic particles. In order to do so, the flows of biogenic carbon in the marine pel...

NW Mediterranean surface water was spiked with picoplankton prey (heterotrophic bacteria or cyanobacteria) or predators (bacterivorous microflagellates or ciliates) to investigate differential grazing pressure on picoplankton populations. Adding a particular prey type did not yield different growth patterns for heterotrophic bacteria and cyanobacte...

A simple and rapid method for microscopic quantification of an abundant class of labile pico- and nano-sized detrital particles (0.2 to 20 mu m), using the fluorochrome stain DAPI, is described. Using an appropriate UV-filter set, examination of DAPI-stained samples revealed not only blue (DNA-containing) but also yellow particulate matter. We show...

Seasonal variations of pico- and nano-sized marine detrital particles (DAPI Yellow Particles, DYP) and their relationships with components of the microbial food web were studied from April 1993 to March 1994 in the NW Mediterranean Sea. A hierarchical flexible clustering distinguished 2 major groups of DYP: less than or equal to 10 mu m and 10-20 m...

Extracellular α-glucosidase, β-glucosidase, and aminopeptidase activity variations (measured by use of fluorogenic substrate analogs) at a coastal station in the Mediterranean Sea were investigated over a 1-year period. A 27-h cycle and daily measurements were made in a summer situation. We observed strong relative diurnal variations, compared to s...

Biological oceanographers generally distinguish between two contrasting trophic pathways in the pelagic environment, i.e. the herbivorous and the microbial food webs. The former goes from large phytoplankton and zooplankton to fish, whereas the latter comprises small eukaryotic algae and cyanobacteria as well as heterotrophic bacteria and protozoa....

Turnover time of orthophosphate, uptake of phosphate into particulate size-fractions (0.2-1, 1-5, 5-10, > 10 µm), and subsequent release from size-fractions were examined using P, in samples from surface waters of Villefranche Bay during the autumnal erosion of the thermocline. Turnover time of orthophosphate increased from 1.6 h in early October t...

In light of evidence suggesting that both phytoplankton and bacteria in the Medi- terranean Sea are limited by the availability of phosphorus rather than of nitrogen, and that most of the P in the photic zone during summer stratification exists as dissolved organic compounds (DOP), we address the question of how these observations may interact with...

Using fluorogenic substrate analogs, the dynamics of extracellular enzyme activities (EEA) of free and particle-bound alpha-glucosidase and aminopeptidase were investigated in mixed cultures of marine phagotrophic nanoflagellates and bacteria. Flagellate growth on either live or heat-killed bacteria led to an expression of free aminopeptidase activ...

The effect of protozoan presence on pico- and nanoplankton growth rates was studied during a whole seasonal cycle, together with several other environmental factors (such as microphytoplankton abundance, temperature, salinity, solar irradiance and nutrients). Pico- and nanoplankton growth rates were enhanced in the presence of protozoa inside a dia...

The seasonal distribution of mixotrophic oligotrichous ciliates was compared to that of the strict heterotrophic oligotrichs and of the autotrophic haptorid Mesodinium rubrum, in surface coastal waters of the Ligurian Sea, from March 1988 to December 1989. The results show that (1) mixotrophic ciliates, which were observed year-round, average 51 %...

Ammonium excretion rates of two marine planktonic protozoa (an oligotrichous ciliate, Strombidium sulcatum , and an aplastidic flagellate, Pseudobodo sp.) both isolated from oligotrophic Mediterranean waters were quantified with heat‐killed bacteria as the food source for the protozoans. Ammonium excretion rates ranged from 0.25 to 2 µ g N mg DW ⁻¹...

Ciliate microzooplankton have long been hypothesized as probably being important in nutrient regeneration. However, few estimates of excretion rates exist, especially with regard to phosphorus, identified as the limiting nutrient in many marine and estuarine systems. The utility of using heat-killed prey, which avoids prey uptake of excretory produ...

The effect of protozoan presence on pico- and nanoplankton growth rates was studied during a whole seasonal cycle, together with several other environmental factors (such as microphytoplankton abundance, temperature, salinity, solar irradiance and nutrients). Pico- and nanoplankton growth rates were enhanced in the presence of protozoa inside a dia...

Ammonium excretion rates of two marine planktonic protozoa (an oligotrichous ciliate, Strombidium sulcatum, and an aplastidic flagellate, Pseudobodo sp.) both isolated from oligotrophic Mediterranean waters were quantified with heat-killed bacteria as the food source for the protozoans. Ammonium excretion rates ranged from 0.25 to 2 mug N mg DW-1 h...

ABSTRACT: Nitrogen transformations were investigated In the Bay of Villefranche-sur-Mer, on the south coast of France, during autumn 1990 and spring 1991, using '"N-isotope techniques on sizefractionated sea water samples. Besides measuring uptake of ammonium and nitrate, and ammonium regeneration, the biological composition in 4 size fractions (<...

Food vacuole contents in naturally occurring populations of tintinnids in surface waters at the mouth of the Bay of Villefranche were examined from November 1988 to December 1989. Three types of prey were considered: cyanobactena ( Synechococcus spp.), plastidic pico- and nanoflagel lates. The relative frequency (%) of these were observed in the di...

Marine zooplankton are a diverse group of organisms. Different species have different foraging strategies, life cycles, patterns of diel vertical migration, etc. Reducing such complexity to a minimal number of compartments in large-scale ecosystem models therefore poses a significant problem. The working group concentrated their discussions on the...

The effect of grazing within microbial food webs on the optical properties of seawater was studied in the laboratory by inoculating a culture of cyanobacteria Synechocystis with an oligotrichous ciliate, Strombidium. During the 4 week experiment sequential measurements of spectral absorption and scattering coefficients were supplemented by determin...

Spatial and size distribution of micro-organisms and their ETS activity has been investigated in Ligurian Sea surface waters along the Nice-Calvi transect across frontal areas from 18 to 37 km offshore (TOMOFRONT 1 and 2 cruises, April 1988 and April-May 1989 respectively). Aplastidic and plastidic nanoflagellates and aplastidic picoflagellates wer...

Plankton production in the Bay of Villefranche was relatively constant during March and April 1986 but the particle size at which the production occurred was more variable. At the beginning of the study, production was dominated by the larger (ca. 6 m) flagellates but towards the end it was more or less equally divided between the nano- and picopla...

Assessed nutrient feedback of oligotrichous ciliates and phagotrophic flagellates on growth in naturally occurring Mediterranean autotrophic microbial populations as well as on the microphytoplankton. Microbial growth half-saturation constants, expressed in terms of protozoan densities added, varied from 11-17 ciliates ml-1 and 2-5 × 103 flagellate...

As protozoan and microbial ecologists our ‘raison d’être’rests with the study of the structure function of ocean systems. To progress this we need field and experimental sampling, observation, and rate measurement techniques. Since the work of Pomeroy (1974), a widespread interest in, and a need for, an understanding of the dynamics of aquatic micr...

Production and decomposition of DMSP and DMS in a microbial food web were investigated by means of a 5-d incubation of prescreened seawater samples (< 1, < 10, < 100-mu-m) from northwestern Mediterranean coastal waters. The major goal of size fractionation was to create predator-free compartments. Aplastidic flagellates, plastidic nanoflagellates (...

The grazing rate of pelagic bacteria was repeatedly determined during 36-h time-course exper- iments with the minicell recapture technique. Die1 variation in the rate of grazing occurred in all water masses investigated. Maximal rates reached 2 x lo5 cells ml-' h-r, whereas minimal values were 20 times lower. Highest grazing rates were found during...

The effect of temperature on length of time for digestion of bacteria was evaluated, by using fluorescently labeled bacteria (FLB), for phagotrophic flagellates and ciliates isolated from coastal northwest Mediterranean waters. Accumulation of FLB in protozoan food vacuoles was followed until a plateau of FLB per cell occurred; then after a 1:10 di...

Planktonic ciliates, principally from the suborders Oligotrichina and Tintinnina, were examined by epifluorescence microscopy. This allowed (1) to check if isolated symbiotic plastids demonstrated by TEM in some species could keep and show an important autofluorescence, (2) to count and identify the species presenting this characteristic, and (3) t...

Food size-range for 13 species of Tintinnina and 18 species of Oligotrichina were studied using electronic particle counting and in situ observation of food vacuole contents. Tintinnids consume nanoplankton in the size range 2–20 μm. Oligotrichous naked ciliates consume particles in the size range 0.5–10 μm. Ciliates smaller than 30 μm take 72% pic...

ATRACT: In an attempt to quantify the organic fluxes within the microbial loop of oligotrophic Mediterranean water, organic pools and production rates were monitored. The production of cyanobacteria and its dynamics dominated the overall productivity in the system. The largest standing stock was that of the bacterioplankton and its growth consumed...