Anammox bacterial populations in deep marine hypersaline gradient systems

To extend the knowledge of anaerobic ammonium oxidation (anammox) habitats, bacterial communities were examined in two hypersaline sulphidic basins in Eastern Mediterranean Sea. The 2 m thick seawater–brine haloclines of the deep anoxic hypersaline basins Bannock and L’Atalante were sampled in intervals of 10 cm with increasing salinity. ¹⁵N isotope pairing incubation experiments showed the production of ²⁹N₂ and ³⁰N₂ gases in the chemoclines, ranging from 6.0 to 9.2 % salinity of the L’Atalante basin. Potential anammox rates ranged from 2.52 to 49.65 nmol N₂ L^?1 day^?1 while denitrification was a major N₂ production pathway, accounting for more than 85.5 % of total N₂ production. Anammox-related 16S rRNA genes were detected along the L’Atalante and Bannock haloclines up to 24 % salinity, and the amplification of the hydrazine synthase genes (hzsA) further confirmed the presence of anammox bacteria in Bannock. Fluorescence in situ hybridisation and sequence analysis of 16S rRNA genes identified representatives of the marine anammox genus ‘Candidatus Scalindua’ and putatively new operational taxonomic units closely affiliated to sequences retrieved in marine environments that have documented anammox activity. ‘Scalindua brodae’ like sequences constituted up to 84.4 % of the sequences retrieved from Bannock. The anammox community in L’Atalante was different than in Bannock and was stratified according to salinity increase. This study putatively extends anammox bacterial habitats to extremely saline sulphidic ecosystems.     

Sulfate-reducing prokaryotic communities in two deep hypersaline anoxic basins in the Eastern Mediterranean deep sea

In the Eastern Mediterranean Sea, deep hypersaline anoxic basins (DHABs) and deep-sea sediment contain anoxic environments where sulfate reduction is an important microbial metabolic process. The objective of this study was to characterize the sulfate-reducing community in the brine and interface of the DHABs L'Atalante and Urania based on a phylogenetic analysis of the dissimilatory sulfite reductase gene (dsrA). Results demonstrated that the sulfate-reducing community was diverse, except for the sulfidogenic brine of the Urania basin. The similarity of the dsrA sequences between different environments was very low demonstrating that each environment had a unique sulfate-reducing community. Sequences had 67.6-93.3% similarity to dsrA sequences from GenBank database and were mostly related to the delta-proteobacteria. Each environment was dominated by a different family within the delta-proteobacteria except for the Urania interface, which was dominated by sequences related to the Gram-positive Peptococcaceae. We conclude that sulfate-reducing communities inhabiting the L'Atalante and Urania basins are highly diverse with low similarities to each other and contain a sulfate-reducing species composition that is very different from sulfate-reducing species compositions in previously studied ecosystems.     

A fundamental difference between macrobiota and microbial eukaryotes: protistan plankton has a species maximum in the freshwater-marine transition zone of the Baltic Sea

Remane's Artenminimum at the horohalinicum is a fundamental concept in ecology to describe and explain the distribution of organisms along salinity gradients. However, a recent metadata analysis challenged this concept for protists, proposing a species maximum in brackish waters. Due to data bias, this literature-based investigation was highly discussed. Reliable data verifying or rejecting the species minimum for protists in brackish waters were critically lacking. Here, we sampled a pronounced salinity gradient along a west–east transect in the Baltic Sea and analysed protistan plankton communities using high-throughput eDNA metabarcoding. A strong salinity barrier at the upper limit of the horohalinicum and 10 psu appeared to select for significant shifts in protistan community structures, with dinoflagellates being dominant at lower salinities, and dictyochophytes and diatoms being keyplayers at higher salinities. Also in vertical water column gradients in deeper basins (Kiel Bight, Arkona and Bornholm Basin) appeared salinity as significant environmental determinant influencing alpha- and beta-diversity patterns. Importantly, alpha-diversity indices revealed species maxima in brackish waters, that is, indeed contrasting Remane's Artenminimum concept. Statistical analyses confirmed salinity as the major driving force for protistan community structuring with high significance. This suggests that macrobiota and microbial eukaryotes follow fundamentally different rules regarding diversity patterns in the transition zone from freshwater to marine waters.     

Microbial eukaryote life in the new hypersaline deep-sea basin Thetis

Only recently, a novel anoxic hypersaline (thalassic) basin in the eastern Mediterranean was discovered at a depth of 3,258 m. The halite-saturated brine of this polyextreme basin revealed one of the highest salt concentrations ever reported for such an environment (salinity of 348‰). Using a eukaryote-specific probe and fluorescence in situ hybridization, we counted 0.6 × 10⁴ protists per liter of anoxic brine. SSU rRNA sequence analyses, based on amplification of environmental cDNA identified fungi as the most diverse taxonomic group of eukaryotes in the brine, making deep-sea brines sources of unknown fungal diversity and hotspots for the discovery of novel metabolic pathways and for secondary metabolites. The second most diverse phylotypes are ciliates and stramenopiles (each 20%). The occurrence of closely related ciliate sequences exclusively in other Mediterranean brine basins suggests specific adaptations of the respective organisms to such habitats. Betadiversity-analyses confirm that microeukaryote communities in the brine and the interface are notably different. Several distinct morphotypes in brine samples suggest that the rRNA sequences detected in Thetis brine can be linked to indigenous polyextremophile protists. This contradicts previous assumptions that such extremely high salt concentrations are anathema to eukaryotic life. The upper salinity limits for eukaryotic life remain unidentified.     

Organic Matter Preservation and Microbial Community Accumulations in Deep-Hypersaline Anoxic Basins

The Eastern Mediterranean Sea hosts several deep hypersaline anoxic basins (DHABs) such as the Bannock, L'Atalante, Discovery, and Urania which, due to strong salinity gradients, have a limited exchange with the overlying seawater. In the present study, a series of environmental variables associated with the origin and quality of organic matter were thoroughly investigated in an attempt to understand the function of these unique ecosystems. The redox potential of sediments collected from the brines as well as from reference sites varied from ?136 to 543 mV and salinity varied from 38 to 380 psu. Principal component analysis of chemical characteristics, including salinity, redox potential, organic carbon and nitrogen content, and C/N ratio grouped the sediments into two major clusters according to their redox state. Aliphatic hydrocarbon analysis revealed that the organic matter in the DHABs was predominantly of terrestrial origin but there was also evidence for petroleum inputs and for organic matter of phototrophic origin. Phospholipid linked fatty acids (PLFA) which were employed to assess the composition of microbial communities were found in greater abundance in stations situated inside the anoxic basins providing also strong evidence for the presence of methanotrophs and sulfate reducers. These results may represent an enhanced preservation of organic matter and an accumulation of microorganisms in these extreme environments. Heterogeneity in microbial community fatty acid profiles was documented between the anoxic sediments and the oxic and suboxic stations. However there were no significant correlations between PLFA and organic matter parameters. Redox conditions appear to influence microbial community composition, highlighting the role of the redox state as a regulator of organic matter preservation and microbial community accumulations in these ancient hypersaline anoxic lakes.     

Microbial eukaryotes in the hypersaline anoxic L'Atalante deep-sea basin

The frontiers of eukaryote life in nature are still unidentified. In this study, we analysed protistan communities in the hypersaline (up to 365 g l⁻¹ NaCl) anoxic L'Atalante deep-sea basin located in the eastern Mediterranean Sea. Targeting 18S ribosomal RNA retrieved from the basin's lower halocline (3501 m depth) we detected 279 protistan sequences that grouped into 42 unique phylotypes (99% sequence similarity). Statistical analyses revealed that these phylotypes account only for a proportion of the protists inhabiting this harsh environment with as much as 50% missed by this survey. Most phylotypes were affiliated with ciliates (45%), dinoflagellates (21%), choanoflagelates (10%) and uncultured marine alveolates (6%). Sequences from other taxonomic groups like stramenopiles, Polycystinea , Acantharea and Euglenozoa , all of which are typically found in non-hypersaline deep-sea systems, are either missing or very rare in our cDNA clone library. Although many DHAB sequences fell within previously identified environmental clades, a large number branched relatively deeply. Phylotype richness, community membership and community structure differ significantly from a deep seawater reference community (3499 m depth). Also, the protistan community in the L'Atalante basin is distinctively different from any previously described hypersaline community. In conclusion, we hypothesize that extreme environments may exert a high selection pressure possibly resulting in the evolution of an exceptional and distinctive assemblage of protists. The deep hypersaline anoxic basins in the Mediterranean Sea provide an ideal platform to test for this hypothesis and are promising targets for the discovery of undescribed protists with unknown physiological capabilities.     

Algal and bacterial processes in platelet ice during late austral summer

 The biota inhabiting layers of platelet ice were investigated in the Weddell Sea during late austral summer. Due to meltwater release, the salinity of the interstitial water between platelets was reduced. Algae and bacteria accumulated within this ice environment attaining concentrations of up to 500 μg in total pigments (chlorophyll a plus phaeopigments) and 2 mg in bacterial biomass per liter. Pennate diatoms of the genus Fragilariopsis were most common in the platelet layer, while ice-free water was dominated by autotrophic nanoflagellates. Protozoa contributed only 5% or less to the total protistan (microalgae plus protozoa) cell concentration in the ice, compared to about 10% in open water, thus suggesting a low grazing pressure within the platelet habitat. The bulk of bacterial biomass occurred within the dense assemblages of pennate diatoms that grew attached to the ice platelets. Algal and bacterial concentrations in the interstitial water between platelets were much lower. Measurements of bacterial growth showed that substantial heterotrophic potential can be established within assemblages inhabiting late summer platelet ice. Small-scale analyses of bacterial activity patterns revealed that those bacteria that were closely associated with ice and/or algae showed considerably less biomass-specific substrate uptake than cells that occurred freely suspended in the interstitial water, indicating that their physiological state differed. Received: 21 October 1995/Accepted: 27 January 1996     

The effect of salinity on the dominance-diversity relations of experimental coastal macrophyte communities

Salinity and water regime have previously been recognised as the main environmental factors controlling the abundance of coastal submerged macrophytes in temporarily-flooded marshes in the Camargue. The effects of these environmental variables, which are considered interrelated, are tested experimentally by subjecting experimental macrophyte communities from six temporarily flooded marshes to different levels of salinity (from 0 to 6 g/1 Cl^?). Communities subjected to high salinity levels (4 and 6 g/1 Cl~) showed a decrease in species richness and in biomass of all species involved. The species that most frequently dominate these communities, Chara áspera and Zannichellia pedunculata, are tolerant of salt and dominate over the entire salinity range. Three species groups can be distinguished based on the distribution of their biomass and centre of gravity of distribution over the salinity range: (1) non-salt-tolerant species, ‘glyco-phytes’, (2) moderately salt-tolerant species and (3) very tolerant species (‘halophytes’). A species ordination based on the experiments appeared to give results close to those previously obtained from field data.     

Structure and formation properties of the haloalkaliphilic community of Lake Khilganta

The structural features of a cyanobacterial mat from Lake Khilganta (Southeastern Transbaikal Region) developing at different values of salinity and pH were determined based on our long-term investigation of the natural community, as well as results obtained during experimentation with its laboratory analogue. At water mineralization of 40–50 g/l, Microcoleus chthonoplastes and Phormidium molle play a key role in the formation of the cyanobacterial mat. As water mineralization increases, the diversity of cyanobacteria in the natural mat increases as well, reaches its maximum at 80 g/l NaCl, and decreases at 100 g/l. In the laboratory community, Nodularia sp. prevailed. It was able to form matlike structures within a broad pH range and at a salinity of up to 50 g/l NaCl. As the water mineralization level increased up to 100 g/l or higher, a replacement of the dominant complexes occurred both in the laboratory and natural communities: cyanobacterial species were substituted with green algae.     

Seed Banks in Arid Wetlands with Contrasting Flooding,Salinity and Turbidity Regimes

Aquatic plant communities in arid zone wetlands underpin diverse fauna populations and ecosystem functions yet are relatively poorly known. Erratic flooding, drying, salinity and turbidity regimes contribute to habitat complexity, creating high spatial and temporal variability that supports high biodiversity. We compared seed bank density, species richness and community composition of aquatic plants (submergent, floating-leaved and emergent) among nine Australian arid zone wetlands. Germinable seed banks from wetlands within the Paroo and Bulloo River catchments were examined at nested scales (site, wetland, wetland type) using natural flooding and salinity regimes as factors with nondormant seed density and species richness as response variables. Salinity explained most of the variance in seed density (95%) and species richness (68%), with flooding accounting for 5% of variance in seed density and 32% in species richness. Salinity-flooding interactions were significant but explained only a trivial portion of the variance (<1%). Mean seed densities in wetlands ranged from 40 to 18,760 m⁻² and were highest in wetlands with intermediate levels of salinity and flooding. Variability of densities was high (CVs 0.61–2.66), particularly in saline temporary and fresh permanent wetlands. Below salinities of c. 30 g l⁻¹ TDS, seed density was negatively correlated to turbidity and connectivity. Total species richness of wetlands (6–27) was negatively correlated to salinity, pH and riverine connectivity. A total of 40 species germinated, comprising submergent (15 species), floating-leaved or amphibious (17 species), emergent (6 species) and terrestrial (6 species) groups. Charophytes were particularly important with 10 species (five Chara spp., four Nitella spp. and Lamprothamnium macropogon), accounting for 68% of total abundance. Saline temporary wetlands were dominated by Ruppia tuberosa, Lamprothamnium macropogon and Lepilaena preissii. Variable flooding and drying regimes profoundly altered water quality including salinity and turbidity, producing distinctive aquatic plant communities as reflected by their seed banks. This reinforces the importance of hydrology in shaping aquatic biological communities in arid systems.     

Diversity of ribulose-1,5-bisphosphate carboxylase/oxygenase large-subunit genes in the MgCl2-dominated deep hypersaline anoxic basin discovery

Partial sequences of the form I (cbbL) and form II (cbbM) of the ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) large subunit genes were obtained from the brine and interface of the MgCl2-dominated deep hypersaline anoxic basin Discovery. CbbL and cbbM genes were found in both brine and interface of the Discovery Basin but were absent in the overlying seawater. The diversity of both genes in the brine and interface was low, which might caused by the extreme saline conditions in Discovery of approximately 5 M MgCl2. None of the retrieved sequences were closely related to sequences deposited in the GenBank database. A phylogenetic analysis demonstrated that the cbbL sequences were affiliated with a Thiobacillus sp. or with one of the RuBisCO genes from Hydrogenovibrio marinus. The cbbM sequences clustered with thiobacilli or formed a new group with no close relatives. The results implicate that bacteria with the potential for carbon dioxide fixation and chemoautotrophy are present in the Discovery Basin. This is the first report demonstrating that RuBisCO genes are present under hypersaline conditions of 5 M MgCl2.     

Effect of environmental factors on the structure and functioning of biocoenoses of hyperhaline water reservoirs in the south of Western Siberia

Based on the perennial monitoring (1995–2010) of hyperhaline water reservoirs in the south of Western Siberia with salinity from 28 to 417 g/L, four seasons in the development of biocoenoses are distinguished and integrated results of studies of the effect of environmental factors on the species composition and productivity of biocoenoses are presented. The decisive role of temperature on the seasonal cycles of hydrocole development and salinity on the productivity and composition of biocoenoses are revealed. The highest species diversity is observed in summer at temperatures above 12°C; the fauna is represented only by artemia cysts at negative temperatures. With an increase of salinity from 28 to 100 g/L the number of species declines. For salinity 100 g/L the fauna is represented only by brine shrimp Artemia. Hyperhaline biocoenoses are the most productive at salinity 150–180 g/L.     

Changes in community structure of active protistan assemblages from the lower Pearl River to coastal Waters of the South China Sea

Protists make up an important component of aquatic ecosystems, playing crucial roles in biogeochemical processes on local and global scales. To reveal the changes of diversity and community structure of protists along the salinity gradients, community compositions of active protistan assemblages were characterized along a transect from the lower Pearl River estuary to the open waters of the South China Sea (SCS), using high-throughput sequencing of the hyper-variable V9 regions of 18S rRNA. This study showed that the alpha diversity of protists, both in the freshwater and in the coastal SCS stations was higher than that in the estuary. The protist community structure also changed along the salinity gradient. The relative sequence abundance of Stramenopiles was highest at stations with lower salinity and decreased with the increasing of salinity. By contrast, the contributions of Alveolata, Hacrobia and Rhizaria to the protistan communities generally increased with the increasing of salinity. The composition of the active protistan community was strongly correlated with salinity, indicating that salinity was the dominant factor among measured environmental parameters affecting protistan community composition and structure.     

The effect of salinity on the reproduction of coastal submerged macrophytes in experimental communities

The effects of salinity on the reproduction of coastal submerged macrophyte species were studied on samples of communities from six seasonal marshes in two outdoor experiments performed in autumn and in spring. The submerged macrophyte communities were submitted to five different salinity levels (0, 1, 2, 4 and 6 g/1 Cl^?1). In a companion paper (Grillas, van Wijck & Bonis 1993) three groups of species were distinguished on the basis of their biomass production over the salinity range 0 to 6 g/1 Cl^?1: (1) glycophytes (non-salt-tolerant species), (2) salt-tolerant species and (3) halo-phytes. This part of the study describes the impact of salinity on the reproduction of the individual species during the two experiments. The species differ in their capacity to reproduce in the autumn; only Zannichelliapedunculata and Tolypella hispánica were able to produce fruits in that season. For all species reproduction was greater in spring and strongly correlated with biomass, except for Chara canescens. Differences in reproductive effort over the salinity range amplified the halophytic nature of Ruppia marítima and Chara canescens and the intolerance of Callitriche truncata and Chara contraria. For the other species, reproductive effort did not differ significantly over the salinity range. Regarding the effect of salinity on biomass and reproductive effort of individual species, there were large differences in the total weight of propagules produced at the community level and in the relative contribution of individual species. The resulting quantitative changes in the species composition of the seed bank could affect the structure of the communities by their effects on the establishment and survival of species populations.     

Tempo and mode of the multiple origins of salinity tolerance in a water beetle lineage

Salinity is one of the most important drivers of the distribution, abundance and diversity of organisms. Previous studies on the evolution of saline tolerance have been mainly centred on marine and terrestrial organisms, while lineages inhabiting inland waters remain largely unexplored. This is despite the fact that these systems include a much broader range of salinities, going from freshwater to more than six times the salinity of the sea (i.e. >200 g/L). Here, we study the pattern and timing of the evolution of the tolerance to salinity in an inland aquatic lineage of water beetles (Enochrus species of the subgenus Lumetus, family Hydrophilidae), with the general aim of understanding the mechanisms by which it was achieved. Using a time‐calibrated phylogeny built from five mitochondrial and two nuclear genes and information about the salinity tolerance and geographical distribution of the species, we found that salinity tolerance appeared multiple times associated with periods of global aridification. We found evidence of some accelerated transitions from freshwater directly to high salinities, as reconstructed with extant lineages. This, together with the strong positive correlation found between salinity tolerance and aridity of the habitats in which species are found, suggests that tolerance to salinity may be based on a co‐opted mechanism developed originally for drought resistance.     

Composition and succession of dinoflagellates and chrysophytes in the upper fast ice of Davis Station, East Antarctica

Little is known of the wider Antarctic distribution of the upper fast ice community now comprehensively described from McMurdo Sound. We determined the fast ice protist community at Davis Station, East Antarctica and compared it with that of McMurdo Sound. As at McMurdo Sound, Davis fast ice is characterised by extreme and transitory salinities (96–2.5 psu) and temperatures (−4.5 to −0.1°C) during the spring/summer transition. Both communities are dominated by Polarella glacialis (an autotrophic dinoflagellate), chrysophytes and their life cycle stages. Furthermore, the physical parameters of brine temperature and salinity at which these successions occurred approximated those of McMurdo Sound. The high degree of similarity between the communities from the geographically disparate locations indicates that this community type has a circum-Antarctic distribution. Confirming the areal extent and seasonality of this community type will assist in future predictions of sea ice productivity.     

Hydrothermally generated aromatic compounds are consumed by bacteria colonizing in Atlantis II Deep of the Red Sea

Hydrothermal ecosystems have a wide distribution on Earth and many can be found in the basin of the Red Sea. Production of aromatic compounds occurs in a temperature window of ∼60–150 °C by utilizing organic debris. In the past 50 years, the temperature of the Atlantis II Deep brine pool in the Red Sea has increased from 56 to 68 °C, whereas the temperature at the nearby Discovery Deep brine pool has remained relatively stable at about 44 °C. In this report, we confirmed the presence of aromatic compounds in the Atlantis II brine pool as expected. The presence of the aromatic compounds might have disturbed the microbes in the Atlantis II. To show shifted microbial communities and their metabolisms, we sequenced the metagenomes of the microbes from both brine pools. Classification based on metareads and the 16S rRNA gene sequences from clones showed a strong divergence of dominant bacterial species between the pools. Bacteria capable of aromatic degradation were present in the Atlantis II brine pool. A comparison of the metabolic pathways showed that several aromatic degradation pathways were significantly enriched in the Atlantis II brine pool, suggesting the presence of aromatic compounds. Pathways utilizing metabolites derived from aromatic degradation were also significantly affected. In the Discovery brine pool, the most abundant genes from the microbes were related to sugar metabolism pathways and DNA synthesis and repair, suggesting a different strategy for the utilization of carbon and energy sources between the Discovery brine pool and the Atlantis II brine pool.     

The occurrence of <Emphasis Type="Italic">Eualus gaimardii gibba</Emphasis> Krøyer 1841 (Crustacea,Decapoda) in the sympagic habitat: an example of bentho-sympagic coupling

We report the first records of a decapod species (Eualus gaimardii gibba) in association with Arctic multiyear sea-ice. Seven specimens inhabiting brine channels in the ice were collected during two field campaigns (September 2003, March/April 2004) by diver operated suction samplers. Salinity tolerance experiments (14–50 PSU) suggest that the species is stenohaline. Gut examinations revealed remains of the sympagic amphipod Gammarus wilkitzkii. All collected individuals were found in ice above relatively shallow waters (60–100 m depth) suggesting bentho-sympagic coupling.     

Morphological study of Cyclotella choctawhatcheeana Prasad (Stephanodiscaceae) from a saline Mexican lake

Background

Extracellular dissolved DNA has been demonstrated to be present in many terrestrial and aquatic environments, actively secreted, or released by decaying cells. Free DNA has the genetic potential to be acquired by living competent cells by horizontal gene transfer mediated by natural transformation. The aim of this work is to study the persistence of extracellular DNA and its biological transforming activity in extreme environments like the deep hypersaline anoxic lakes of the Mediterranean Sea. The brine lakes are separated from the upper seawater by a steep chemocline inhabited by stratified prokaryotic networks, where cells sinking through the depth profile encounter increasing salinity values and osmotic stress.

Results

Seven strains belonging to different taxonomic groups isolated from the seawater-brine interface of four hypersaline lakes were grown at medium salinity and then incubated in the brines. The osmotic stress induced the death of all the inoculated cells in variable time periods, between 2 hours and 144 days, depending on the type of brine rather than the taxonomic group of the strains, i.e. Bacillaceae or gamma-proteobacteria. The Discovery lake confirmed to be the most aggressive environment toward living cells. In all the brines and in deep seawater dissolved plasmid DNA was substantially preserved for a period of 32 days in axenic conditions. L'Atalante and Bannock brines induced a decrease of the supercoiled form up to 70 and 40% respectively; in the other brines only minor changes in plasmid conformation were observed. Plasmid DNA after incubation in the brines maintained the capacity to transform naturally competent cells of Acinetobacter baylii strain BD413.

Conclusion

Free dissolved DNA is likely to be released by the lysis of cells induced by osmotic stress in the deep hypersaline anoxic lakes. Naked DNA was demonstrated to be preserved and biologically active in these extreme environments, and hence could constitute a genetic reservoir of traits acquirable by horizontal gene transfer.     

The microbial plankton of Lake Fryxell,southern Victoria Land,Antarctica during the summers of 1992 and 1994

 Samples collected from Lake Fryxell, southern Victoria Land, Antarctica in January 1992 and 1994 were analysed for the abundance of bacterioplankton and the diversity and abundance of protistan plankton. At the times of sampling, 14 ciliate species and 10 species of autotrophic flagellate were recorded. The samples contained two species of rotifer (Philodina spp.), which formed the first record of planktonic metazoans in the Dry Valley lakes of this region of Antarctica. Bacterial concentrations ranged between 1.0 and 3.8×10⁸ l^-1 in the upper oxic waters increasing to 20×10⁸ l^-1 in the anoxic waters. Heterotrophic flagellates decreased in abundance down the oxygenated water column, disappearing completely at 9 m, and ranged between 0.28 and 7.39×10⁵ l^-1 in abundance. Autotrophic flagellates were much more abundant exhibiting a number of distinct peaks down the water column (1.89–25.3×10⁸ l^-1). The ciliated protozoa were very abundant (up to 7720 l^-1) in relation to flagellate and bacterial numbers, typical of oligotrophic lakes world-wide. The distribution of the protistan plankton showed marked zonation, probably in response to the differing salinity and temperature gradients in the water column. Possible trophic interactions are discussed and comparisons with other continental Antarctic lakes made. Received: 29 November 1995/Accepted: 18 February 1996