Microbial diversity under extreme euxinia: Mahoney Lake, Canada

Mahoney Lake, British Columbia, Canada, is a stratified, 15-m deep saline lake with a euxinic (anoxic, sulfidic) hypolimnion. A dense plate of phototrophic purple sulfur bacteria is found at the chemocline, but to date the rest of the Mahoney Lake microbial ecosystem has been underexamined. In particular, the microbial community that resides in the aphotic hypolimnion and/or in the lake sediments is unknown, and it is unclear whether the sulfate reducers that supply sulfide for phototrophy live only within, or also below, the plate. Here we profiled distributions of 16S rRNA genes using gene clone libraries and PhyloChip microarrays. Both approaches suggest that microbial diversity is greatest in the hypolimnion (8 m) and sediments. Diversity is lowest in the photosynthetic plate (7 m). Shallower depths (5 m, 7 m) are rich in Actinobacteria, Alphaproteobacteria, and Gammaproteobacteria, while deeper depths (8 m, sediments) are rich in Crenarchaeota, Natronoanaerobium, and Verrucomicrobia. The heterogeneous distribution of Deltaproteobacteria and Epsilonproteobacteria between 7 and 8 m is consistent with metabolisms involving sulfur intermediates in the chemocline, but complete sulfate reduction in the hypolimnion. Overall, the results are consistent with the presence of distinct microbial niches and suggest zonation of sulfur cycle processes in this stratified system.     

Carbon isotope fractionation by anoxygenic phototrophic bacteria in euxinic Lake Cadagno

Anoxygenic phototrophic bacteria utilize ancient metabolic pathways to link sulfur and iron metabolism to the reduction of CO₂. In meromictic Lake Cadagno, Switzerland, both purple sulfur (PSB) and green sulfur anoxygenic phototrophic bacteria (GSB) dominate the chemocline community and drive the sulfur cycle. PSB and GSB fix carbon utilizing different enzymatic pathways and these fractionate C‐isotopes to different extents. Here, these differences in C‐isotope fractionation are used to constrain the relative input of various anoxygenic phototrophs to the bulk community C‐isotope signal in the chemocline. We sought to determine whether a distinct isotopic signature of GSB and PSB in the chemocline persists in the settling fraction and in the sediment. To answer these questions, we also sought investigated C‐isotope fractionation in the water column, settling material, and sediment of Lake Cadagno, compared these values to C‐isotope fractionation of isolated anoxygenic phototroph cultures, and took a mass balance approach to investigate relative contributions to the bulk fractionation signature. We found a large C‐isotope fractionation between dissolved inorganic carbon (DIC) and particulate organic carbon (POC) in the Lake Cadagno chemocline. This large fractionation between the DIC and POC was also found in culture experiments carried out with anoxygenic phototrophic bacteria isolated from the lake. In the Lake Cadagno chemocline, anoxygenic phototrophic bacteria controlled the bulk C‐isotope fractionation, but the influence of GSB and PSB differed with season. Furthermore, the contribution of PSB and GSB to bulk C‐isotope fractionation in the chemocline could be traced in the settling fraction and in the sediment. Taken together with other studies, such as lipid biomarker analyzes and investigations of other stratified lakes, these results offer a firmer understanding of diagenetic influences on bacterial biomass.     

In situ abundance and carbon fixation activity of distinct anoxygenic phototrophs in the stratified seawater lake Rogoznica

Sulphide-driven anoxygenic photosynthesis is an ancient microbial metabolism that contributes significantly to inorganic carbon fixation in stratified, sulphidic water bodies. Methods commonly applied to quantify inorganic carbon fixation by anoxygenic phototrophs, however, cannot resolve the contributions of distinct microbial populations to the overall process. We implemented a straightforward workflow, consisting of radioisotope labelling and flow cytometric cell sorting based on the distinct autofluorescence of bacterial photopigments, to discriminate and quantify contributions of co-occurring anoxygenic phototrophic populations to in situ inorganic carbon fixation in environmental samples. This allowed us to assign 89.3% ± 7.6% of daytime inorganic carbon fixation by anoxygenic phototrophs in Lake Rogoznica (Croatia) to an abundant chemocline-dwelling population of green sulphur bacteria (dominated by Chlorobium phaeobacteroides), whereas the co-occurring purple sulphur bacteria (Halochromatium sp.) contributed only 1.8% ± 1.4%. Furthermore, we obtained two metagenome assembled genomes of green sulphur bacteria and one of a purple sulphur bacterium which provides the first genomic insights into the genus Halochromatium, confirming its high metabolic flexibility and physiological potential for mixo- and heterotrophic growth.     

Anoxygenic phototrophic bacteria from gypsum karst lakes of Lithuania

The spatial distribution and composition of anoxygenic phototrophic bacteria in the enriched bacterial communities from different depths of karst lakes Kirkilai and Ramunelis was studied using spectrophotometric analysis, as well as microbiological and molecular methods. In Lake Kirkilai, the highest bacterial abundance was measured in the metalimnion and near the bottom (up to 10.7 × 10⁶ cell/mL); in Lake Ramunelis it was in the anoxic hypolimnion (up to 22.4 × 10⁶ cell/mL). Increased water mineralization (0.5–1.2 g/L) with the domination of SO ₄ ^2? and Ca²⁺ ions created favorable conditions for the development of sulfate-reducing bacteria; hydrogen sulfide produced as a result of their life activity facilitated the development of sulfur-oxidizing bacteria. The pigment analysis of phototrophic green and purple sulfur bacteria showed the domination of green sulfur bacteria in the enrichment culture. The results of phylogenetic analysis showed that Chlorobium limicola dominated in the enrichment culture for the green sulfur bacteria, whereas purple nonsulfur bacteria of the genus Rhodopseudomonas dominated in the enrichment culture for the purple sulfur bacteria.     

Anoxygenic phototrophic bacteria from microbial communities of Goryachinsk thermal spring (Baikal Area,Russia)

Species composition of anoxygenic phototrophic bacteria in microbial mats of the Goryachinsk thermal spring was investigated along the temperature gradient. The spring belonging to nitrogenous alkaline hydrotherms is located at the shore of Lake Baikal 188 km north-east from Ulan-Ude. The water is of the sulfate-sodium type, contains trace amounts of sulfide, and salinity does not exceed 0.64 g/L, pH 9.5. The temperature at the outlet of the spring may reach 54°C. The cultures of filamentous anoxygenic phototrophic bacteria, nonsulfur and sulfur purple bacteria, and aerobic anoxygenic phototrophic bacteria were identified using the pufLM molecular marker. The fmoA marker was used for identification of green sulfur bacteria. Filamentous cyanobacteria predominated in the mats, with anoxygenic phototrophs comprising a minor component of the phototrophic communities. Thermophilic bacteria Chloroflexus aurantiacus were detected in the samples from both the thermophilic and mesophilic mats. Cultures of nonsulfur purple bacteria similar to Blastochloris sulfoviridis and Rhodomicrobium vannielii were isolated from the mats developed at high (50.6–49.4°C) and low temperatures (45–20°C). Purple sulfur bacteria Allochromatium sp. and Thiocapsa sp., as well as green sulfur bacteria Chlorobium sp., were revealed in low-temperature mats. Truly thermophilic purple and green sulfur bacteria were not found in the spring. Anoxygenic phototrophic bacteria found in the spring were typical of the sulfur communities, for which the sulfur cycle is mandatory. The presence of aerobic bacteriochlorophyll a-containing bacteria identified as Agrobacterium (Rhizobium) tumifaciens in the mesophilic (20°C) mat is of interest.     

Distribution of iron in Lake Banyoles in relation to the ecology of purple and green sulfur bacteria

The distribution of iron both in suspended sediment and in the water column has been studied during summer stratification in Lake Banyoles. In this lake, near bottom springs, a very fine material suspended sediment remains in suspension. Dissolved Fe²⁺ in interstitial water of this suspended sediment, is related to redox potential and to the bottom water inflow. In the water column, soluble iron is present in the hypolimnion of the six different basins forming Lake Banyoles. Under those conditions Fe²⁺ is partially removed by sulfide produced in the anoxic sediment. In addition, a peak of Fe²⁺ found at the density gradient level in basins C-III, C-IV and C-VI. A three compartment model on the dynamics of the processes involving iron in Lake Banyoles is proposed. The bottom springs supply oxygen to the anoxic hypolimnion affecting chemical processes of the iron cycle. The presence of phototrophic sulfur bacteria in the anoxic monimolimnion of basins C-III and C-IV can be related to the kinetics of Fe²⁺ and sulfide. In C-III sulfide concentration exceeds Fe²⁺ concentration whereas in C-IV sulfide is not detectable and iron reached values up to 60 mM. The presence of phototrophic sulfur bacteria in iron-containing environments with no detectable sulfide is explained by the ability of such microorganisms to use FeS as electron donor instead of H₂S.     

Bacterial Community Composition in the Water Column of a Lake Formed by a Former Uranium Open Pit Mine

Mining of pyrite minerals is a major environmental issue involving both biological and geochemical processes. Here we present a study of an artificial lake of a former uranium open pit mine with the aim to connect the chemistry and bacterial community composition (454-pyrosequencing of 16S rRNA genes) in the stratified water column. A shift in the water chemistry from oxic conditions in the epilimnion to anoxic, alkaline, and metal and sulfide-rich conditions in the hypolimnion was corresponded by a strong shift in the bacterial community, with few shared operational taxonomic units (OTU) between the water layers. The epilimnetic bacterial community of the lake (～20?years old) showed similarities to other temperate freshwater lakes, while the hypolimnetic bacterial community showed similarity to extreme chemical environments. The epilimnetic bacterial community had dominance of Actinobacteria and Betaproteobacteria. The hypolimnion displayed a higher bacterial diversity and was dominated by the phototrophic green sulphur bacterium of the genus Chlorobium (ca. 40?% of the total community). Deltaproteobacteria were only represented in the hypolimnion and the most abundant OTUs were affiliated with ferric iron and sulfate reducers of the genus Geobacter and Desulfobulbus, respectively. The chemistry is clearly controlling, especially the hypolimnetic, bacterial community but the community composition also indicates that the bacteria are involved in metal cycling in the lake.     

Development of a real-time PCR method for the detection of fossil 16S rDNA fragments of phototrophic sulfur bacteria in the sediments of Lake Cadagno

Lake Cadagno is a crenogenic meromictic lake situated in the southern range of the Swiss Alps characterized by a compact chemocline that has been the object of many ecological studies. The population dynamics of phototrophic sulfur bacteria in the chemocline has been monitored since 1994 with molecular methods such as 16S rRNA gene clone library analysis. To reconstruct paleo-microbial community dynamics, we developed a quantitative real-time PCR methodology for specific detection of 16S rRNA gene sequences of purple and green sulfur bacteria populations from sediment samples. We detected fossil 16S rDNA of nine populations of phototrophic sulfur bacteria down to 9-m sediment depth, corresponding to about 9500 years of the lake's biogeological history. These results provide the first evidence for the presence of 16S rDNA of anoxygenic phototrophic bacteria in Holocene sediments of an alpine meromictic lake and indicate that the water column stratification and the bacterial plume were already present in Lake Cadagno thousands of years ago. The finding of Chlorobium clathratiforme remains in all the samples analyzed shows that this population, identified in the water column only in 2001, was already a part of the lake's biota in the past.     

Dominance of a clonal green sulfur bacterial population in a stratified lake

For many years, the chemocline of the meromictic Lake Cadagno, Switzerland, was dominated by purple sulfur bacteria. However, following a major community shift in recent years, green sulfur bacteria (GSB) have come to dominate. We investigated this community by performing microbial diversity surveys using FISH cell counting and population multilocus sequence typing [clone library sequence analysis of the small subunit (SSU) rRNA locus and two loci involved in photosynthesis in GSB: fmoA and csmCA ]. All bacterial populations clearly stratified according to water column chemistry. The GSB population peaked in the chemocline ( c . 8 × 10⁶ GSB cells mL⁻¹) and constituted about 50% of all cells in the anoxic zones of the water column. At least 99.5% of these GSB cells had SSU rRNA, fmoA , and csmCA sequences essentially identical to that of the previously isolated and genome-sequenced GSB Chlorobium clathratiforme strain BU-1 (DSM 5477). This ribotype was not detected in Lake Cadagno before the bloom of GSB. These observations suggest that the C. clathratiforme population that has stabilized in Lake Cadagno is clonal. We speculate that such a clonal bloom could be caused by environmental disturbance, mutational adaptation, or invasion.     

Light‐responsive current generation by phototrophically enriched anode biofilms dominated by green sulfur bacteria

The objective of this study was to employ microbial electrochemical cells (MXCs) to selectively enrich and examine anoxygenic photosynthetic bacteria for potential anaerobic respiration capabilities using electrodes. In the process, we designed a novel enrichment strategy that manipulated the poised anode potential, light, nitrogen availability, and media supply to promote growth of phototrophic bacteria while minimizing co‐enrichment of non‐phototrophic anode‐respiring bacteria (ARB). This approach resulted in light‐responsive electricity generation from fresh‐ and saltwater inocula. Under anoxic conditions, current showed a negative light response, suggesting that the enriched phototrophic consortia shifted between phototrophic and anaerobic respiratory metabolism. Molecular, physical, and electrochemical analyses elucidated that anode biofilms were dominated by green sulfur bacteria, and biofilms exhibited anode respiration kinetics indicative of non‐mediated electron transfer, but kinetic parameters differed from values previously reported for non‐phototrophic ARB. These results invite the utilization of MXCs as microbiological tools for exploring anaerobic respiratory capabilities among anoxygenic photosynthetic bacteria. Biotechnol. Bioeng. 2013; 110: 1020–1027. © 2012 Wiley Periodicals, Inc.     

Dynamics of purple sulfur bacteria in a meromictic saline Lake Shunet (Khakassia,Siberia) in 2007–2013

According to the results of seasonal monitoring, in 2007–2013 purple sulfur bacteria morphologically similar to Thiocapsa sp. Shira_1 (AJ633676 in EMBL/GenBank) predominated in the anoxygenic phototrophic community of the water column of the meromictic Lake Shira (Khakassia, Siberia). No pronounced seasonal periodicity in the total cell number in the water column was revealed during the period of observation. In some years cell number during the period when the lake was covered with ice was reliably higher than in summer. The absence of seasonal periodicity was probably due to the low amplitude of seasonal variations in temperature and illumination in the redox zone, resulting from its relatively deep location (12–16 m). The year-to-year dynamics was characterized by a reliable decrease of the total cell number in 2009–2010 and maxima in 2007 and 2011–2012. Canonical correlation analysis revealed that water temperature in the redox zone was the best predictor of the PSB abundance in Lake Shira. Water temperature, in turn, depended on the depth of mixing of the water column. Intense mixing in 2009–2011 was probably responsible for decreased PSB abundance in the lake. On the other hand, the absence of deep winter mixing, resulting in stable conditions in the chemocline, favored the preservation of relatively high PSB biomass. Prediction of circulation depth, which depends mainly on the weather conditions and dynamics of the water level, is required for prediction of PSB abundance in Lake Shira. These results may be useful for paleolimnological reconstructions of the history of the lake based on the remnants of purple sulfur bacteria in bottom sediments.     

Microbiological and isotopic-geochemical investigations of meromictic lakes in Khakasia in winter

Microbiological and isotopic-geochemical investigations of the brackish meromictic lakes Shira and Shunet were performed in the steppe region of Khakasia in winter. Measurements made with a submersed sensor demonstrated that one-meter ice transmits light in a quantity sufficient for oxygenic and anoxygenic photosynthesis. As in the summer season, in the community of phototrophic bacteria found in Lake Shira, the purple sulfur bacteria Amoebobacter purpureus dominated, whereas, in Lake Shunet, the green sulfur bacteria Pelodictyon luteolum were predominant. Photosynthetic production, measured using the radioisotopic method, was several times lower than that in summer. The rates of sulfate reduction and production and oxidation of methane in the water column and bottom sediments were also lower than those recorded in summer. The process of anaerobic methane oxidation in the sediments was an exception, being more intense in winter than in summer. The data from radioisotopic measurements of the rates of microbial processes correlate well with the results of determination of the isotopic composition of organic and mineral carbon (delta13C) and hydrogen sulfide and sulfate (delta34S) and suggest considerable seasonal variations in the activity of the microbial community in the water bodies investigated.     

Diversity of extremophilic purple phototrophic bacteria in Soap Lake, a Central Washington (USA) Soda Lake

Culture-based and culture-independent methods were used to explore the diversity of phototrophic purple bacteria in Soap Lake, a small meromictic soda lake in the western USA. Among soda lakes, Soap Lake is unusual because it consists of distinct upper and lower water bodies of vastly different salinities, and its deep waters contain up to 175 mM sulfide. From Soap Lake water new alkaliphilic purple sulfur bacteria of the families Chromatiaceae and Ectothiorhodospiraceae were cultured, and one purple non-sulfur bacterium was isolated. Comparative sequence analysis of pufM, a gene that encodes a key photosynthetic reaction centre protein universally found in purple bacteria, was used to measure the diversity of purple bacteria in Soap Lake. Denaturing gradient gel electrophoresis and subsequent phylogenetic analyses of pufMs amplified from Soap Lake water revealed that a significant diversity of purple bacteria inhabit this soda lake. Although close relatives of several of the pufM phylotypes obtained from cultured species could also be detected in Soap Lake water, several other more divergent pufM phylotypes were also detected. It is possible that Soap Lake purple bacteria are major contributors of organic matter into the ecosystem of this lake, especially in its extensive anoxic and sulfidic deep waters.     

Seasonal Variations in the Structure of an Anoxygenic Phototrophic Bacterial Community from the Meromictic Lake Trekhtsvetnoe (Kandalaksha Bay,White Sea)

Microbiology - The community of anoxygenic phototrophic bacteria (APB) from the water column of the meromictic Lake Trekhtsvetnoe (Kandalaksha Bay, White Sea, Russia) was studied in March 2012 and...     

Succession processes in the anoxygenic phototrophic bacterial community in Lake Kislo-Sladkoe (Kandalaksha Bay,White Sea)

The community of anoxygenic phototrophic bacteria (APB) in the water column of Lake Kislo- Sladkoe (Kandalaksha Bay, White Sea), which has recently become separated from the sea, was investigated in March?April 2012, March?April 2013, and in September 2013. The lake, which was previously considered meromictic, was in fact mixed and was strongly affected by the sea. In winter the lake is sometimes washed off with seawater, and this together with the seasonal cycles of succession processes determines the succession of the community. The consequences of the mixing in autumn 2011 could be observed in the APB community as late as autumn 2013. Green-colored green sulfur bacteria (GSB) usually predominated in the chemocline. In winter 2013 stagnation resulted in turbidity of water under the ice, which was responsible for both predominance of the brown GSB forms and the changes ratio of the species of purple sulfur bacteria (PSB) in anoxic water layers. Production of anoxygenic photosynthesis in the lake was at least 240 mg C m^-2 day^-1 in September and 0–20 mg C m^–2 day^–1 in March—April, which corresponded to 40 and 69%, respectively, of oxygenic photosynthesis. Okenone-containing purple sulfur bacteria, strain TcakPS12, were isolated in 2012 from lake water. The ells of this strain form filaments of not separated cells. Strain TcakPS12 exhibited 98% similarity with the type strains of Thiocapsa pendens DSM 236 and Thiocapsa bogorovii BBS, as well as with the strains AmPS10 and TcyrPS10, which were isolated from Lake Kislo-Sladkoe in 2010.     

Predation impact of ciliated and flagellated protozoa during a summer bloom of brown sulfur bacteria in a meromictic coastal lake

Anaerobic phagotrophic protozoa may play an important role in the carbon flux of chemically stratified environments, especially when phototrophic sulfur bacteria account for a high proportion of the primary production. To test this assumption, we investigated the vertical and temporal distribution of microbial heterotrophs and of autotrophic picoplankton throughout the water column of the meromictic coastal lake Faro (Sicily, Italy), in the summer of 2004, coinciding with a bloom of brown-colored green sulfur bacteria. We also assessed the grazing impact of ciliated and flagellated protozoa within the sulfur bacteria plate using a modification of the fluorescently labeled bacteria uptake approach, attempting to minimize the biases intrinsic to the technique and to preserve the in situ anoxic conditions. Significant correlations were observed between ciliate biomass and bacteriochlorophyll e concentration, and between heterotrophic nanoflagellate biomass and chlorophyll a concentration in the water column. The major predators of anaerobic picoplankton were pleuronematine ciliates and cryptomonad flagellates, with clearances of 26.6 and 9.5 nL per cell h⁻¹, respectively, and a cumulative impact on the picoplankton gross growth rate ranging between 36% and 72%. We concluded that protozoan grazing channels a large proportion of anaerobic picoplankton production to higher trophic levels without restraining photosynthetic bacteria productivity.     

Microbially — related redox changes in a subtropical lake I.In situ monitoring of the annual redox cycle

Using a recently developedin situ multiprobe the redox development in the water column of warm-monomictic Lake Kinneret was investigated during three annual cycles. During the time when sulfide release into the meta- and hypolimnion is initiated, our measurements show a linear relationship, close to the thermodynamic function, between the platinum electrode potential and the amount of sulfide produced by the sulfate reducing bacteria. A change of this relationship during summer stratification coincides with the bloom of the phototrophic sulfur bacteriumChlorobium phaeobacteroides.     

Detection of microbial biomass by intact polar membrane lipid analysis in the water column and surface sediments of the Black Sea

The stratified water column of the Black Sea produces a vertical succession of redox zones, stimulating microbial activity at the interfaces. Our study of intact polar membrane lipids (IPLs) in suspended particulate matter and sediments highlights their potential as biomarkers for assessing the taxonomic composition of live microbial biomass. Intact polar membrane lipids in oxic waters above the chemocline represent contributions of bacterial and eukaryotic photosynthetic algae, while anoxygenic phototrophic bacteria and sulfate-reducing bacteria comprise a substantial amount of microbial biomass in deeper suboxic and anoxic layers. Intact polar membrane lipids such as betaine lipids and glycosidic ceramides suggest unspecified anaerobic bacteria in the anoxic zone. Distributions of polar head groups and core lipids show planktonic archaea below the oxic zone; methanotrophic archaea are only a minor fraction of archaeal biomass in the anoxic zone, contrasting previous observations based on the apolar derivatives of archaeal lipids. Sediments contain algal and bacterial IPLs from the water column, but transport to the sediment is selective; bacterial and archaeal IPLs are also produced within the sediments. Intact polar membrane lipid distributions in the Black Sea are stratified in accordance with geochemical profiles and provide information on vertical successions of major microbial groups contributing to suspended biomass. This study vastly extends our knowledge of the distribution of complex microbial lipids in the ocean.     

Vertical and temporal shifts in microbial communities in the water column and sediment of saline meromictic Lake Kaiike (Japan), as determined by a 16S rDNA-based analysis, and related to physicochemical gradients

The vertical and temporal changes in microbial communities were investigated throughout the water column and sediment of the saline meromictic Lake Kaiike by PCR-denaturing gradient gel electrophoresis (DGGE) of 16S rDNA. Marked depth-related changes in microbial communities were observed at the chemocline and the sediment-water interface. However, no major temporal changes in the microbial community below the chemocline were observed during the sampling period, suggesting that the ecosystem in the anoxic zone of Lake Kaiike was nearly stable. Although the sequence of the most conspicuous DGGE band throughout the anoxic water and in the top of the microbial mat was most similar to that of an anoxic, photosynthetic, green sulphur bacterium, Pelodyction luteolum DSM273 (97% similarity), it represented a new phylotype. A comparison of DGGE banding patterns of the water column and sediment samples demonstrated that specific bacteria accumulated on the bottom from the anoxic water layers, and that indigenous microbial populations were present in the sediment. The measurements of bicarbonate assimilation rates showed significant phototrophic assimilation in the chemocline and lithoautotrophic assimilation throughout the anoxic water, but were not clearly linked with net sulphide turnover rates, indicating that sulphur and carbon metabolisms were not directly correlated.     

Dominant bacterioplankton populations in the meromictic Lake Suigetsu as determined by denaturing gradient gel electrophoresis of 16S rRNA gene fragments

Lake Suigetsu is a typical meromictic lake in Japan characterized by a permanent chemocline at a depth of between 3 and 8 m separating the oxic freshwater mixolimnion from anoxic saline sulfidogenic monimolimnion. Dominant bacterioplankton populations in Lake Suigetsu were investigated using PCR-denaturing gradient gel electrophoresis (DGGE) of 16S rRNA gene fragments. The bacterial population was vertically stratified, and temporal shifts in the microbial communities were observed in both the oxic and anoxic layers of Lake Suigetsu during the sampling period. Several dominant DGGE bands were excised and sequenced. In the chemocline, green sulfur bacteria phylogenetically related to the genera Prosthecochloris, Pelodyctyon, and Chlorobium within the phylum Chlorobi were dominant; the colorless sulfur bacteria closely related to the genus Thiomicrospira were detected. These sulfur bacterial groups appear to be important in the biogeochemical cycling of sulfur and/or carbon in Lake Suigetsu. Bacterial sequences affiliated with the Bacteroidetes phylum were frequent among the dominant fragments in the DGGE profiles throughout the water column. Populations possessing a fermentative metabolism exist in Bacteroidetes, suggesting they may contribute to the degradation of organic matter in the anoxic environment of Lake Suigetsu.