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.     

Dominance of green sulfur bacteria in the chemocline of the meromictic Lake Suigetsu, Japan, as revealed by dissimilatory sulfite reductase gene analysis

This study investigated the spatiotemporal abundance and diversity of the α-subunit of the dissimilatory sulfite reductase gene (dsrA) in the meromictic Lake Suigetsu for assessing the sulfur-oxidizing bacterial community. The density of dsrA in the chemocline reached up to 3.1 × 10⁶ copies ml^?1 in summer by means of quantitative real-time PCR and it was generally higher than deeper layers. Most of the dsrA clones sequenced were related to green sulfur bacteria such as Chlorobium phaeovibrioides, C. limicola, and C. luteolum. Below the chemocline of the lake, we also detected other dsrA clones related to the purple sulfur bacterium Halochromatium salexigens and some branching lineages of diverse sequences that were related to chemotrophic sulfur bacterial species such as Magnetospirillum gryphiswaldense, Candidatus Ruthia magnifica, and Candidatus Thiobios zoothamnicoli. The abundance and community compositions of sulfur-oxidizing bacteria changed depending on the water depth and season. This study indicated that the green sulfur bacteria dominated among sulfur-oxidizing bacterial population in the chemocline of Lake Suigetsu and that certain abiotic environmental variables were important factors that determined sulfur bacterial abundance and community structure.     

Vertical distributions of stable isotopic compositions and bacteriochlorophyll homologues in suspended particulate matter in saline meromictic Lake Abashiri

We determined chloropigment composition as well as stable carbon and nitrogen isotopic compositions of dissolved and particulate species in saline meromictic Lake Abashiri. We observed a sharp peak of bacteriochlorophyll e in a narrow redox boundary zone and the upper monimolimnion, indicating a dense population of brown-colored strains of green sulfur bacteria around the chemocline. Nitrogen isotopic records of particulate nitrogen and dissolved ammonium suggested that the green sulfur bacteria in the redox boundary zone assimilated either ammonium or dinitrogen through the nitrogen fixation pathway. In the anoxic monimolimnion, several lines of evidence suggest that a major portion of particulate organic matter originated from the overlying mixolimnion and redox boundary zone.     

Anoxygenic phototrophic bacteria of the high-altitude meromictic Lake Gek-Gel,Azerbaijan

The anoxygenic phototrophic bacterial community of the high-altitude meromictic Lake Gek-Gel (Azerbaijan) was investigated in September 2003. The highest concentration of bacteriochlorophyll e (48 μg/l) was detected at a depth of 30 m; the peak of bacteriochlorophyll a (4.5 μg/l) occurred at 29 m. Phylogenetic analysis revealed that brown-colored green sulfur bacteria Chlorobium phaeobacteroides predominated in the lake. Nonsulfur purple bacteria phylogenetically close to Blastochloris sulfoviridis were found in insignificant amounts; these organisms have not been previously reported in Lake Gek-Gel.     

Characterization and In Situ Carbon Metabolism of Phototrophic Consortia

A dense population of the phototrophic consortium “Pelochromatium roseum” was investigated in the chemocline of a temperate holomictic lake (Lake Dagow, Brandenburg, Germany). Fluorescence in situ hybridization revealed that the brown epibionts of “P. roseum” constituted up to 37% of the total bacterial cell number and up to 88% of all green sulfur bacteria present in the chemocline. Specific amplification of 16S rRNA gene fragments of green sulfur bacteria and denaturing gradient gel electrophoresis fingerprinting yielded a maximum of four different DNA bands depending on the year of study, indicating that the diversity of green sulfur bacteria was low. The 465-bp 16S rRNA gene sequence of the epibiont of “P. roseum” was obtained after sorting of individual consortia by micromanipulation, followed by a highly sensitive PCR. The sequence obtained represents a new phylotype within the radiation of green sulfur bacteria. Maximum light-dependent H¹⁴CO₃⁻ fixation in the chemocline in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea suggested that there was anaerobic autotrophic growth of the green sulfur bacteria. The metabolism of the epibionts was further studied by determining stable carbon isotope ratios (δ¹³C) of their specific biomarkers. Analysis of photosynthetic pigments by high-performance liquid chromatography revealed the presence of high concentrations of bacteriochlorophyll (BChl) e and smaller amounts of BChl a and d and chlorophyll a in the chemocline. Unexpectedly, isorenieratene and β-isorenieratene, carotenoids typical of other brown members of the green sulfur bacteria, were absent. Instead, four different esterifying alcohols of BChl e were isolated as biomarkers of green sulfur bacterial epibionts, and their δ¹³C values were determined. Farnesol, tetradecanol, hexadecanol, and hexadecenol all were significantly enriched in ¹³C compared to bulk dissolved and particulate organic carbon and compared to the biomarkers of purple sulfur bacteria. The difference between the δ¹³C values of farnesol, the major esterifying alcohol of BChl e, and CO₂ was −7.1%, which provides clear evidence that the mode of growth of the green sulfur bacterial epibionts of “P. roseum” in situ is photoautotrophic.     

Physiology and Phylogeny of Green Sulfur Bacteria Forming a Monospecific Phototrophic Assemblage at a Depth of 100 Meters in the Black Sea

The biomass, phylogenetic composition, and photoautotrophic metabolism of green sulfur bacteria in the Black Sea was assessed in situ and in laboratory enrichments. In the center of the western basin, bacteriochlorophyll e (BChl e) was detected between depths of 90 and 120 m and reached maxima of 54 and 68 ng liter⁻¹. High-pressure liquid chromatography analysis revealed a dominance of farnesyl esters and the presence of four unusual geranyl ester homologs of BChl e. Only traces of BChl e (8 ng liter⁻¹) were found at the northwestern slope of the Black Sea basin, where the chemocline was positioned at a significantly greater depth of 140 m. Stable carbon isotope fractionation values of farnesol indicated an autotrophic growth mode of the green sulfur bacteria. For the first time, light intensities in the Black Sea chemocline were determined employing an integrating quantum meter, which yielded maximum values between 0.0022 and 0.00075 μmol quanta m⁻² s⁻¹ at the top of the green sulfur bacterial layer around solar noon in December. These values represent by far the lowest values reported for any habitat of photosynthetic organisms. Only one 16S rRNA gene sequence type was detected in the chemocline using PCR primers specific for green sulfur bacteria. This previously unknown phylotype groups with the marine cluster of the Chlorobiaceae and was successfully enriched in a mineral medium containing sulfide, dithionite, and freshly prepared yeast extract. Under precisely controlled laboratory conditions, the enriched green sulfur bacterium proved to be capable of exploiting light intensities as low as 0.015 μmol quanta m⁻² s⁻¹ for photosynthetic ¹⁴CO₂ fixation. Calculated in situ doubling times of the green sulfur bacterium range between 3.1 and 26 years depending on the season, and anoxygenic photosynthesis contributes only 0.002 to 0.01% to total sulfide oxidation in the chemocline. The stable population of green sulfur bacteria in the Black Sea chemocline thus represents the most extremely low-light-adapted and slowest-growing type of phototroph known to date.     

Analysis of Subfossil Molecular Remains of Purple Sulfur Bacteria in a Lake Sediment

Molecular remains of purple sulfur bacteria (Chromatiaceae) were detected in Holocene sediment layers of a meromictic salt lake (Mahoney Lake, British Columbia, Canada). The carotenoid okenone and bacteriophaeophytin a were present in sediments up to 11,000 years old. Okenone is specific for only a few species of Chromatiaceae, including Amoebobacter purpureus, which presently predominates in the chemocline bacterial community of the lake. With a primer set specific for Chromatiaceae in combination with denaturing gradient gel electrophoresis, 16S rRNA gene sequences of four different Chromatiaceae species were retrieved from different depths of the sediment. One of the sequences, which originated from a 9,100-year-old sample, was 99.2% identical to the 16S rRNA gene sequence of A. purpureus ML1 isolated from the chemocline. Employing primers specific for A. purpureus ML1 and dot blot hybridization of the PCR products, the detection limit for A. purpureus ML1 DNA could be lowered to 0.004% of the total community DNA. With this approach the DNA of the isolate was detected in 7 of 10 sediment layers, indicating that A. purpureus ML1 constituted at least a part of the ancient purple sulfur bacterial community. The concentrations of A. purpureus DNA and okenone in the sediment were not correlated, and the ratio of DNA to okenone was much lower in the subfossil sediment layers (2.7 · 10⁻⁶) than in intact cells (1.4). This indicates that degradation rates are significantly higher for genomic DNA than for hydrocarbon cell constituents, even under anoxic conditions and at the very high sulfide concentrations present in Mahoney Lake.     

Seasonal Changes in the Structure of the Anoxygenic Phototrophic Bacterial Community in Lake Mogilnoe,a Relict Lake on Kil'din Island in the Barents Sea

An anaerobic phototrophic bacterial community in Lake Mogilnoe, a relict lake on Kil'din Island in the Barents Sea, was studied in June 1999 and September 2001. Irrespective of the season, the upper layer of the anaerobic zone of this lake had a specific species composition of sulfur phototrophic bacteria, which were dominated by the brown-colored green sulfur bacterium Chlorobium phaeovibrioides. The maximum number of sulfur phototrophic bacteria was observed in June 1999 at a depth of 9 m, which corresponded to a concentration of bacteriochlorophyll (Bchl) e equal to 4.6 mg/l. In September 2001, the maximum concentration of this pigment (3.4 mg/l) was found at a depth of 10 m. In both seasons, the concentration of Bchl a did not exceed 3 μg/l. Purple sulfur bacteria were low in number, which can be explained by their poor adaptation to the hydrochemical and optical conditions of the Lake Mogilnoe water. In June 1999, the water contained a considerable number of Pelodictyon phaeum microcolonies and Prosthecochloris phaeoasteroides cell chains, which was not the case in September 2001. A 16S rDNA-based phylogenetic analysis of pure cultures of phototrophic bacteria isolated from the lake water confirmed that the bacterial community is dominated by Chl. phaeovibrioides and showed the presence of three minor species, Thiocystis gelatinosa, Thiocapsa sp., and Thiorhodococcus sp., the last of which is specific to Lake Mogilnoe.     

Investigation of the sulfate-reducing bacterial community in the aerobic water and chemocline zone of the Black Sea by the fish technique

Fluorescent in situ hybridization (FISH) was used to analyze the abundance and phylogenetic composition of sulfate-reducing bacteria in the aerobic waters and in the oxic/anoxic transitional zone (chemocline) of the Black Sea, where biogenic formation of reduced sulfur compounds was detected by radioisotope techniques. Numerous sulfate-reducing bacteria of the genera Desulfotomaculum (30.5% of detected bacterial cells), Desulfovibrio (29.6%), and Desulfobacter (6.7%) were revealed in the aerobic zone at a depth of 30 m, while Desulfomicrobium-related bacteria (33.5%) were prevalent in the upper chemocline waters at 150-m depth. Active cells of sulfate-reducing bacteria were much more abundant in the samples collected in summer than in the winter samples from the deep-sea zone. The presence of physiologically active sulfate reducers in oxic and chemocline waters of the Black Sea correlates with the hydrochemical data on the presence of reduced sulfur compounds in the aerobic water column.     

Densities and distribution of flagellates and ciliates in the chemocline of saline,meromictic Lake Shunet (Siberia,Russia)

The vertical and seasonal distributions of the phytoflagellate Cryptomonas spp., and its most common, the planktonic ciliate predators (Oligotrichida, Scuticociliatida, Hypotrichida and Prostomatida) were investigated in chemocline region of small saline, meromictic lake Shunet (Siberia, Russia) during 2003 and 2005. The lake has a pronounced chemocline, with abundance of purple and green sulphur bacteria. Vertical distribution of the Cryptomonas populations near the oxic/anoxic boundary layer was studied at close intervals in water sampled using a hydraulically operated thin-layer sampler. In both summer and winter, Cryptomonas peaked in water stratum 5–10 cm above anoxic zone or in the anoxic zone water column in the chemocline (about 5 m). Ciliate densities and biomass were also much higher in chemocline than in mixolimnion. The range of diurnal migration of Cryptomonas population was not very wide, and it was restricted to layers with high light intensity. The ciliates were sometimes detected above the upper border of the anoxic zone but also several centimetres below this zone.     

Sedimentation of photosynthetic pigments during the bloom of the green sulfur bacterium <Emphasis Type="Italic">Chlorobium phaeobacteroides</Emphasis> in Lake Kinneret: spatial patterns

Sediment fluxes were studied in the subtropical Lake Kinneret (Israel) in 2006, 2007, and 2008 from mid-June to October, when lake was chemically stratified and the green sulfur bacterium Chlorobium phaeobacteroides formed a dense population in the anoxic metalimnion. The rate of seston accumulation in traps was measured with sedimentation traps positioned along an offshore transect connecting the littoral zone and lake center. The sediment fluxes increased from the lake center towards the littoral, while the percentage of organic material (OM) decreased correspondingly. High fluxes of bacteriochlorophyll e (BChl e—a signature pigment of Chl. phaeobacteroides) and chlorophyll a (Chl a—a marker for eukaryotic algae and cyanobacteria) were detected in all locations. The relative contributions of Chl a and BChl e to the bulk of the accumulated OM were higher in traps positioned below the thermocline in the pelagic zone than in traps located near the shore line. The presence of BChl e in traps exposed to oxic conditions in the littoral, where Chl. phaeobacteroides does not develop, implies horizontal translocation of cells from the lake center towards its periphery. We assume that seiche-mediated movement of particles embedded in the metalimnetic waters is the most probable explanation for the existence of Chl. phaeobacteroides tracer in an oxic environment, but do not exclude the possibility of resuspension of settled particles as source for BChl e in littoral traps. The green sulfur bacteria are potentially important component of the sediment flux of photosynthesizing organisms in a thermally stratified lake and should be taken into account when carbon budget are constructed.     

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.     

Community Shift from Phototrophic to Chemotrophic Sulfide Oxidation following Anoxic Holomixis in a Stratified Seawater Lake

Most stratified sulfidic holomictic lakes become oxygenated after annual turnover. In contrast, Lake Rogoznica, on the eastern Adriatic coast, has been observed to undergo a period of water column anoxia after water layer mixing and establishment of holomictic conditions. Although Lake Rogoznica''s chemistry and hydrography have been studied extensively, it is unclear how the microbial communities typically inhabiting the oxic epilimnion and a sulfidic hypolimnion respond to such a drastic shift in redox conditions. We investigated the impact of anoxic holomixis on microbial diversity and microbially mediated sulfur cycling in Lake Rogoznica with an array of culture-independent microbiological methods. Our data suggest a tight coupling between the lake''s chemistry and occurring microorganisms. During stratification, anoxygenic phototrophic sulfur bacteria were dominant at the chemocline and in the hypolimnion. After an anoxic mixing event, the anoxygenic phototrophic sulfur bacteria entirely disappeared, and the homogeneous, anoxic water column was dominated by a bloom of gammaproteobacterial sulfur oxidizers related to the GSO/SUP05 clade. This study is the first report of a community shift from phototrophic to chemotrophic sulfide oxidizers as a response to anoxic holomictic conditions in a seasonally stratified seawater lake.     

Microbial processes of the carbon and sulfur cycles in an ice‐covered,iron‐rich meromictic lake Svetloe (Arkhangelsk region,Russia)

Biogeochemical, isotope geochemical and microbiological investigation of Lake Svetloe (White Sea basin), a meromictic freshwater was carried out in April 2014, when ice thickness was ～0.5 m, and the ice‐covered water column contained oxygen to 23 m depth. Below, the anoxic water column contained ferrous iron (up to 240 μμM), manganese (60 μM), sulfide (up to 2 μM) and dissolved methane (960 μM). The highest abundance of microbial cells revealed by epifluorescence microscopy was found in the chemocline (redox zone) at 23–24.5 m. Oxygenic photosynthesis exhibited two peaks: the major one (0.43 μmol C L^?1 day^?1) below the ice and the minor one in the chemocline zone, where cyanobacteria related to Synechococcus rubescens were detected. The maximum of anoxygenic photosynthesis (0.69 μmol C L^?1 day^?1) at the oxic/anoxic interface, for which green sulfur bacteria Chlorobium phaeoclathratiforme were probably responsible, exceeded the value for oxygenic photosynthesis. Bacterial sulfate reduction peaked (1.5 μmol S L^?1 day^?1) below the chemocline zone. The rates of methane oxidation were as high as 1.8 μmol CH₄ L^?1 day^?1 at the oxi/anoxic interface and much lower in the oxic zone. Small phycoerythrin‐containing Synechococcus‐related cyanobacteria were probably involved in accumulation of metal oxides in the redox zone.     

Bacterial Communities of Three Saline Meromictic Lakes in Central Asia

Meromictic lakes located in landlocked steppes of central Asia (~2500 km inland) have unique geophysiochemical characteristics compared to other meromictic lakes. To characterize their bacteria and elucidate relationships between those bacteria and surrounding environments, water samples were collected from three saline meromictic lakes (Lakes Shira, Shunet and Oigon) in the border between Siberia and the West Mongolia, near the center of Asia. Based on in-depth tag pyrosequencing, bacterial communities were highly variable and dissimilar among lakes and between oxic and anoxic layers within individual lakes. Proteobacteria, Bacteroidetes, Cyanobacteria, Actinobacteria and Firmicutes were the most abundant phyla, whereas three genera of purple sulfur bacteria (a novel genus, Thiocapsa and Halochromatium) were predominant bacterial components in the anoxic layer of Lake Shira (~20.6% of relative abundance), Lake Shunet (~27.1%) and Lake Oigon (~9.25%), respectively. However, few known green sulfur bacteria were detected. Notably, 3.94% of all sequencing reads were classified into 19 candidate divisions, which was especially high (23.12%) in the anoxic layer of Lake Shunet. Furthermore, several hydro-parameters (temperature, pH, dissolved oxygen, H₂S and salinity) were associated (P< 0.05) with variations in dominant bacterial groups. In conclusion, based on highly variable bacterial composition in water layers or lakes, we inferred that the meromictic ecosystem was characterized by high diversity and heterogenous niches.     

Ecology of purple sulfur bacteria in the highly stratified meromictic Lake Shunet (Siberia, Khakassia) in 2002–2009

Phototrophic sulfur bacteria form dense accumulations in the chemocline zones of stratified lakes where light reaches the sulfide-containing layers of water. Many works are dedicated to the ecophysiology of these microorganisms in meromictic lakes. However, the role of these microorganisms in the trophic network of these ecosystems, the ways of biomass utilization, and the contribution to the turnover of biogenic elements have so far been insufficiently understood. This work deals with the analysis of many years?? seasonal dynamics of the biomass of purple sulfur bacteria and the physicochemical conditions of their environment in Lake Shunet (Siberia, Khakassia, Russia), unraveling the causes of their anomalous development in the chemocline of this lake, as well as the comparative analysis of such type of ecosystems. Lake Shunet is characterized by markedly pronounced stratification and the high density of purple sulfur bacteria (PSB) in the chemocline, which is comparable to that of Lake Mahoney (Canada) where the number of PSB is the greatest among those known in the world. It was shown that, in the period 2002?C2009, the total amount of bacterio-chlorophyll a in the water column of Lake Shunet increased and did not correlate with the seasonal variations in temperature and illumination in the chemocline. It was established that PSB cells in the purple layer experienced the effect of self-shading. The sedimentation rate of purple sulfur bacteria in Lake Shunet was low due to the pronounced density gradient in the chemocline zone. Thus, the high number of PSB in the chemocline was due to the combination of strong illumination, a high sulfide concentration, and a high water density gradient, which was responsible for stable stratification and contributed to the accumulation of the cells in a narrow layer. The data obtained could be useful for the paleoreconstruction of climatically deter-mined changes in the level of the lake and its periods of meromixis by the presence of carotenoids and bacte-riochlorophylls in the bottom sediments.     

Isolation, Characterization, and Ecology of Cold-Active, Chemolithotrophic, Sulfur-Oxidizing Bacteria from Perennially Ice-Covered Lake Fryxell, Antarctica

Novel strains of obligately chemolithoautotrophic, sulfur-oxidizing bacteria have been isolated from various depths of Lake Fryxell, Antarctica. Physiological, morphological, and phylogenetic analyses showed these strains to be related to mesophilic Thiobacillus species, such as T. thioparus. However, the psychrotolerant Antarctic isolates showed an adaptation to cold temperatures and thus should be active in the nearly freezing waters of the lake. Enumeration by most-probable-number analysis in an oxic, thiosulfate-containing medium revealed that the sulfur-oxidizing chemolithotroph population peaks precisely at the oxycline (9.5 m), although viable cells exist well into the anoxic, sulfidic waters of the lake. The sulfur-oxidizing bacteria described here likely play a key role in the biogeochemical cycling of carbon and sulfur in Lake Fryxell.     

Grazing of the copepod Diaptomus connexus on purple sulphur bacteria in a meromictic salt lake

A meromictic lake ecosystem (Mahoney Lake, BC, Canada) was investigated to elucidate the significance of chemocline bacteria in the total carbon cycle under natural conditions. In this lake, primary production by oxygenic phototrophs was insufficient to support the observed net secondary production of the calanoid copepod Diaptomus connexus and the rotifer Brachionus plicatilis , indicating the presence of additional food sources for consumers. Mahoney Lake harbours the densest population of phototrophic sulphur bacteria ever reported in a natural body of water. This layer is located at the interface between oxic and anoxic water layers and is dominated by the purple sulphur bacterium Amoebobacter purpureus . The transfer rates of A. purpureus carbon to D. connexus determined in stratified mesocosms were very low (0.71 ngC copepod⁻¹ day⁻¹) and accounted for only 0.6% of the observed net biomass increase in the zooplankter. Stable stratification within the mesocosms prevented an upwelling of A. purpureus into the oxic part. However, measurements of carbon fluxes, infrared fluorescence microscopy and stable carbon analysis provided cumulative evidence that, under in situ conditions, the cell carbon of purple sulphur bacteria indeed enters the aerobic food chain via the grazing activity of D. connexus . Based on a two-source isotopic mixing model, A. purpureus represents at least 75–85% of the diet of D . connexus . Autumnal upwelling into oxic water layers and aggregation of A . purpureus cells appear to be the main factors determining the high carbon flux from purple sulphur bacteria to zooplankton under natural conditions, and most probably also play a key role in other aquatic ecosystems. Through this pathway, over 53% of the reduced organic matter of purple sulphur bacteria trapped in anoxic bottom waters is returned to the oxic realm.     

Sulfur-Oxidizing Bacteria in Soap Lake (Washington State), a Meromictic, Haloalkaline Lake with an Unprecedented High Sulfide Content

Culture-dependent and -independent techniques were used to study the diversity of chemolithoautotrophic sulfur-oxidizing bacteria in Soap Lake (Washington State), a meromictic, haloalkaline lake containing an unprecedentedly high sulfide concentration in the anoxic monimolimnion. Both approaches revealed the dominance of bacteria belonging to the genus Thioalkalimicrobium, which are common inhabitants of soda lakes. A dense population of Thioalkalimicrobium (up to 10⁷ cells/ml) was found at the chemocline, which is characterized by a steep oxygen-sulfide gradient. Twelve Thioalkalimicrobium strains exhibiting three different phenotypes were isolated in pure culture from various locations in Soap Lake. The isolates fell into two groups according to 16S rRNA gene sequence analysis. One of the groups was closely related to T. cyclicum, which was isolated from Mono Lake (California), a transiently meromictic, haloalkaline lake. The second group, consisting of four isolates, was phylogenetically and phenotypically distinct from known Thioalkalimicrobium species and unique to Soap Lake. It represented a new species, for which we suggest the name Thioalkalimicrobium microaerophilum sp. nov.     

Effect of winter conditions on distributions of anoxic phototrophic bacteria in two meromictic lakes in Siberia,Russia

The year-to-year variations of vertical distribution and biomass of anoxic phototrophic bacteria were studied during ice periods 2003–2005 and 2007–2008 in meromictic lakes Shira and Shunet (Southern Siberia, Russian Federation). The bacterial layers in chemocline of both lakes were sampled with a thin-layer hydraulic multi-syringe sampler. In winter, biomass of purple sulphur bacteria varied considerably depending on the amount of light penetrating into the chemocline through the ice and snow cover. In relatively weakly stratified, brackish Shira Lake, the depth of chemocline varied between winters, so that light intensity for purple sulphur bacteria inhabiting this zone differed. In Shira Lake, increased transparency of mixolimnion in winter, high chemocline position and absence of snow resulted in light intensity and biomass of purple sulphur bacteria exceeding the summer values in the chemocline of the lake. We could monitor snow cover at the lake surface using remote sensing and therefore estimate dynamics and amount of light under ice and its availability for phototrophic organisms. In Shunet Lake, the light intensities in the chemocline and biomasses of purple sulphur bacteria were always lower in winter than in summer, but the biomasses of green sulphur bacteria were similar.