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A Chemoautotrophically Based Cave Ecosystem
Abstract
Microbial mats discovered in a ground-water ecosystem in southern Romania contain chemoautotrophic bacteria that fix inorganic carbon, using hydrogen sulfide as an energy source. Analysis of stable carbon and nitrogen isotopes showed that this chemoautotrophic production is the food base for 48 species of cave-adapted terrestrial and aquatic invertebrates, 33 of which are endemic to this ecosystem. This is the only cave ecosystem known to be supported by in situ autotrophic production, and it contains the only terrestrial community known to be chemoautotrophically based.
... Terrestrial extreme environments are widely used to study the limits of life [1,2]. They may also serve as analogue environments for studying the potential for the habitation of other planets [2,3]. ...
... A unique groundwater ecosystem exists in Movile Cave, in which microbial communities serve as primary biomass producers [1] (in Section 4.2, we will further discuss how these chemolithotrophic organisms may survive the cave environments). ...
... For the last 5.5 million years, significant input of meteoritic water and organics from the surface has been prevented by impermeable layers of clay and loess that cover the limestone in which the cave was formed [4]. The absence from the waters of Movile Cave of radioactive cesium and strontium isotopes that were widespread in the region due to the Chernobyl disaster in 1986 indicated that the cave is essentially sealed off from the surface [1]. ...
Movile Cave, situated in Romania close to the Black Sea, constitutes a distinct and challenging environment for life. Its partially submerged ecosystem depends on chemolithotrophic processes for its energetics, which are fed by a continuous hypogenic inflow of mesothermal waters rich in reduced chemicals such as hydrogen sulfide and methane. We sampled a variety of cave sublocations over the course of three years. Furthermore, in a microcosm experiment, minerals were incubated in the cave waters for one year. Both endemic cave samples and extracts from the minerals were subjected to 16S rRNA amplicon sequencing. The sequence data show specific community profiles in the different subenvironments, indicating that specialized prokaryotic communities inhabit the different zones in the cave. Already after one year, the different incubated minerals had been colonized by specific microbial communities, indicating that microbes in Movile Cave can adapt in a relatively short timescale to environmental opportunities in terms of energy and nutrients. Life can thrive, diversify and adapt in remote and isolated subterranean environments such as Movile Cave.
... The temperature of a hydrothermal cave depends on the temperature of the thermal water forming the cave and approximates 21-23°C in both Movile aquifer and Molnár János Cave. The ecosystems in the studied hydrothermal caves are highly isolated from the surface and likely rely on the endogenous production of organic matter by chemoautotrophic bacteria (Sarbu et al., 1996;Herczeg et al., 2020Herczeg et al., , 2022. It is assumed that the amount of available food is higher in hydrothermal caves than in epigenic caves (Poulson and Lavoie, 2000). ...
... We note that two of the studied cave populations were not typical in the sense that they were not cold-water epigenic caves relying on organic material of exogenous origin. The Molnár János Cave in Hungary and the Movile aquifer in Romania are both hypogenic hydrothermal cave systems with no exogenous organic material being present, with communities relying on endogenous bacteria of chemoautotroph origin (proven for Movile aquifer: Sarbu et al., 1996;FIGURE 3 Lack of deviation from isometry in male-biased sexual size dimorphism in Asellus aquaticus. Ordinary Least Squares (Model I) regression line (β = 0.96) and its 95% confidence interval (shaded area) are shown. ...
... The water temperature is similar in the caves (between 21°C and 23°C) and both caves are characterized by stable and low flow velocity. Water chemistry is intuitively harsher in Movile aquifer with extremely low oxygen and high sulfur (Sarbu et al., 1996;Brad et al., 2021). Still, the Movile aquifer population had the largest body size among all populations tested. ...
Body size is a trait of fundamental ecological and evolutionary importance that is often different between males and females (sexual size dimorphism; SSD). The island rule predicts that small-bodied species tend to evolve larger following a release from interspecific competition and predation in insular environments. According to Rensch’s rule, male body size relative to female body size increases with increasing mean body size. This allometric body size – SSD scaling is explained by male-driven body size evolution. These ecogeographical rules are rarely tested within species, and has not been addressed in a cave–surface context, even though caves represent insular environments (small and isolated with simple communities). By analyzing six cave and nine surface populations of the widespread, primarily surface-dwelling freshwater isopod Asellus aquaticus with male-biased SSD, we tested whether cave populations evolved larger and showed higher SSD than the surface populations. We found extensive between-population variation in body size (maximum divergence being 74%) and SSD (males being 15%–50% larger than females). However, habitat type did not explain the body size and SSD variation and we could not reject isometry in the male–female body size relationship. Hence, we found no support for the island or Rensch’s rules. We conclude that local selective forces stemming from environmental factors other than island vs. mainland or the general surface vs. cave characteristics are responsible for the reported population variation.
... However, most human-accessible caves lack chemoautotrophic production. Chemoautotrophy-based karst ecosystems are often limited not only by energy, but also by the supply of elements such as iron, nitrogen, and sulfur (Sarbu et al., 1996). The dynamics of these elements have a strong influence on the magnitude and location of the energy base of food webs in the cave ecosystems, with zones such as the water-air interface acting as hotspots for productivity. ...
... The diverse hygropetric fauna of Movile is composed mainly of new species which are congeners of genera present in the epigeic surroundings that penetrated the "dry bells" of that cave system possibly in a relatively recent geological past (Sârbu, 2001). The first use of environmental isotopes to study a chemoautotrophic cave system was performed at Movile by Sarbu et al. (1996). ...
... Previous studies have used stable environmental isotopes to study chemoautotrophic cave ecosystems. In Movile and Frasassi caves, organisms with a chemolithoautotrophic diet have δ 13 C values ranging from −38 to −46‰ and from −30 to −40‰ respectively (Sarbu et al., 1996(Sarbu et al., , 2000. We hypothesize that such negative values are expected also in other similar chemoautotrophic caves, such as Ayyalon. ...
The stable isotopes composition of chemolithoautotrophic cave ecosystems is known to differ from epigenic caves. Here we show that in addition, dead carbon (devoid of ¹⁴C), is utilized and transferred throughout this ecosystem, rendering it unsuitable for radiocarbon dating. The connectivity of the Ayyalon Cave ecosystem with the surface is studied, along with its sources of energy and carbon, as well as the interconnections between its constituents. We use isotopic evidence to show that its ancient resilient ecosystem is based on an underground food web depending on rich biomass production by chemolithoautotrophic nutrient supplies, detached from surface photosynthesis. Carbon isotopic values indicate that: (1) the microbial biota use bicarbonate from the groundwater (23.34 pMC [% of modern carbon]) rather than the atmospheric CO2 above the water (71.36 pMC); (2) the depleted ¹⁴C signal is transferred through the entire ecosystem, indicating that the ecosystem is well-adapted and based on the cave biofilm which is in turn based on groundwater-dissolved inorganic carbon. Incubation of Ayyalon biofilm with ¹⁴C-labelled bicarbonate indicates uptake of the radio-labeled bicarbonate by sulfur-oxidizing proteobacteria Beggiatoa, suggesting that these sulfur-oxidizing microorganisms use the water-dissolved inorganic carbon for chemolithoautotrophic carbon fixation. Organic matter in the cave is much lighter in its stable nitrogen and carbon isotopes compared with respective surface values, as expected in chemolithoautotrophic systems. This evidence may be applicative to subsurface voids of ancient Earth environments and extraterrestrial systems.
... Different phyla of invertebrates inhabit the GW's pore spaces and fissures and resulting in diverse biological communities with high levels of endemism (Sarbu et al., 1996;Thulin and Juergen, 2008;Tobler et al., 2016). However, crustaceans (amphipods, decapods, copepods, ostracods, isopods, harpacticoids) are particularly rich and diverse and dominate the GW in general, but also those underground habitats with life-threatening environmental conditions (De Jong et al., 2005;Engel, 2007;Thulin and Juergen, 2008;Wilkens et al., 2009;Papi and Pipan, 2011;Por et al., 2013;Greenway et al., 2014;Mulec et al., 2014;Galassi et al., 2016;Khalaji-Pirbalouty et al., 2018;Borko et al., 2019;Fatemi et al., 2019;Negus et al., 2020;Popa et al., 2020;Brad et al., 2021). ...
... Sulfidic caves have been examined the most in terms of primary production, sulfide-oxidizing, methanotrophic, and nitrifying bacteria providing the trophic base for the entire local community in these habitats (Sarbu et al., 1996;Sarbu, 2000; ...
... Movile cave Romania 10-43 21 Sarbu et al. (1994aSarbu et al. ( , 1996 Frasassi caves Italy 17.05 13 Galdenzi and Jones (2017) Melissotrypa Frontiers in Ecology and Evolution 13 frontiersin.org Porter et al., 2009;Bauermeister et al. 2013;Flot et al., 2014;Greenway et al., 2014). ...
Groundwater ecosystems from cold polar and circumpolar regions, hot springs, as well as those developed in salt, gypsum or in volcanic rocks are one of the environments considered to exhibit extreme environmental conditions such as low (below 0°C) or high (over 45°C) temperatures, hypersaline waters, or with elevated content of toxic gases like hydrogen sulfide or methane. They represent the “unseen ecosystem beneath our feet” and are inhabited by a large diversity of organisms, persisting and flourishing under severe environmental conditions that are usually hostile to the majority of organisms. These types of groundwater ecosystems are remarkable “evolutionary hotspots” that witnessed the adaptive radiation of morphologically and ecologically diverse species, whereas the organisms living here are good models to understand the evolutionary processes and historical factors involved in speciation and adaptation to severe environmental conditions. Here, we provide an overview of the groundwater invertebrates living in continental groundwater habitats prone to extreme environmental conditions in one or more physico-chemical parameters. Invertebrates are represented by a wide variety of taxonomic groups, however dominated by crustaceans that show specific adaptations mostly metabolic, physiologic, and behavioral. Symbiotic associations among bacteria and invertebrates are also discussed enlightening this biological interaction as a potential adaptation of different groundwater invertebrates to cope with severe environmental conditions. Given the high pressures that anthropogenic activities pose on groundwater habitats worldwide, we predict that several of these highly specialized organisms will be prone to extinction in the near future. Finally, we highlight the knowledge gaps and future research approaches in these particular groundwater ecosystems by using integrative-omic studies besides the molecular approach to shed light on genetic variation and phenotypic plasticity at species and populational levels.
GRAPHICAL ABSTRACT
... Several studies have underlined how some subterranean ecosystems do not depend entirely on organic matter coming from the surface [7][8][9][10][11][12][13] but instead, on chemoautotrophic in situ production of organic matter by microorganisms processing different inorganic elements to obtain energy and nutrients. These organisms represent de facto the only possible primary producers inhabiting the subterranean ecosystem. ...
... Moville Cave in Romania represents one of the most interesting and well known examples of a chemoautotrophic hypogean system, characterized by hydrogen sulfide-rich groundwater [18]. As demonstrated by carbon and nitrogen stable isotope analyses [7], the cave is a closed system, fully supported by chemosynthesis. Here Vlasceanu et al. [19] reported the presence of Thiobacillus thioparus, a bacterium capable of oxidizing hydrogen sulfide to sulfuric acid. ...
... We used Stable Isotope Ratio Analysis (SIRA) of carbon and nitrogen to determine the level of independence from the surface of the food-web as suggested by Sarbu et al. [7] (see also De Niro and Epstein [28], Vlasceanu et al. [29], Engel et al. [30], Paoletti et al. [31] and Michener and Lajtha [32]. SIRA represents an effective tool to determine food sources in a given ecosystem because organisms fractionate isotopes of carbon ( 13 C/ 12 C) and nitrogen ( 15 N/ 13 N) in predictable ways. ...
The great diversity of the invertebrate community thriving in the deepest sections of the gypsum karst system of the Monte Conca sinkhole (Sicily, Italy) suggests the existence of a complex food web associated with a sulfidic pool and chemoautotrophic microbial activity. To shed light on the peculiarity of this biological assemblage, we investigated the species composition of the invertebrate community and surveyed trophic interactions by stable isotope analysis. The faunal investigation conducted by visual censuses and hand sampling methods led to the discovery of a structured biological assemblage composed of both subterranean specialized and non-specialized species, encompassing all trophic levels. The community was remarkably diverse in the sulfidic habitat and differed from other non-sulfidic habitats within the cave in terms of stable isotope ratios. This pattern suggests the presence of a significant chemoautotrophic support by the microbial communities to the local food web, especially during the dry season when the organic input from the surface is minimal. However, when large volumes of water enter the cave due to local agricultural activities (i.e., irrigation) or extreme precipitation events, the sulfidic habitat of the cave is flooded, inhibiting the local autotrophic production and threatening the conservation of the entire ecosystem.
... Moreover, the cave has been sealed off from the surface for close to 6 million years (Lascu, 1989). The conditions were right for nonphotosythetic fauna to thrive and for species to adapt, with ∼30 of the described invertebrate species (∼70%) endemic to the cave (Sarbu et al., 1996;Fišer et al., 2015). Chapter 1 -Introduction ...
... The role of microorganisms in the cave has been investigated in order to unravel the specific food web system and its key players. Sarbu et al. (1994) identified sulfide-oxydizing chemoautotrophic bacteria as the primary producers in this enclosed ecosystem and later showed the in situ autotrophic production sustaining life for many different (micro-)organisms (Sarbu et al., 1996). Rohwerder et al. (2003) discovered that the majority of the metabolic activity takes place on microbial mats floating on the water surface and harboring the elemental sulfur and the primary producers i.e. sulfur oxidisers. ...
... Located in a limestone area close to the Black Sea in Southeast Romania, it encompasses several inundated galleries fed by thermal (21 C) sulfidic waters. The first explorations of these galleries showed that some of them contained oxygendepleted air pockets ('airbells') and floating whitish microbial mats apparently formed of bacteria and fungi (Sarbu et al., 1994;Sarbu et al., 1996). Early stable isotope labelling experiments showed that this subsurface ecosystem is chemosynthetic (Sarbu et al., 1996). ...
Microbial ecology is the science of micro-organisms and their biotic and abiotic interactions in a given ecosystem. As technology has advanced, molecular techniques have been widely used to overcome the limitations of classical approaches such as culturing and microscopy. Indeed, the development of Next Generation Sequencing (NGS) technologies in the past twenty years has largely helped to unravel the phylogenetic diversity and functional potential of microbial communities across ecosystems.Nonetheless, most of the environments studied through these techniques concentrated on relatively easily accessible, tractable and host-related ecosystems such as plankton (especially in marine ecosystems), soils and gut microbiomes. This has contributed to the rapid accumulation of a wealth of environmental diversity and metagenomic data along with advances in bioinformatics leading to the development of myriads of tools. Oxygen-depleted environments and especially their microbial eukaryote components are less studied and may lead to future phylogenetic and metabolic discoveries.In order to address this, we conducted analyses on two poorly studied suboxic ecosystems: Movile Cave (Romania) and lake Baikal sediments (Siberia, Russia). In this task, we aimed at unveiling the taxonomic and functional diversity of microorganims in these environments.To do so, I first evaluated the available bioinformatics tools and implemented a bioinformatics pipeline for 16S/18S rRNA gene-based metabarcoding analysis, making reasoned methodological choices. Then, as a case study, I carried out metabarcoding analyses of the water and floating microbial mats found in Movile Cave in order to investigate its protist diversity. Our study showed that Movile Cave, a sealed off chemosynthetic ecosystem, harbored a substantial protist diversity with species spanning most of the major eukaryotic super groups. The majority if these protists were related to species of freshwater and marine origins. Most of them were putatively anaerobic, in line with the cave environment, and suggesting that in addition to their predatory role, they might participate in prokaryote-protist symbioses.In a second study, I applied my metabarcoding pipeline to explore unique and relatively unexplored environment of Lake Baikal sediments. I first applied a metabarcoding approach using 16S and 18S rRNA genes to describe prokaryotic as well as protist diversity. Overall, the communities within these ecosystems were very diverse and enriched in ammonia-oxidizing Thaumarchaeota. We also identified several typical marine taxa which are likely planktonic but accumulate in sediments. Finally, our sampling plan allowed us to test whether differences across depth, basin or latitude affected microbial community structure. Our results showed that the composition of sediment microbial communities remained relatively stable across the samples regardless of depth or latitude.In a third study, we applied metagenomics to study the metabolic potential of communities associated to Baikal sediments and to reconstruct metagenome-assembled genomes (MAGs) of dominant organisms. This revealed the considerable ecological importance of Thaumarchaeota lineages in lake Baikal sediments, which were found to be the major autotrophic phyla and also very implicated in the nitrogen cycle. Chloroflexi and Proteobacteria-related species also appeared ecologically important.This PhD thesis reveals the taxonomic diversity of poorly studied suboxic ecosystems and therefore contributes to our knowledge of microbial diversity on Earth. Additionally, the analyses of surface sediment samples in lake Baikal adds new light on freshwater-marine transitions. The metagenomic analyses reported here allowed us to postulate a model of nutrient cycle carried out by microorganismsin these sediments. Overall, this work sheds light on the microbial ecology of oxygen-depleted environments, and most notably lake Baikal surface sediments.
... These multidisciplinary approaches have revealed that chemolithoautotrophic microorganisms are of major importance within the trophic chain of subterranean niches, since they act as primary producers, further supporting the development of heterotrophic microorganisms. They, together with the entry of nutrients from outside through water percolation, airflow and the intrusion of plant roots located on the surface, provide the necessary nutritional supply for their development in caves (Jobbagy & Jackson, 2001;McCulley et al., 2004;Saiz-Jimenez & Hermosin, 1999;Sarbu et al., 1996;Schenk & Jackson, 2002). It should also be noted the presence of sulfur, organic carbon and nitrogen associated with the chemical composition of the basaltic substrate in the lava tubes, which favours the presence of sulfur and nitrogen oxidizing and reducing microorganisms (Chen et al., 2009;Hathaway et al., 2014). ...
... In sample 7G-yellow, complete nitrification and denitrification pathways were predicted, with all K0 categories observed. The genus Nitrospira, present in these samples, is composed of nitrite-oxidizing bacteria, which can be involved in the CO 2 -fixationcoupled ammonia oxidation process, as reported in other cave ecosystems (Sarbu et al., 1996). However, samples 3C-yellow and 5E-red lacked complete denitrification pathways, as they were missing the K00376 (gene nosZ), crucial for converting nitrous oxide (N 2 O) to nitrogen (N 2 ). ...
Cueva del Viento, located in the Canary Islands, Spain, is the Earth's sixth‐longest lava tube, spanning 18,500 m, and was formed approximately 27,000 years ago. This complex volcanic cave system is characterized by a unique geomorphology, featuring an intricate network of galleries. Despite its geological significance, the geomicrobiology of Cueva del Viento remains largely unexplored. This study employed a combination of culture‐dependent techniques and metabarcoding data analysis to gain a comprehensive understanding of the cave's microbial diversity. The 16S rRNA gene metabarcoding approach revealed that the coloured microbial mats (yellow, red and white) coating the cave walls are dominated by the phyla Actinomycetota, Pseudomonadota and Acidobacteriota. Of particular interest is the high relative abundance of the genus Crossiella, which is involved in urease‐mediated biomineralization processes, along with the presence of genera associated with nitrogen cycling, such as Nitrospira. Culture‐dependent techniques provided insights into the morphological characteristics of the isolated species and their potential metabolic activities, particularly for the strains Streptomyces spp., Paenarthrobacter sp. and Pseudomonas spp. Our findings underscore the potential of Cueva del Viento as an ideal environment for studying microbial diversity and for the isolation and characterization of novel bacterial species of biotechnological interest.
... In the absence of photosynthesis, most cave ecosystems rely on a combination of imported surface carbon and in situ chemosynthesis. One potential source of carbon in caves is methane and methane-derived carbon has been shown to drive food-web interactions in the chemosynthetic ecosystem of Movile cave (Sarbu et al., 1996;Hutchens et al., 2004;Kumaresan et al., 2018). Karst caves have also been identified as a sink for atmospheric methane with studies showing they have a consistently lower than atmospheric levels (Mattey et al., 2013;Waring et al., 2017). ...
... The only surveyed metagenomes which had a higher proportion of methanogens than MOB classified reads were from the anaerobic sediment of Alinen Mustajärvi lake (Buck et al., 2021). Movile cave is isolated from surface inputs and the primary production is via chemolithoautotrophic microorganisms including MOB (Sarbu et al., 1996). The higher relative abundance of low-affinity MOB in Movile cave microbial mat metagenomes is due to the cave's methane-enriched atmosphere fed by thermal waters (Figure 1; Sarbu, 2000). ...
Karst ecosystems represent up to 25% of the land surface and recent studies highlight their potential role as a sink for atmospheric methane. Despite this, there is limited knowledge of the diversity and distribution of methane-oxidizing bacteria (MOB) or methanogens in karst caves and the sub-surface environment in general. Here, we performed a survey of 14 shotgun metagenomes from cave ecosystems covering a broad set of environmental conditions, to compare the relative abundance and phylogenetic diversity of MOB and methanogens, targeting biomarker genes for methane monooxygenase (pmoA and mmoX) and methyl-coenzyme M reductase (mcrA). Taxonomic analysis of metagenomes showed 0.02–1.28% of classified reads were related to known MOB, of which Gammaproteobacterial MOB were the most abundant making up on average 70% of the surveyed caves’ MOB community. Potential for biogenic methane production in caves was also observed, with 0.008–0.39% of reads classified to methanogens and was dominated by sequences related to Methanosarcina. We have also generated a cave ecosystems protein database (CEPD) based on protein level assembly of cave metagenomes that can be used to profile genes of interest.
... Primary production is carried out by autotrophic bacteria, which corroborates the survival of many chemoorganotrophic bacteria (Cañveras et al. 2001). As primary producers, cave microbes maintain the nutrient cycle and obtain energy from the processes of chemolithoautotrophy, which is common in cave ecosystems (Desai et al., 2013;Northup and Lavoie, 2001;Sarbu et al., 1996;Simon, 2019). Such extreme environment forces microbial communities to adopt novel metabolic mechanisms to oxidize reduced metals, fix gases, and utilize organic compounds (Barton, 2006). ...
... Such bacteria have also been reported in chemolithotrophic cave (Chen et al. 2009), and chemolithotrophy might be a strategy for this bacteria to survive in cave ecosystems. Nitrospira genus has been documented to perform autotrophic nitrification, which is an indicator of CO 2 fixation and NH 3 oxidation in cave environment (Sarbu et al. 1996). The presence of Nitrospira genus (which can perform complete nitrification) in cave ecosystems has nitrite oxidoreductase encoding genes (narG/narZ), and they are all considered to be nitrite oxidizers (Pester et al. 2014). ...
Microbial communities in cave ecosystems have specific survival strategies, which is far from being well explicated. Here, we reported the genetic and functional diversity of bacteria and archaea in typical limestone (Kashmir Cave) and silicate-containing (Tiser Cave) caves. X-ray diffraction (XRD) and Fourier transform infrared spectroscopic (FTIR) analyses revealed the different geochemical and mineral compositions of the two caves. Amplicon barcode sequencing revealed the dominancy of Actinobacteria and Proteobacteria in Kashmir and Tiser Caves. Bacteroidetes and Firmicutes were the dominant phyla in Tiser Cave, and the abundance is relatively small in Kashmir Cave. Archaea was also abundant prokaryotes in Kashmir Cave, but it only accounted for 0.723% of the total prokaryote sequences in Tiser Cave. Functional analysis based on metagenomic sequencing data revealed that a large number of functional potential genes involved in nutrient metabolism and biosynthesis of bioactive compounds in Tiser and Kashmir Cave samples could significantly influence the biogeochemical cycle and secondary metabolite production in cave habitats. In addition, the two caves were also found to be rich in biosynthetic genes, encoding bioactive compounds, such as monobactam and prodigiosin, indicating that these caves could be potential habitats for the isolation of antibiotics. This study provides a comprehensive insight into the diversity of bacteria and archaea in cave ecosystems and helps to better understand the special survival strategies of microorganisms in cave ecosystems.
Key points
• Geochemically distinct caves possess unique microbial community structure.• Cavernicoles could be important candidates for antibiotic production.• Cavernicoles are important for biogeochemical cycling.
... Since then, some evidence for trophic specialization within trophic levels has been identified (Ercoli et al., 2019;Francois et al., 2016Francois et al., , 2020Saccò, Blyth, Humphreys, Karasiewicz, et al., 2020;Saccò, Humphreys, et al., 2022), including the discovery of autotrophic systems based on chemoautotrophic bacteria that serve as primary producers (Sarbu et al., 1996). ...
Groundwater is a vital ecosystem of the global water cycle, hosting unique biodiversity and providing essential services to societies. Despite being the largest unfrozen freshwater resource, in a period of depletion by extraction and pollution, groundwater environments have been repeatedly overlooked in global biodiversity conservation agendas. Disregarding the importance of groundwater as an ecosystem ignores its critical role in preserving surface biomes. To foster timely global conservation of groundwater, we propose elevating the concept of keystone species into the realm of ecosystems, claiming groundwater as a keystone ecosystem that influences the integrity of many dependent ecosystems. Our global analysis shows that over half of land surface areas (52.6%) has a medium-to-high interaction with groundwater, reaching up to 74.9% when deserts and high mountains are excluded. We postulate that the intrinsic transboundary features of groundwater are critical for shifting perspectives towards more holistic approaches in aquatic ecology and beyond. Furthermore, we
... Other aquatic invertebrate taxa, besides the cyclopoids, have been found in these sulfidic waters, including cladocerans, ostracods, harpacticoids, isopods, oligochaetes, nematodes, and chironomids. The presence of biota, either microbes or invertebrates, in these sulfidic springs, having affinities for high content of H 2 S in groundwater, opens up the possibility of the existence at Techirghiol Lake of a chemoautotrophic spot within the large upper Sarmatian aquifer, slightly similar to that first described from Movile Cave near Mangalia (Sarbu et al., 1996). Furthermore, we speculate that the maze cave attested by the presence of the hillocks, might still preserve passages on the lake shore or underwater in ROSAC0197 Eforie Nord-Eforie Sud submerged beaches and its surroundings. ...
Underwater sea caves form a relatively under-examined habitat type within the marine regions of Europe, although they provide unique physical conditions such as reduced light and wave energy, in addition to reduced temperature amplitude. This study aimed at revealing the characteristics of submerged cavities on the southern Romanian continental shelf where six protected areas exist. We used high-resolution bathymetry data and side-scan sonar imaging to identify limestone outcrops where cavities would be most probable to form and then performed visual observation during SCUBA diving activities. We found that submerged cavities appear in all marine-protected areas and their neighboring unprotected areas from the shore to ~20-m depth mostly in the form of caverns, niches, and overhangs that meet the physical requirements of the habitat type 8330 as defined by the European Commission. We propose that habitat type 8330 should be listed in the Natura 2000 Standard Data Forms of the protected areas where it is missing, and we also propose that some protected areas should be expanded to include important areas with the occurrence of this habitat type. Finally, we note here for the first time the occurrence of Movile-type karst and an associated sulfidic water spring on the southern shore of Lake Techirghiol, similar to the region near Mangalia where it was first described in the literature. Although our finding prompts for a more detailed study, it indicates the existence of an underground ecosystem similar to that from the well-studied Movile Cave, but isolated by a distance of over 20 km.
... Caves represent specific biotopes characterized by stable, typically low temperatures, high humidity, and scarcity of organic substrates. The absence of light imposes restrictions on primary productivity, which either absent [1] or associated with chemoautotrophs, such as ammonium, nitrite, hydrogen sulfide, methane, and manganese-iron oxidizers [2][3][4][5]. These conditions form special microbial communities consisting predominantly of psychrophilic oligotrophs [6,7]. ...
Background
Cave biotopes are characterized by stable low temperatures, high humidity, and scarcity of organic substrates. Despite the harsh oligotrophic conditions, they are often inhabited by rich microbial communities. Abundant fouling with a wide range of morphology and coloration of colonies covers the walls of the Shulgan-Tash cave in the Southern Urals. This cave is also famous for the unique Paleolithic painting discovered in the middle of the last century. We aimed to investigate the diversity, distribution, and potential impact of these biofilms on the cave’s Paleolithic paintings, while exploring how environmental factors influence the microbial communities within the cave.
Results
The cave’s biofilm morphotypes were categorized into three types based on the ultrastructural similarities. Molecular taxonomic analysis identified two main clusters of microbial communities, with Actinobacteria dominating in most of them and a unique “CaveCurd” community with Gammaproteobacteria prevalent in the deepest cave sections. The species composition of these biofilms reflects changes in environmental conditions, such as substrate composition, temperature, humidity, ventilation, and CO 2 content. Additionally, it was observed that cave biofilms contribute to biocorrosion on cave wall surfaces.
Conclusions
The Shulgan-Tash cave presents an intriguing example of a stable extreme ecosystem with diverse microbiota. However, the intense dissolution and deposition of carbonates caused by Actinobacteria pose a potential threat to the preservation of the cave’s ancient rock paintings.
... One of the most interesting groundwater habitats are waterbodies rich in hydrogen sulfide. While the sulfide is toxic to most metazoans, it is also a source of energy to chemolithoautotrophic bacteria, resulting in primary production in the absence of light (Sarbu et al., 1996). On some occasions, chemolithoautotrophic primary production is sufficient to establish and maintain isolated, local ecosystems (Sarbu, 2000). ...
Cave systems fed with hydrogen sulfide-rich groundwater are unique chemoautotrophy-dependent
ecosystems. Although globally widespread and known to harbor unique subterranean metazoan
communities, they have mostly been studied in Europe and North America, less so in Asia. Here, we report
on a discovery of a new species of amphipod crustacean from sulfidic waters of Tashan-Chah Kabootari
aquifer from Zagros Mountains in Iran. The new species corresponds morphologically to the melitid
genus Tegano Barnard & Karaman, 1982, and Tegano tashanensis sp. nov. is the first amphipod from the
superfamily Hadzioidea found exclusively in sulfidic water. Phylogenies derived from the mitochondrial
coi and the nuclear 28S rRNA sequences recover T. tashanensis sp. nov. in a clade that contains
representatives of the currently para- or polyphyletic genera Barnardomelita, Brachina, Josephosella,
and Tegano indicating the need for a revised melitid genus level systematics. This clade comprises
marine, freshwater and subterranean species globally distributed in regions that were connected by the
ancient Tethys Sea. As such, these taxa may be suitable biogeographic models for studying past dispersal,
vicariance and multiple colonization of inland groundwaters.
... Worldwide, authors have used the SAS model [29] to explain the morphology and mineralogy of caves from the Guadalupe Mountains, New Mexico [30], as well as from Mexico (Cueva de Villa Luz [31]), Italy (Grotte di Frasassi [32]), Romania (Peştera Movile [33]; Cerna River Valley [34][35][36]), and Wyoming (Lower Kane Cave [37]). ...
Carbonic acid and sulfuric acid speleogenesis describe a dichotomy between epigenetic and hypogenetic caves and carbon and sulfur cycling in karst, but do not acknowledge the global spectrum of cave formation. This paper, part one of a two-part investigation, tests and revises speleogenetic models from a classic karst landscape using dissolved ion concentrations δ13CDIC, and δ34S in water samples collected at four sites across the Bluespring and Lost River karst basins in the Mitchell Plateau, Indiana, USA. Analyses revealed elevated sulfur in both karst basins but differently sourced; H2S (δ34S = −14.2‰) evolved from petroleum seeps in Bluespring Caverns accounted for up to 61% of sulfur in the cave stream, while evaporite beds (δ34S = [+14.50‰, +17.91‰]) of the St. Louis Limestone contributed up to 100% of sulfur at Orangeville Rise, a terminal spring of the Lost River karst basin. These results have implications for carbon–sulfur cycle linkages, particularly the potential acceleration of carbon flux from sulfuric acid dissolution in otherwise epigenetic settings. We suggest a new paradigm for speleogenesis in the North American midcontinent—speleogenesis in the Mitchell Plateau and similar settings is not epigenetic or hypogenetic, but instead polygenetic with competing chemical processes varying across space and time.
... The decrease in δ 15 N values with increasing BAFA proportions and decreasing δ 34 S values in juvenile catfish indicates that the low δ 15 N values were caused by bacterial activity under anoxic conditions. Extremely low δ 15 N values have been observed in methanotrophic and/or chemoautotrophic communities in freshwater 44,45 , caves 46 , and the deep sea 47 , which likely results from extensive nitrogen isotope fractionation during the assimilation of dissolved inorganic nitrogen (i.e., ammonium and nitrate) by bacteria under anoxic conditions. However, despite the high proportion of BAFA, juvenile catfish also contained PUFA that are typically absent in bacteria 19,48 . ...
Dietary information from aquatic organisms is instrumental in predicting biological interactions and understanding ecosystem functionality. In freshwater habitats, generalist fish species can access a diverse array of food sources from multiple food chains. These may include primary photosynthetic production and detritus derived from both oxic and anoxic decomposition. However, the exploitation of anoxic decomposition products by fish remains insufficiently explored. This study examines feeding habits of striped catfish ( Pangasianodon hypophthalmus ) at both adult and juvenile stages within a tropical reservoir, using stable carbon, nitrogen, and sulfur isotope ratios ( δ ¹³ C, δ ¹⁵ N, and δ ³⁴ S, respectively) and fatty acid (FA) analyses. The adult catfish exhibited higher δ ¹⁵ N values compared to primary consumers that feed on primary photosynthetic producers, which suggests ingestion of food sources originating from primary photosynthetic production-based food chains. On the other hand, juvenile catfish demonstrated lower δ ¹⁵ N values than primary consumers, correlating with low δ ³⁴ S value and large proportions of bacterial FA but contained small proportions of polyunsaturated FA. This implies that juveniles utilize food sources from both anoxic decomposition and primary photosynthetic production-based food chains. Our results indicate that food chains based on anoxic decomposition can indeed contribute to the dietary sources of tropical fish species.
... The higher trophic levels of anchialine ecosystems largely comprise cave-adapted invertebrates with high rates of endemism [5,6]. Earlier investigations into these anchialine food webs indicated that they may be supported, at least in part, by chemosynthetic microbes [7][8][9]. There is now growing interest in surveying the microbial communities that inhabit subterranean estuaries, and in particular, characterising their niche-adaptive metabolisms [1,10]. ...
Background
Anchialine environments, in which oceanic water mixes with freshwater in coastal aquifers, are characterised by stratified water columns with complex physicochemical profiles. These environments, also known as subterranean estuaries, support an abundance of endemic macro and microorganisms. There is now growing interest in characterising the metabolisms of anchialine microbial communities, which is essential for understanding how complex ecosystems are supported in extreme environments, and assessing their vulnerability to environmental change. However, the diversity of metabolic strategies that are utilised in anchialine ecosystems remains poorly understood.
Results
Here, we employ shotgun metagenomics to elucidate the key microorganisms and their dominant metabolisms along a physicochemical profile in Bundera Sinkhole, the only known continental subterranean estuary in the Southern Hemisphere. Genome-resolved metagenomics suggests that the communities are largely represented by novel taxonomic lineages, with 75% of metagenome-assembled genomes assigned to entirely new or uncharacterised families. These diverse and novel taxa displayed depth-dependent metabolisms, reflecting distinct phases along dissolved oxygen and salinity gradients. In particular, the communities appear to drive nutrient feedback loops involving nitrification, nitrate ammonification, and sulphate cycling. Genomic analysis of the most highly abundant members in this system suggests that an important source of chemotrophic energy is generated via the metabolic coupling of nitrogen and sulphur cycling.
Conclusion
These findings substantially contribute to our understanding of the novel and specialised microbial communities in anchialine ecosystems, and highlight key chemosynthetic pathways that appear to be important in these energy-limited environments. Such knowledge is essential for the conservation of anchialine ecosystems, and sheds light on adaptive processes in extreme environments.
... But Movile Cave presents unique trophic environmental conditions not typical of most cave systems. It"s reasonable to argue that the ecological dynamics reported from Movile Cave are likely restricted to this system or at the very least, to similar chemoautotrophic-based environments (Brad et al. 2021;Sarbu et al. 1996). Therefore, advancing our understanding of the ecology of cave-dwelling nematodes requires additional information on the community dynamics (i.e., temporal changes in community composition) and distribution of nematodes in cave habitats. ...
... Bacterial-dominated communities are most frequently reported as colonizing the cave walls or speleothems apparently depleted in organics (Lavoie et al., 2017) and playing key roles in speleothem genesis or limestone erosion (Cuezva et al., 2009). Cave ecosystems might harbor chemotrophic-based primary production as in sulfidic Movile Cave (Sarbu et al., 1996). Microbial diversity could be shaped by geochemical composition and/ or anthropic activity. ...
Introduction
Karst caves are characterized by relatively constant temperature, lack of light, high humidity, and low nutrients availability. The diversity and functionality of the microorganisms dwelling in caves micro-habitats are yet underexplored. Therefore, in-depth investigations of these ecosystems aid in enlarging our understanding of the microbial interactions and microbially driven biogeochemical cycles. Here, we aimed at evaluating the diversity, abundance, distribution, and organic substrate preferences of microbial communities from Peștera cu Apă din Valea Leșului (Leșu Cave) located in the Apuseni Mountains (North-Western Romania).
Materials and Methods
To achieve this goal, we employed 16S rRNA gene amplicon sequencing and community-level physiological profiling (CLPP) paralleled by the assessment of environmental parameters of cave sediments and water.
Results and Discussion
Pseudomonadota (synonym Proteobacteria ) was the most prevalent phylum detected across all samples whereas the abundance detected at order level varied among sites and between water and sediment samples. Despite the general similarity at the phylum-level in Leșu Cave across the sampled area, the results obtained in this study suggest that specific sites drive bacterial community at the order-level, perhaps sustaining the enrichment of unique bacterial populations due to microenvironmental conditions. For most of the dominant orders the distribution pattern showed a positive correlation with C-sources such as putrescine, γ-amino butyric acid, and D-malic acid, while particular cases were positively correlated with polymers (Tween 40, Tween 80 and α-cyclodextrin), carbohydrates (α-D-lactose, i-erythritol, D-mannitol) and most of the carboxylic and ketonic acids. Physicochemical analysis reveals that sediments are geochemically distinct, with increased concentration of Ca, Fe, Al, Mg, Na and K, whereas water showed low nitrate concentration. Our PCA indicated the clustering of different dominant orders with Mg, As, P, Fe, and Cr. This information serves as a starting point for further studies in elucidating the links between the taxonomic and functional diversity of subterranean microbial communities.
... Sulfur inclusions were also shown to form when the supply of O 2 is limited and as a result the sulfur cannot be entirely oxidized to soluble sulfite, thiosulfate, or sulfate. Complete depletion of sulfur inclusions from cells is not likely in Movile Cave where abundant H 2 S (< 1 mM) [21]) is available continuously and where O 2 is scarce in most habitats, and specifically in Air Bell 2 [83]. The analysis of the 6 Thiovulum genomes identified the SQR gene responsible for the oxidation of sulfide to elemental sulfur, which was also highly expressed in the Movile Cave transcripts. ...
Thiovulum spp. (Campylobacterota) are large sulfur bacteria that form veil-like structures in aquatic environments. The sulfidic Movile Cave (Romania), sealed from the atmosphere for ~5 million years, has several aqueous chambers, some with low atmospheric O 2 (~7%). The cave’s surface-water microbial community is dominated by bacteria we identified as Thiovulum . We show that this strain, and others from subsurface environments, are phylogenetically distinct from marine Thiovulum . We assembled a closed genome of the Movile strain and confirmed its metabolism using RNAseq. We compared the genome of this strain and one we assembled from public data from the sulfidic Frasassi caves to four marine genomes, including Candidatus Thiovulum karukerense and Ca . T. imperiosus, whose genomes we sequenced. Despite great spatial and temporal separation, the genomes of the Movile and Frasassi Thiovulum were highly similar, differing greatly from the very diverse marine strains. We concluded that cave Thiovulum represent a new species, named here Candidatus Thiovulum stygium. Based on their genomes, cave Thiovulum can switch between aerobic and anaerobic sulfide oxidation using O 2 and NO 3 ⁻ as electron acceptors, the latter likely via dissimilatory nitrate reduction to ammonia. Thus, Thiovulum is likely important to both S and N cycles in sulfidic caves. Electron microscopy analysis suggests that at least some of the short peritrichous structures typical of Thiovulum are type IV pili, for which genes were found in all strains. These pili may play a role in veil formation, by connecting adjacent cells, and in the motility of these exceptionally fast swimmers.
... The inorganic compounds necessary to sustain life in this environment (H2S, CH4, NH4 + ) are provided by the mesothermal waters that run through the cave system (Sarbu et al. 1996). Due to the lack of input of organic matter and energy from the surface, the food chain in Movile Cave is likely based on the chemoautotrophic microorganisms that live freely in the water of the submerged passages as bacterioplankton, gather in microbial biofilms at the surface of the water or attach to the cave walls and the sediments (Sarbu and Kane, 1995;Chiciudean et al., 2022). These microorganisms become the food for unicellular eukaryotes as well as rotifers, nematodes, gastropods and crustaceans. ...
Chemoautotroph bacteria have evolved symbiosis with different organisms, invertebrates in particular, allowing them, among other, to colonize new habitats that are extreme for most animals. In this work we aim to detect if a symbiosis occurs between sulfur-oxidizing bacteria of the genus Thiothrix and microcrustaceans especially groundwater cyclopoids and ostracods that live in sulfidic subterranean waters from the mesothermal aquifer of Mangalia, Romania and compare them with the already known associations between sulfidic groundwater amphipods and Thiothrix.
... Globally, studies have identified Proteobacteria, Actinobacteria, Acidobacteria, Firmicutes, Nitrospirae, Gemmatimonadetes, Chloroflexi, Verrucomicrobia and Planctomycetes throughout cave and karst aquifers, associated in-cave sediments and on cave surfaces, including speleothems (Dong et al., 2020;Ma et al., 2021;Ortiz et al., 2013;Yun et al., 2016). The lack of sunlight within cave systems means that these microbes have to be adapted to utilize alternative energy sources, such as the chemosynthetic fixation of inorganic carbon, degradation of aromatic hydrocarbons and the use of hydrogen sulphide (Marques et al., 2019;Sarbu et al., 1996;Wu et al., 2015). On that account, the microbial interactions within caves were found to be intense and cooperative to support mutually in an underground karstic environment (Dong et al., 2020). ...
Caves are complex ecosystems with various microbial habitats. Understanding the individual community structures in the key source environments (soils, waters) and those in the sinks (speleothems, cave muds) can help elucidate the extent to which in‐cave communities are a function of their source communities, or if they can successfully adapt and diversify locally. Here, we assess the network of microbial communities existing within a unique British cave system. Poole's Cavern is characterized by alkalinity or even hyperalkalinity (pH > 9) in its drip waters, creating a series of challenging ecological niches for microbes to survive. Samples of soil, drip water, speleothem, and cave sediment were collected for chemical characterization, cell culture assays and DNA sequencing analysis. We show that microbial abundance and diversity were highest in the soils proximal to the cave, and the microbiotas of the hyperalkaline cave chamber were less abundant and less diverse than the cave chamber with lower pH. Proteobacteria and Planctomycetota were the most prevalent bacterial phyla throughout the cave system irrespective of pH, indicating their members are comparatively more metabolically versatile. Predicted essential metabolic pathways were still dominant within all sampling sites, and microorganisms were shown to be capable of utilizing various inorganic or simple organic compounds to survive. Interestingly, co‐occurrence between Poole's samples was limited, never demonstrating more than 50% similarity. This work highlights the diversity of microbial communities within this extreme environment and the development of microbial niches which reflects the adaptation strategy of microorganisms under alkaline‐hyperalkaline conditions.
... The Movile Cave (SE Romania) was the first to be mentioned as defying the conventional view of subterranean ecosystems as supported by aboveground photosynthesis. Movile Cave is a small-surfaced, closed chemoautotrophic system [8] driven by in-house sulfur and methane oxidation and CO 2 fixation as primary production processes. The cave, located in south-eastern Romania (Dobrogea region), developed in oolitic and fossil-rich limestone of the Sarmatian age (Late Miocene) and was sealed off during the Quaternary by a thick and impermeable layer of clays and loess [17]. ...
Background
Movile Cave (SE Romania) is a chemoautotrophically-based ecosystem fed by hydrogen sulfide-rich groundwater serving as a primary energy source analogous to the deep-sea hydrothermal ecosystems. Our current understanding of Movile Cave microbiology has been confined to the sulfidic water and its proximity, as most studies focused on the water-floating microbial mat and planktonic accumulations likely acting as the primary production powerhouse of this unique subterranean ecosystem. By employing comprehensive genomic-resolved metagenomics, we questioned the spatial variation, chemoautotrophic abilities, ecological interactions and trophic roles of Movile Cave’s microbiome thriving beyond the sulfidic-rich water.
Results
A customized bioinformatics pipeline led to the recovery of 106 high-quality metagenome-assembled genomes from 7 cave sediment metagenomes. Assemblies’ taxonomy spanned 19 bacterial and three archaeal phyla with Acidobacteriota , Chloroflexota , Proteobacteria , Planctomycetota , Ca. Patescibacteria, Thermoproteota , Methylomirabilota, and Ca. Zixibacteria as prevalent phyla. Functional gene analyses predicted the presence of CO 2 fixation, methanotrophy, sulfur and ammonia oxidation in the explored sediments. Species Metabolic Coupling Analysis of metagenome-scale metabolic models revealed the highest competition-cooperation interactions in the sediments collected away from the water. Simulated metabolic interactions indicated autotrophs and methanotrophs as major donors of metabolites in the sediment communities. Cross-feeding dependencies were assumed only towards 'currency' molecules and inorganic compounds (O 2 , PO 4 ³⁻ , H ⁺ , Fe ²⁺ , Cu ²⁺ ) in the water proximity sediment, whereas hydrogen sulfide and methanol were assumedly traded exclusively among distant gallery communities.
Conclusions
These findings suggest that the primary production potential of Movile Cave expands way beyond its hydrothermal waters, enhancing our understanding of the functioning and ecological interactions within chemolithoautotrophically-based subterranean ecosystems.
... Given cenote morphology with a slope of about 150° from the entrance to the cave zone we speculate that food source is not dependent of allochthonous organic material only, but from other sources such a chemoautotrophy. At global scale, in cave environments have been observed that chemical compounds such as hydrogen sulfide (H2S) (Sarbu et al. 1996), methane and ammonium (Pohlman et al. 1997) can be synthetized by bacterial mats and transformed into organic products that become the main food source for subterranean organisms. In the Yucatán Peninsula, such processes have been observed in cenotes of coastal areas (which constitute subterranean estuaries), such as Ox Bel Ha Cave System (Brankovits et al. 2017). ...
... Given cenote morphology with a slope of about 150° from the entrance to the cave zone we speculate that food source is not dependent of allochthonous organic material only, but from other sources such a chemoautotrophy. At global scale, in cave environments have been observed that chemical compounds such as hydrogen sulfide (H2S) (Sarbu et al. 1996), methane and ammonium (Pohlman et al. 1997) can be synthetized by bacterial mats and transformed into organic products that become the main food source for subterranean organisms. In the Yucatán Peninsula, such processes have been observed in cenotes of coastal areas (which constitute subterranean estuaries), such as Ox Bel Ha Cave System (Brankovits et al. 2017). ...
We studied benthic assemblages through X-Batún, a continental freshwater cenote and its associated submerged cave located in San Antonio Mulix (Yucatán, Mexico). Using cave diving techniques, we collected sediment samples at four zones of the system. We extracted and counted individuals of benthic species in three replicates of 5 grams of wet sediment at each site. The biological composition was integrated by 15 species from eight higher taxonomic groups. Non-metric multidimensional scaling distinguished four assemblages that coincided with surface, open water, cavern and cave zones. ANOSIM test revealed significant differences between the assemblages. In the deeper zones of the cenote characterized by twilight and total darkness, Ostracoda and Gastropoda show the highest diversity and abundance, with practical absence in surface sediments. This pattern may suggest ecological interactions with chemosynthetic bacterial activity. Surface shows an assemblage typical of epigean environments. Environmental variables along the cenote varied little from the upper layers to bottom. Linear correlation and detrended canonical analysis revealed that light is the main driver of benthic species assemblages. Temperature, pH, and dissolved oxygen exert higher influence at individual biological benthic assemblage in X-Batún.
... The differentially abundant bacterial taxa in caves tended to be specialists that have also been found in high abundance in other cave ecosystems across geographic regions [26,[69][70][71]. Knowledge of the ecology of many of these taxa remains limited, but some include chemolithoautotrophs (e.g., Nitrospirales, Nitrosococcales) [72][73][74] and chemoheterotrophs (e.g., Pirellulales, Blastocatellales) [75][76][77][78] that can use inorganic compounds to generate energy. The relatively high abundance of multiple ammonia-oxidising bacterial taxa belonging to the order Nitrosococcales [79] also suggest that these taxa might play key roles through facilitating the nitrogen cycle within these cave ecosystems. ...
Background
Subarctic regions are particularly vulnerable to climate change, yet little is known about nutrient availability and biodiversity of their cave ecosystems. Such knowledge is crucial for predicting the vulnerability of these ecosystems to consequences of climate change. Thus, to improve our understanding of life in these habitats, we characterized environmental variables, as well as bacterial and invertebrate communities of six subarctic caves in Northern Norway.
Results
Only a minuscule diversity of surface-adapted invertebrates were found in these caves. However, the bacterial communities in caves were compositionally different, more diverse and more complex than the nutrient-richer surface soil. Cave soil microbiomes were less variable between caves than between surface communities in the same area, suggesting that the stable cave environments with tougher conditions drive the uniform microbial communities. We also observed only a small proportion of cave bacterial genera originating from the surface, indicating unique cave-adapted microbial communities. Increased diversity within caves may stem from higher niche specialization and levels of interdependencies for nutrient cycling among bacterial taxa in these oligotrophic environments.
Conclusions
Taken together this suggest that environmental changes, e.g., faster melting of snow as a result of global warming that could alter nutrient influx, can have a detrimental impact on interactions and dependencies of these complex communities. This comparative exploration of cave and surface microbiomes also lays the foundation to further investigate the long-term environmental variables that shape the biodiversity of these vulnerable ecosystems.
... Caves are underground environments in which the absence of light impedes the presence of photoautotrophic organisms and determines a dependence on different allochthonous organic matter transfer processes for the biota maintenance (Simon et al., 2007). The primary autochthonous production rarely occurs, mainly through chemoautotrophic bacteria (Sarbu et al., 1996;Chivian et al., 2008). The allochthonous organic matters are brought in to the caves by sinking surface streams or rivers, by runoffs and by water that percolates from the roof or wall, through openings or fissures (Simon et al., 2003). ...
A number of arthropods, predominantly insects, are habitual inhabitants of caves, having evolved marked characteristics or specializations for the extreme conditions of their habitat. Colonization of the caves by the arthropods was mainly by pre-adapted lineages, with high proportions of eutroglophile species. Over time these have dominated this subterranean ecosystem becoming a major constituent of the fauna inhabiting the caves. Faunal samples from Krem Lawkhlieng invariably consists, predominantly, of terrestrial arthropods species mainly insects (cave crickets, cave beetles, fungus gnats, springtails, cockroaches, etc.), woodlice, millipedes, harvestmen, spiders, etc. These species are taxonomically unique, have a low population, present high levels of population fragmentation with restricted or narrow distribution ranges, and are likely candidates for imperiled species requiring urgent conservation measures. Moreover, with the recent modification of this Cave as Krem Arwah that is fast emerging as a major tourist destination in the Sohra (Cherrapunjee) tourist circuit, has already affected the fragile cave fauna, about which a brief discussion is added in the paper.
... Chemoautotrophically-based ecosystems known so far comprise deep sea hydrothermal vents and cold seeps [1][2][3][4], reduced sulfur-rich water and sediments of circumneutral saline and soda lakes [5][6][7] and sulfidic cave [8][9][10]. Only a handful of sulfidic cave ecosystems that rely totally or partially on chemosynthesis are explored to date, including Ayyalon Cave (Israel), Movile Cave (Romania), Frasassi Cave (Italy), Lower Kane Cave and Cesspool Cave (USA), and Villa Luz Cave (Mexico) [11-14, 9, 15, 16]. ...
Background
Movile Cave (Dobrogea, SE Romania) hosts a subterranean chemoautotrophically-based ecosystem supported by a sulfidic thermal aquifer analogous to the deep-sea hydrothermal ecosystems. Our current understanding of Movile Cave microbiology has been confined to the thermal water proximity (no more than 2 m distant), with most studies focusing on the water-floating mat, which likely acts as the primary production powerhouse in this sulfidic ecosystem. To gain more insightful information on the functioning of the sulfidic Movile Cave ecosystem, we employed a metagenomics-resolved approach to reveal the microbiome diversity, metabolic potential, and interactions and infer its roles within the food webs in the sediments beyond the sulfidic thermal waters.
Results
A customized bioinformatics pipeline led to the recovery of 106 high-quality metagenome-assembled genomes from 7 cave sediment metagenomes. Assemblies’ taxonomy spanned 19 bacterial and three archaeal phyla with Acidobacteriota, Chloroflexota, Proteobacteria, Planctomycetota, Ca . Patescibacteria, Thermoproteota, Methylomirabilota , and Ca . Zixibacteria as prevalent phyla. Functional gene analyses allowed prediction of CO 2 fixation, methanotrophy, sulfur and ammonia oxidation as possibly occurring in the explored sediments. Species Metabolic Coupling Analysis of metagenome-scale metabolic models revealed the highest competition-cooperation interactions in the sediments collected at the farthest distance from the sulfidic water. As a result of simulated metabolic interactions, autotrophs and methanotrophs were hypothesized as major donors of exchanged metabolites in the sediment communities. Cross-feeding dependencies were assumed only towards ‘currency’ molecules and inorganic compounds (O 2 , PO 4 ³⁻ , H ⁺ , Fe ²⁺ , Cu ²⁺ ) in the sediment nearby sulfidic water, whereas hydrogen sulfide and methanol are predictably traded exclusively among communities dwelling in the distant gallery.
Conclusions
These findings suggest that the primary production potential of the Movile Cave expands way beyond its hydrothermal waters, enhancing our understanding of ecological interactions inside chemolithoautotrophically based subterranean ecosystems and their functioning.
... There is no exogenous food in the cave, only endogenous bacteria forming mats. Therefore, this cave's community is most likely based on organic matter produced by chemoautotrophic bacteria living in complete darkness, similarly to the Romanian Movile Cave (Sarbu et al. 1996). The cave community is extremely simple, and there are no predators of A. aquaticus present. ...
Behavioural innovativeness is important for colonising new habitats; however, it is also costly. Along the colonisation event of a simple, stable and isolated habitat offering only new food sources, one could hypothesize that the colonising individuals are more innovative than the average in their source population, showing preference to the new resource, while after colonisation, the adapted population will lose its innovativeness and become specialised to the new resource. To test this hypothesis, we compared food preference and food type innovation of a cave-dwelling waterlouse ( Asellus aquaticus ) population (genetically isolated for at least 60 000 years) to three surface-dwelling populations, also sampling individuals that have recently entered the cave (‘colonists’). In the cave, the only food sources are endogenous bacterial mats, while surface populations feed on various living and dead plant material together with their fungal and bacterial overgrow. We assayed all populations with the familiar and unfamiliar food types from the natural habitats and two novel food types not occurring in the natural habitats of the species. We found that all populations preferred surface to cave food and consumed the unnatural novel food types. Surface populations avoided cave food and colonists spent the most time with feeding on surface food. We conclude that the cave population maintained its preference for surface food and did not lose its food type innovativeness. We suggest that adapting to the special cave food was a major challenge in colonising the cave.
Significance statement
Behavioural innovativeness is a key trait for adapting to environmental changes or to colonise new habitats. However, it has developmental and maintenance costs due to the high energy need of the necessary sensory and neural organs. Therefore, we asked whether behavioural innovativeness decreases after colonising an isolated, stable and highly specialised habitat. By comparing food type innovativeness of surface-dwelling populations of waterlouse ( Asellus aquaticus ) to a population that has colonised a cave at least 60 000 years ago, we found that the high innovativeness towards unnatural food was retained in the cave population. Further, all populations preferred surface food (decaying leaves), with surface populations almost completely avoiding cave food (endogenous bacteria mats). We suggest that (i) food type innovativeness is evolutionary rigid in our system and (ii) the cave food was rather an obstacle against than a trigger of cave colonisation.
... 细菌是洞穴生物多样性的 主要组成部分, 在维持洞穴生态系统中起着关键作 用 [5] . 同时洞穴细菌与矿物的相互作用 [6~8] 以及放线菌 在新药物开发中的重要地位 [9,10] 洞穴中存在多种小生境, 前期人们对洞穴微生物的研 究主要集中在上覆土壤 [14] 、滴水、岩壁 [15] 、沉积物 [16] 和蝙蝠粪便 [17] [19] , 利用离子色谱(ICS-600, Thermo, 美国)和ICP-OES(iCAP 7600+, Thermo, 美国)测定样品 阴离子和阳离子的含量 [20] . TOC含量用元素分析仪 ...
... Surface A. aquaticus are detritivores. Organic debris in caves is presumably of low quality in comparison with biofilm (Poulson, 2012), thus scrapping of biofilm might be a relatively more important feeding strategy for cave populations (Francois et al., 2016) especially in closed systems such as Movile-and Molnár János Cave Sarbu et al., 1996). We found that cave populations had narrower propodus (pereopod I) than surface populations, irrespective of sex. ...
Abstract Studying parallel evolution (repeated, independent evolution of similar phenotypes in similar environments) is a powerful tool to understand environment‐dependent selective forces. Surface‐dwelling species that repeatedly and independently colonized caves provide unique models for such studies. The primarily surface‐dwelling Asellus aquaticus species complex is a good candidate to carry out such research, because it colonized several caves in Europe. By comparing 17 functional morphological traits between six cave and nine surface populations of the A. aquaticus species complex, we investigated population divergence in morphology and sexual dimorphism. We found habitat‐dependent population divergence in 10 out of 17 traits, likely reflecting habitat‐driven changes in selection acting on sensory systems, feeding, grooming, and antipredator mechanisms. Sexual dimorphism was present in 15 traits, explained by sexual selection acting on male traits important in male–male agonistic behavior or mate guarding and fecundity selection acting on female traits affecting offspring number and nursing. In eight traits, the degree of sexual dimorphism was habitat dependent. We conclude that cave‐related morphological changes are highly trait‐ and function‐specific and that the strength of sexual/fecundity selection strongly differs between cave and surface habitats. The considerable population variation within habitat type warrants further studies to reveal cave‐specific adaptations besides the parallel patterns.
The cosmos, replete with the elemental diversity of Earth, continues to cloak the existence of extraterrestrial life in mystery. Vital to the search for life is the identification of environments that can support biological processes. This review synthesizes current knowledge on the conditions necessary for life, with a special focus on extremophiles. These organisms, thriving under extreme conditions, illustrate the robust adaptability required for life beyond Earth. We examine the evolutionary adaptations on Earth to provide a framework for potential analogues on other celestial bodies. Importantly, this paper reports the initial identification of approximately three hundred three (∼ 303) sites on Mars as potential habitats intriguingly considered as potential Mars Cave Candidates (MCC) based on meticulous visual interpretations, and require detailed investigation to confirm their nature, whether as caves, other geological features, or craters. These investigations are currently ongoing, highlighting the dynamic and exploratory nature of Martian research. The findings are preliminary and serve to inform the development of robotic exploration strategies aimed at in-depth study of these environments, thereby advancing the astrobiological search for life. This review sets strategic directions for future research, aiming to refine our approach to uncovering where and how life could exist across the cosmos. By guiding future missions, both robotic and astrobiological, this work seeks to deepen our understanding of potential extraterrestrial habitats and to foster a systematic exploration of these promising sites.
Chemosynthetic microorganisms facilitate microbialite development in many caves throughout the world. In Youqin Cave and Tian'e Cave, located in the Carboniferous–Triassic carbonates on the South China Block, five Quaternary speleothems (stalagmite, stalactite and cave pearl) that are 2.3 to 11.0 cm long were examined for their petrographic, geochemical and microbiological features to reveal how chemotrophs contribute to microbialite growth. In the speleothems, millimetre‐sized stromatolites, thrombolites and calcified microbial mats are characterized by alternating light, calcitic microlaminae and dark, clay and organic‐rich calcite microlaminae. Filamentous (reticulate, smooth, nodular and helical), coccoid and bacilliform microbes, originally carried into the caves from surface soils, are more common in the dark microlaminae/clots than in the light microlaminae. 16S rRNA gene sequencing shows that the biotas in the microbialites are dominated by chemoorganotrophic heterotrophic bacteria, including primarily Sphingomonas , Crossiella and Acinetobacter , and rare Archaea. Diverse metabolic pathways of these prokaryotes, including ureolysis, denitrification and nitrite ammonification, contributed to increases in localized pH and/or dissolved inorganic carbon in these microenvironments, prompting carbonate precipitation. Development of the cave microbialites was probably controlled by the evolution of the cave microbial community as environmental conditions changed. Microbialite growth was probably mediated by the microorganisms that flourished on the speleothem surfaces during periods of low drip water rates and slow calcite precipitation. The change from microstromatolites to microthrombolites was probably linked to a decrease in cell populations in the microbial communities. These cave microbialites provide clear insights regarding the biogenicity and growth mechanisms of chemosynthetic microbialites. Given their association with chemolithotrophic activities that can date back to the Meso‐Archean, cave microbialites provide insights into the biogenicity and growth mechanisms of chemosynthesis‐based microbialites throughout geological history.
Space is full of immense possibilities, it is a treasure of every element that exists on Earth today, yet life has not yet been discovered anywhere beyond the Earth. It is essential to have a favorable environment for life to flourish and survive. To discover life, it is also necessary to explore an environment suitable and favorable for life. Through this review paper, we will briefly describe life, the necessity of life, the environment conducive to life, the possibility of extraterrestrial life as extremophiles, and a wonderful cosmic journey undertaken to adapt to life on Earth. This review will serve to guide robotic and astrobiological exploration in the search for extraterrestrial life beyond Earth especially on Mars.
The conquest of land by arthropods is commonly believed to be a surface phenomenon associated with the arrival of photosynthetic plants, atmospheric oxygenation, and an ozone shield in the mid-Paleozoic Era. However, recent molecular and fossil evidence suggests terrestrial fauna may have first appeared in the Cambrian, before the proliferation of plants and ozone, which are thought to be essential for survival. This raises the question-how could arthropods survive on land without established plants and an ozone shield? We propose a hypothesis that chemolithoautotrophic cave ecosystems, independent of photosynthesis, may have served as a subsurface stepping stone, providing a possible explanation for the land invasion enigma. Chemolithoautrophic caves have offered abundant food and radiation protection, enabling ancient arthropods to evolve strategies to adapt to new frontiers through gradual dispersion from the sea to shielded cave waters, then to cave hygropetric margins of cave waters, and, finally, to the surface.
Current food systems reduce, deplete and pollute our limited global resources. Radical changes are required to ensure future food security and safety. Worldwide biodiversity losses and mass extinction of species, increasing urban populations, growing human vulnerability and climate change are extending the challenges to achieve food security.
Groundwater is a vital ecosystem of the global water cycle, hosting unique biodiversity and providing essential services to societies. Despite being the largest unfrozen freshwater resource, in a period of depletion by extraction and pollution, groundwater environments have been repeatedly overlooked in global biodiversity conservation agendas. Disregarding the importance of groundwater as an ecosystem ignores its critical role in preserving surface biomes. To foster timely global conservation of groundwater, we propose elevating the concept of keystone species into the realm of ecosystems, claiming groundwater as a keystone ecosystem that influences the integrity of many dependent ecosystems. Our global analysis shows that over half of land surface areas (52.6%) has a medium-to-high interaction with groundwater, reaching up to 74.9% when deserts and high mountains are excluded. We postulate that the intrinsic transboundary features of groundwater are critical for shifting perspectives towards more holistic approaches in aquatic ecology and beyond. Furthermore, we propose eight key themes to develop a science-policy integrated groundwater conservation agenda. Given ecosystems above and below the ground intersect at many levels, considering groundwater as an essential component of planetary health is pivotal to reduce biodiversity loss and buffer against climate change.
Sulfidic cave ecosystems are remarkable evolutionary hotspots that have witnessed adaptive radiation of their fauna represented by extremophile species having particular traits. Ostracods, a very old group of crustaceans, exhibit specific morphological and ecophysiological features that enable them to thrive in groundwater sulfidic environments. Herein, we report a peculiar new ostracod species Pseudocandonamovilaensis sp. nov. thriving in the chemoautotrophic sulfidic groundwater ecosystem of Movile Cave (Romania). The new species displays a set of homoplastic features specific for unrelated stygobitic species, e.g., triangular carapace in lateral view with reduced postero-dorsal part and simplification of limb chaetotaxy (i.e., loss of some claws and reduction of secondary male sex characteristics), driven by a convergent or parallel evolution during or after colonization of the groundwater realm. P.movilaensis sp. nov. thrives exclusively in sulfidic meso-thermal waters (21 °C) with high concentrations of sulphides, methane, and ammonium. Based on the geometric morphometrics-based study of the carapace shape and molecular phylogenetic analyses based on the COI marker (mtDNA), we discuss the phylogenetic relationship and evolutionary implication for the new species to thrive in groundwater sulfidic groundwater environments.
The Asellus aquaticus species complex is an excellent model for understanding subterranean evolution. Here, we review its
phylogeography and population structure, the phenotypic differences between its subterranean and surface ecomorphs, the
genetic basis of subterranean traits, progress toward becoming an evo-devo model, and genomic resources. The above
information has allowed for comparisons to be made between A. aquaticus species complex and the most commonly
studied model of subterranean evolution, Astyanax mexicanus. Ultimate goals are to understand the specifics of how and
why particular characteristics have evolved in the mentioned complex as well as to achieve a more global understanding
of the evolution of subterranean-related traits, bringing together information from multiple models.
One of the most prominent features of the Grand Canyon is the iron-stained Redwall Limestone, which has been documented to contain extensive cave systems with significant storage potential in the karstic Redwall-Muav (R-M) aquifer. The hydrogeologic relationships between the R-M aquifer and cave formation in the Grand Canyon via descending (epigenic) surface waters have been well documented. However; the potentially large role of deeply sourced (hypogenic) waters rising from depth in relation to cave formation is poorly understood. The Bopper Cave System (BCS) is a relict feature of the ancient abandoned hydrologic system at Grand Canyon and displays unique network maze cave morphologies formed under phreatic conditions with rich and diverse mineral associations of allochthonous sediments. There are competing hypotheses on confined network maze cave development in the Grand Canyon, with observations supporting both epigenic and hypogenic processes. The nature of expansive phreatic passages, cupolas, solution pockets, and lack of vadose features suggest that the BCS was formed from hypogene speleogenesis. X-ray diffraction identifies vast amounts of speleogenetic gypsum deposits, other varieties of sulfates, and corrosion residue, all of which indicate that the dominant hypogene fluid was sulfuric acid-bearing. 𝛿34S values for gypsum flowers from Double Bopper range from 0.6‰ to -13.9‰ (VCDT), with an average of -6.4‰ (n=25). Sulfur isotopes identify three plausible sources of sulfuric acid in the Grand Canyon region including magmatic sulfur, sulfide oxidation from pyrite, and sulfide oxidation from hydrocarbons. Identifying sulfuric acid speleogenesis (SAS) and the ancient hydrogeologic conditions necessary for network maze cave development holds implications for better understanding the evolution and dynamics of the modern aquifer. Present-day hypogene speleogenesis may contribute to significant previously unrecognized mega-conduit storage capacity in the modern Redwall-Muav aquifer.
The increased knowledge on caves, a better understanding of the physical and chemical processes at work in the underground, and the evolution of hydrology as a science brought about a better understanding of speleogenesis in the last 60 years. This chapter mainly deals with the formation of caves in carbonate rocks. It focuses on the geological properties of carbonate rocks (limestones and dolostones) that host most of the cave systems in the world. The hydrogeological factors can be influenced by the topographic characteristics of the area, climate, and the long‐term landscape evolution occurring over the period between initial enlargement of fissures and the final configuration of the cave. Condensation of water vapor from moist air masses onto colder cave walls, or at the contact between warm humid and cold air masses in the cave atmosphere are responsible for the formation of water droplets and films, which are initially very low in dissolved load.
Dissolution is the main process responsible for the development of the specific geomorphic and hydrologic features found in karst terrains. Most karst rocks are essentially monomineralic, hence the problem of their karstification mainly concerns the dissolution of the dominant constituent mineral. This chapter is focused on the dissolution of carbonate rocks in shallow meteoric environments, where CO 2 from the atmosphere and especially from the soil is the main source of water acidity. Dissolution of carbonate rocks and cave development by sulfuric acid are known since the first half of the twentieth century. The chapter describes chemical and physical processes that affect the solubility and saturation state of soluble minerals in water, and therefore influence their dissolution and precipitation. There is a large gap between dissolution processes investigated in laboratory experiments and numerical models, and those that operate in real karst systems over geological time scales, as the comparison of some results with field data reveals.
The karst underground river ecosystem of Yucatan peninsula is composed of cave systems and sinkholes. The microbial diversity of water from this underground river has been studied, but, structure of the microbial community in its cave sediments remained largely unknown. Here we describe how the microbial community structure of these sediments changes due to different environmental conditions found in sediment zones along the caves of a coastal and an inland sinkhole. We found that dominant microbial groups varied according to the type of sinkhole (Coastal: Chloroflexi and Crenarchaeota; inland: Methylomirabilota and Acidobacteriota) and that the community structures differed both among sinkhole types, and within the sediment zones that were studied. These microorganisms are associated with different types of metabolism, and differed from a microbial community dominated by sulfate reducers at the coastal sinkhole, to one dominated by methylotrophs at the inland sinkhole, suggesting there are biogeochemical processes in the coastal and inland sinkholes that lead to changes in the microbial composition of the underground river ecosystem’s sediments. Our results suggest sediments from unexplored sinkhole caves are unique environmental niches with distinct microbial assemblages that putatively play an important role in the biogeochemical cycles of these ecosystems.
The 2021 GSA Northeastern, Southeastern, joint North-Central/South-Central, and Cordilleran Section Meet-ings were held virtually in spring 2021 during continued restrictions on travel and large gatherings due to COVID-19. Eleven groups put together field guides, taking participants on treks to states from Connecticut to Nevada in the United States, to Mexico, and to Italy, and covering topics as varied as bedrock geologic map-ping, geochemistry, paleodrainage, barrier islands, karst, spring systems, a southern Appalachian transect, Ordo-vician and Mississippian stratigraphy, high-energy events, Cretaceous arc granites and dextral shear zones, and Mesoproterozoic igneous rocks. This volume serves as a valuable resource for those wishing to discover, learn more about, and travel through these geologically fascinating areas.
Sulphidic cave ecosystems are remarkable evolutionary hotspots that have witnessed adaptive radiation of their fauna represented by extremophile species having particular traits. Ostracods, a very old group of crustaceans, exhibit specific morphological and ecophysiological features that enable them to thrive in groundwater sulphidic environments. Herein, we report a peculiar new ostracod species Pseudocandona movilaensis sp. nov. thriving in the chemoautotrophic sulphidic groundwater ecosystem of Movile Cave (Romania). The new species displays a set of homoplastic features specific for unrelated stygobitic species, for e.g., triangular carapace in lateral view with reduced postero–dorsal part and simplification of limb chaetotaxy (i.e., loss of some claws and reduction of secondary male sex characteristics), driven by a convergent or parallel evolution during or after colonization of the groundwater realm. P. movilaensis sp. nov. thrives exclusively in sulphidic meso-thermal waters (21°C) with high concentrations of sulphides, methane, and ammonium. Based on the geometric morphometrics-based study of the carapace shape and molecular phylogenetic analyses based on the COI marker (mtDNA), we discuss the phylogenetic relationship and evolutionary implication for the new species to thrive in groundwater sulphidic groundwater environments.
Nutritional interactions among invertebrates at 3 vent sites on the Galapagos Rift are examined through the use of stable carbon and nitrogen isotopes. A large number of individuals of several vent species were analyzed and this provides previously unavailable insights into the variability within various groups. Stable nitrogen isotope contents (delta(15)N values) of vent invertebrates are below 11 parts per thousand. This is significantly lower than all non-vent deep-sea fauna examined to date, which makes delta(15)N values an excellent tool in identifying vent-dependent fauna. However, the large range in delta(15)N values among both primary producers and many Vent consumer species renders it of Limited use in determining either the trophic level of most individual species or the total number of trophic levels in vent communities. Stable carbon isotope content (delta(13)C values) in vent invertebrates from the Galapagos Rift range from -9 to -37 parts per thousand, which encompasses the range of non-vent deep-sea fauna (-17 to -22 parts per thousand) and extends significantly beyond it. Many vent invertebrates have delta(13)C values which overlap that of ambient deep-sea fauna, however in several cases the extreme values can be linked to specific endogenous vent sources. When both nitrogen and carbon are taken together, robust trophic links can be established between several vent taxa, and considerable nutritional plasticity in other taxa becomes evident.
Movile Cave, recently discovered in southern Romania, contains sulfide‐rich thermal waters in submerged passages, as well as isolated air pockets. The water surfaces within the air pockets are covered by substantial microbial biofilms, while the air bells contain an abundant and diverse community of terrestrial and aquatic animal species. Based on the results of dehydrogenase activity, fecal streptococci counts, and stable carbon isotope ratios, we propose that the cave community is biologically isolated and receives little, if any, organic carbon inputs from the surface environment. Several sulfide‐oxidizing chemoautotrophic bacteria were isolated from the cave waters. One putative Thiosphaera sp. strain, LV‐43, was further characterized. The presence and high level activity of RuBisCO was clearly demonstrated in this strain.
Mention the word 'isotopes' and most people will think of short-lived radioactive isotopes. Yet far more abundant than the radioactive forms and perhaps much more useful for ecological studies are the stable isotopes. Early interest in stable isotope analyses developed in the geological sciences, and their application in environmental biology has developed slowly until recently. Over the past few years, innovative applications of stable isotope analyses to studies of biological processes have been expanding rapidly, and there is every indication that stable isotope approaches will lead to major advances over the next decade in our understanding of physiological processes and fluxes through ecological systems. Studies of plant and animal physiological ecology and food webs will benefit greatly.
Laboratory-grown strains of chemoautotrophic Thiomicrospira sp. strain L-12 and Thiobacillus neapolitanus produced cell carbon that was 24.6 to 25.1 ppt (24.6 to 25.1 mg/g) lower in C isotope abundance than the ambient source of carbon dioxide and bicarbonate. This degree of C isotope depletion was comparable to that found in organic material produced in deep-sea hydrothermal-vent communities.
The submarine hydrothermal activity on and near the Galápagos Rift has been explored with the aid of the deep submersible Alvin. Analyses of water samples from hydrothermal vents reveal that hydrothermal activity provides significant or dominant sources and sinks for several components of seawater; studies of conductive and convective heat transfer suggest that two-thirds of the heat lost from new oceanic lithosphere at the Galápagos Rift in the first million years may be vented from thermal springs, predominantly along the axial ridge within the rift valley. The vent areas are populated by animal communities. They appear to utilize chemosynthesis by sulfur-oxidizing bacteria to derive their entire energy supply from reactions between the seawater and the rocks at high temperatures, rather than photosynthesis.
Les auteurs présentent les résultats des analyses aux rayons X qui attestent, dans la Grotte de la Movile, de la présence d'une association minéralogique constituée de calcite, aragonite, ankérite et quartz. Les auteurs envisagent également les conditions d'une genèse spécifique pour cette grotte particulièrement affectée par des eaux thermominérales sulfureuses.
The thermomineral sulfurous waters at Mangalia in southeastern Dobrogea, Romania, have been known and used as spa facilities for well over 2,000 years (Feru and Capotà 1991). Hydrogeologieal studies performed during the last 60 years (Macovei 1912; Ciocîrdel and Protopopescu-Pache 1955; Moissiu 1968; Feru and Capotà 1991) identified a deep captive sulfurous aquifer located in Barremian-Jurassic limestones, extending 15 km to the North and 50 km to the South of Mangalia. In the Mangalia region, a system of geological faults allows the deep water to ascend toward the surface and mix with the Sarmatian oxygenated waters (Lascu et al. 1993). The biological investigation of the subsurface ecosystems associated with the sulfurous waters at Mangalia commenced in the late eighties, after the discovery of Movile Cave and its unique subterranean chemoautotrophically based ecosystem (Sarbu, 1990).
In 1947, in his classical paper on the thermodynamic properties of isotopic substances, H. C. Urey [1] laid the foundation of modern isotope geochemistry. At the same time, A. O. Nier [2] designed a new mass spectrometer which allowed the measurement of small differences in isotope abundance ratios. A modification in the Nier-type mass spectrometer and a refinement in instrumentation techniques by McKinney
et al. [3] finally initiated stable isotope studies of the type that will be discussed in this review.
Bacterial communities were detected in deep crystalline rock aquifers within the Columbia River Basalt Group (CRB). CRB ground waters contained up to 60 {mu}M dissolved H{sub 2} and autotrophic microorganisms outnumbered heterotrophs. Stable carbon isotope measurements implied that autotrophic methanogenesis dominated this ecosystem and was coupled to the depletion of dissolved inorganic carbon. In laboratory experiments, H{sub 2} a potential energy source for bacteria, was produced by reactions between crushed basalt and anaerobic water. Microcosms containing only crushed basalt and ground water supported microbial growth. These results suggest that the CRB contains a lithoautotrophic microbial ecosystem that is independent of photosynthetic primary production. 38 refs., 4 figs., 3 tabs.
Biologists have been studying the biota of hydrothermal vents for 15 yr. Life is supported initially by bacteria which oxidise hydrogen sulphide released at the vents by chemosynthesis. Some 300 species have been identified living around the vents. Contention over the evolutionary pathway taken by vent communities is strong, and the paper presents hypotheses for their primitivity. One suggests an ancient origin for vent life-forms, and that they have developed little with time, given support by finds of creatures with "fossil' characteristics. Alternatively, it is possible that the vent habitat was invaded recently and that evolution proceeded rapidly. The isolation of the vent regions may have prevented their subjection to mass extinctions and hence removed the impetus for significant evolution. -R.Gower
The δ13C and δ15N of the soft tissues of two Calyptogena phaseoliformis specimens sampled from the floor of Ascension Fan-Valley, off the Monterey Peninsula, California, ranged from -35.0 to -33.6% and -6.2 to -0.6%, respectively. The carbon isotope abundances are similar to those of other Calyptogena known or believed to be nutritionally dependent on sulfide-based chemoautotrophic symbionts. Calyptogena δ15N values and their geographic variation indicate an unusual source of dietary nitrogen for these animals. The δ13C and δ18O values for Calyptogena shells from the Ascension and Monterey Fan-Valleys approximate those reported for Calyptogena from three other locations in the Pacific and appear to reflect shell formation from constituents within ambient deep-ocean water.
Nepa anophthalma Decu, Gruia, Keffer & Sarbu, n. sp., a blind, caveadapted waterscorpion from a mesothermal sulfurous cave near Mangalia, Romania, is described and illustrated. Structure of the genitalia places this new taxon in the cinerea species group. This new species is the first aquatic, stygobiotic heteropteran discovered.
Communities of abundant organisms, similar to those which surround the hydrothermal vents of the East Pacific Rise (EPR), occur in 3,266 m of water in the Gulf of Mexico. They were discovered with the Alvin at 26° 02′ N, 84° 55′ W in an area where pore waters seep from the Cretaceous limestones of the adjacent platform at the contact between the Florida Escarpment and the Holocene hemipelagic sediments of the Abyssal Gulf. Because of the unusual abundance of organisms in this deep-sea environment, and the taxonomic similarities to the chemosynthetically based communities which surround the hydrothermal vents on the EPR, the seep communities are suspected to exist on a local non-photosynthetic food source1. We have now measured isotope ratios in tissues of organisms which surround these saline seeps to determine the origin of the local food chain. The tissues have extremely negative δ
13C (−42 to −77‰, PDB) and δ
15N (−2.72 to −9.34‰ air) values. Such highly fractionated carbon and nitrogen isotopes are unknown in food chains based on photosynthesis, suggesting that these communities are chemosynthetic. The cause of this extreme fractionation is attributed either to assimilation from local supplies of isotopically depleted methane and ammonium or to biochemical fractionation. The tissues of these animals contain significant amounts of 14C (~60% modern), so the source of this fractionated carbon is not predominantly fossil methane.
This paper presents data on the isotopic compositions of individual carbohydrates which have been isolated from both living organisms and from a depositional environment. Through recent developments in chromatography, isolation of monosaccharides quantity for isotopic analysis is possible. Carbohydrates isolated from marine and terrestrial plants and animals have carbon isotopic compositions which clearly indicate biosynthetic processes which are recognized to be associated with isotopic fractionations during the incorporation and metabolism of carbon. Stable nitrogen isotopic compositions of N-acetyl-d-glucosamine, isolated from a variety of chitins, indicate a common fractionation of nitrogen which may be associated with transamination. This fractionation is estimated to be about –9%0 relative to the whole organism. Through the isotopic label, the source or history of an organic material may be traced through diagenesis. Further, production of non-indigenous carbohydrates, which may be associated with fungal or bacterial action, can be identified through depleted isotopic compositions. Xylose, for example, isolated from a peat, was approximately 8% more depleted in carbon than the whole peat or mannose isolated from the same peat. The isotopic distributions of carbohydrates isolated from living organisms are essential for the interpretation of those characterized in natural mixtures from an environment.
Explains isotope terminology and fractionation, and summarises isotopic distributions in the C, N and S biogeochemical cycles. Five case studies (delta 15N measures of N2 fixation; the global carbon cycle and the CO2 problem; sulphur and acid deposition; use in archaeology; and detrital organic matter in saltmarshes) show how stable isotope measurements can provide crucial information for ecosystem analysis.-P.J.Jarvis
STABLE ISOTOPE EVIDENCE FOR CHEMOSYNTHESIS IN AN ABYSSAL SEEP COMMUNITY
- C K Paull
- PAULL C.K.
CARBON ISOTOPE FRACTIONATION IN PLANTS
- H Oleary M
- Carbon
- Fractionation In
- Plants
- OLEARY M.H.
FRACTIONATION OF STABLE CARBON ISOTOPES DURING CHEMOAUTOTROPHIC GROWTH OF SULFUR-OXIDIZING BACTERIA
- G Ruby E
- Fractionation
- Stable
- Isotopes
- Chemoautotrophic
- Of Sulfur-Oxidizing
- Bacteria
- RUBY E.G.