Meromictic soda and saline lakes are unique ecosystems characterized by the stability of physical, chemical and biological parameters. The high physical stability of the water masses, the clearly separated compartments and relatively constant vertical stratification of the bacterial populations, a compact and stable transition zone between the oxic mixolimnion and the anoxic monimolimnion, and the presence of a dense microbial community at the redox transition zone(Overmann et al., 1991)make them excellent model systems in limnological studies. Meromictic soda and saline lakes are distributed all over the world and are located in arid and semiarid regions(Lake Magadi, Africa; Mono Lake, Big Soap Lake, America, etc.) and in regions with a permanently negative air temperature(Lake V and a, Organic Lake, Ace Lake, Antarctica, etc.)or average annual negative air temperature(Lake A, Canada; Lakes Shira, Shunet, and Doroninskoe, Siberia). Siberian soda and saline lakes are characterized by the presence of more than one period with intensive and dynamic processes, the so-called biological summer and the long ice season with the biological spring. Meromixis is maintained both in the open water, and ice periods, therefore the investigation of the meromictic process in such lakes is of particular interest.

Lake Doroninskoe is one of the three known meromictic soda lakes located in Siberia. It is located in the permafrost zone, and belongs to the rare type of salted soda lakes(Zamana and Borzenko, 2007). The main climate factors of the area are: a wide annual range of air temperature(hot summers with temperatures up to 40°C and cold winters with temperatures down to -40°C), significant differences in the daily air temperature especially in the spring and autumn seasons, low cloudiness and low annual precipitation with a maximum in summer, and frequent strong winds. During up to seven months per year the lake is covered by ice, and stable meromictic conditions are preserved(Zamana and Borzenko, 2009; Gorlenko et al., 2010). Generally, at the end of the ice period a hypoxic state of the mixolimnion with the presence of a high content of sulfide is observed(Borzenko and Zamana, 2011). One more peculiarity in which the lake differs from the two other meromictic Siberian lakes Shira and Shunet(Rogozin et al., 2010a, b)is the extremely low level of light penetration into the chemocline zone, which in the winter season amounts to no more than 0.001%. This individuality might be interconnected with high turbidity of the lake water which formed by physical properties of water(Lukyanov et al., 2014), geochemical factors of the formation of salted soda lake(Borzenko et al., 2014a), and /or climate condition. It was shown that the absence of a pink-colored water zone, characteristic for other meromictic lakes, was due to this low light intensity(Matyugina et al., 2014).

The main microbiological investigations of Lake Doroninskoe were performed in September during the open water period(Gorlenko et al., 2010). Two aerobic alkaliphilic chemoorganotrophic strains of the genus Halomonas were isolated from surface water samples. 16S rRNA gene sequencing revealed that one strain was closely related to Halomonas salina and Halomonas ventosae, while the other was identified as Halomonas campisalis . The ability of chemolithoheterotrophic growth coupled with the oxidation of thiosulfate was detected in anoxygenic phototrophic bacteria of the genus Roseinatronobacter isolated from the aerobic zone(Gorlenko et al., 2010). The main oxygenic phototrophic microorganisms in the mixolimnion were the filamentous cyanobacteria Phormidium sp., Nodularia sp. with heterocysts, and the unicellular Synechococcus sp. and Synechocystis sp., as well as the diatom Nitzschia sp.(Gorlenko et al., 2010). In the chemocline the alkaliphilic sulfuroxidizing bacterium Thioalkalivibrio sp. was detected. No well-marked layer with anoxygenic phototrophic bacteria on the border with the sulfide zone was observed, but purple sulfur bacteria were isolated from deep mud sediments. They were identified by ribosomal rRNA phylogeny as Thioalkalicoccus limnaeus, Ectothiorhodospira variabilis, “ E . magna ”, and E . shaposhnikovii(Gorlenko et al., 2010). Thus, the conditions in the water column of Lake Doroninskoe during the open water period were favorable for the development of alkaliphilic species of microorganisms. The finding of reduced sulfur compounds not only in the anaerobic zone but also in the upper layers of water column led us to assume an important role of microorganisms in the sulfur cycling in the lake. During the studied period the light intensity in the chemocline zone was extremely low, which determined the predominant presence of lightindependent microbial processes at the contact zone of sulfide and oxygen(Gorlenko et al., 2010). Simulation of light conditions in a laboratory experiment with microcosms proved that the level of light and its penetration into the anaerobic zone defined the dominant composition of the microbial community in the chemocline zone, and therefore the dominant type of metabolism-phototrophic or chemotrophic(Matyugina et al., 2014).

Next-generation ‘-omics’ technologies such as high-throughput amplicon sequencing allow collection of billions of sequences(Green et al., 2008; DeLong, 2009) and application of statistical methods enables the detection of the numerically dominant as well as uncommon organisms in a system(Bent and Forney, 2008; Gonzalez et al., 2012). The first group may be responsible for the majority of metabolic activity and energy fl ux, but uncommon organisms serve as a reservoir of genetic and functional diversity(Yachiand Loreau, 1999; Nandiet al., 2004), they often play key roles in ecosystems(Phillips et al., 2000), and they can become numerically important if environmental conditions change(Bent and Forney, 2008).

The main aim of the present study was the identification of the microbial diversity of Lake Doroninskoe by high-throughput 16S rRNA gene amplicon sequencing to probe the vertical distribution of major and minor bacterial taxa during the ice season.

Lake Doroninskoe is located in Eastern Transbaikalia(Fig. 1). The water column of the lake during the ice period can be divided into three zones: an upper oxic mixolimnion(from the surface to 3.15 m), a sharp chemocline(depth about 0.15– 0.20 m), and the lower oxygen-free monimolimnion(from 3.2 to 6.2 m). The chemical composition of the water during the ice period is similar to the other seasons of the year and on average consists of 70% Na ₂ CO ₃ +NaHCO ₃ ; 29% NaCl, 1% Na ₂ SO ₄, an alkaline pH(9.7–10.0), and a stable increased salt concentration of the water and sediments up to 32.3– 35.0 g/L(Zamana and Borzenko, 2007). Chemical stratification of salinity and the main ions in the mixo and monimolimnion during the ice period are associated with the processes of ice formation and melting and with climate peculiarities during the year(Zamana and Borzenko, 2009). During the ice period in 2013 a stable chemocline was located at the depth of 3.15–3.35 m, characterized by sharp gradients of oxygen and sulfide(Borzenko, 2013).

Fig. 1 Location of Lake Doroninskoe and the sampling site a. location map; b. schematic map of the area marked with a square in panel (a); c. photograph of the sampling site in March 2013 (Photo by S.V.Tsyrenzapov).

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Water samples were collected in March 2013 at the central station of the lake(51°14′N; 112°14′E, ice thickness was 1.19 m, 6.2 m of water depth)from 0, 1.0, 2.0, 2.6, 3.15, 3.6, 4.0, 5.0, and 6.0 m depth. For metagenomic analysis, water samples were filtered through 0.22 μm pore size polycarbonate filters(Millipore) and stored at -20°С. For microscopic analysis, aliquots of 10 mL of water were filtered through nitrocellulose filters with a pore diameter of 0.22 μm in the field immediately after sampling. Microscopic analysis of water samples was performed using a Micmed light microscope P-13/2(with transmitted light system, halogen lamp 12 V, ×1 350)according to st and ard procedures(Kuznetsov and Dubinina, 1989). The filters were stained with erythrosine(Kuznetsov and Dubinina, 1989). The total number of bacteria for each water sample was determined by direct cell enumeration in 20 to 40 fields chosen r and omly.

DNA was isolated with the Bacterial Genomic DNA isolation kit(Axygen), and 16S rRNA gene amplicon sequencing was performed at the Novosibirsk “Genomika” center. Bioinformatics analysis was done using the Ribosomal Database Project web site(https://rdp.cme.msu.edu/index.jsp)(Wang et al., 2007; Cole et al., 2014).

In the period of the study in March 2013, the physico-chemical characteristics of the lake were similar to those earlier recorded in the lake in this season(Zamana and Borzenko, 2009; Gorlenko et al., 2010). The water temperature in the mixolimnion(0–3.15 m)was negative and ranged from -1.2 to -0.6°C under the ice at a depth of 1.0 m, and further decreased to -1.3°C at depths of 2.0 and 3.0 m. The chemocline was 0.15–0.20 m in depth, at a water temperature of about -1.0°C. The temperature increased in the anoxic monimolimnion to -0.5°C at a depth of 4.0 m and was positive(+0.4°C)at the bottom(Fig. 2). An important feature during the study period was the oxygen-deficient(hypoxic)state of the mixolimnion(unpublished data), which was usually observed only at the end of the ice period in the end of April(Borzenko, 2013). The light level in the chemocline zone was about 0.001% of the surface value(Fig. 3). The distribution of the total bacterial abundance along the water column of the lake was similar to determined previously by Gorlenko et al.(2010)with maximum in the upper layer of the moninolimnion and was positively correlated with the temperature(Fig. 2).

Fig. 2 Distribution of water temperature ( T ) (°C) and total bacterial abundance (N, × 10 6 cells/mL) along the water column of Lake Doroninskoe in September 2007 and March 2013

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Fig. 3 Light intensities (lx) in the water of Lake Doroninskoe detected in March 2013 (from Lukyanov et al., 2014) Measurements were performed through the water column (ice thickness was 1.19 m) on March 29, 2013 from 2 to 3 p.m.

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Metagenomic analysis of the microbial communities of the water column revealed the presence of representatives of 11 major Eubacteria phyla: Cyanobacteria, Acidobacteria, Actinobacteria, Proteobacteria, Verrucomicrobia, Tenericutes, Firmicutes, Bacteroidetes, Planctomycetes, Spirochaetes, Deinococcus-Thermus, 5 phantom phyla: TM7, SR1, BRC1, WS3, OD1, and one phylum of Archaea—the Euryarchaeota. Five phyla were dominant: Proteobacteria, Bacteroidetes, Actinobacteria, Cyanobacteria, and Firmicutes(Fig. 4). A negative correlation was found between the distribution of the Proteobacteria-Actinobacteria and the Bacteroidetes-Firmicutes-Cyanobacteria at different depths in the water column.

Fig. 4 Correlation of distribution of five dominant phyla along the water column of Lake Doroninskoe during the ice period in March 2013

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In the aerobic mixolimnion representatives of the phylum Actinobacteria numerically dominated, constituting 29.1% and 30.8% of the community in the upper and lower layers, respectively(Fig. 5). Moreover, the ratio of the dominant families in the upper and lower layers was similar. In the upper layer the Microbacteriaceae dominated(78.0%), while in the lower layers their fraction was reduced by almost 2-fold, whereas the fraction of Nitriliruptor increased and consisted of 1/3 of the total phylum Actinobacteria. In the redox transition zone of the chemocline their fraction in the community was sharply reduced to 9.0%, remaining at the level of 4.2% in the sulfide layer. In the anoxic monimolimnion the fraction of Actinobacteria in the community was 25.5%, dominated by the Nitriliruptor and family Microbacteriaceae, and their diversity was low.

Fig. 5 Distribution of Actinobacteria along the water column of Lake Doroninskoe during the ice period in March 2013 Relative abundance of selected taxons is presented in % of total bacteria.

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The Firmicutes constituted about 1.5% in the upper layer of the aerobic mixolimnion(Fig. 6). At the depths of 1.0 and 2.0 m the fraction of Firmicutes increased significantly and consisted of 14.9% and 24.5%, respectively, and Clostridia was the dominant class. In the lower layer of the oxic mixolimnion their abundance was reduced to 0.9%, but it increased to 13.4% in the chemocline and reached 19.7% at the upper layer of the anoxic monimolimnion. A low content of Firmicutes was again detected at 4.0 and 5.0 m depth(3.0% and 0.7%, respectively), and it increased sharply to 16.0% in the lower layer of the monimolimnion. Interestingly, with increasing of their abundance, the diversity of Firmicutes increased significantly with Clostridia as the dominant class(59.0%).

Fig. 6 Distribution of Firmicutes along the water column of Lake Doroninskoe during the ice period in March 2013 Relative abundance of selected taxons is presented in % of total bacteria.

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The dominant classes of the phylum Proteobacteria in an oxic zone were Alpha-, Beta- and Gammaproteobacteria; their fractions of the total microbial community differed significantly in the upper and the lower layers and decreased from 49.6% to 39.1%, 2.7% to 2.1%, and 4.5% to 2.1%, respectively(Fig. 7). Representatives of the orders Rhodobacterales and Rhizobiales of the Alphaproteobacteria were dominant at 2.6 m depth, and their fractions significantly decreased in the chemocline and in the sulfide layer. The main group consisted of purple nonsulfur bacteria of the family Rhodobacteraceae(Rhodobaca), which are able to grow anaerobically in light or aerobically in the dark with various organic compounds as electron donors and nitrogen sources. The heterogeneity and dynamics of the ecological conditions in the redox zone led to an increase in the diversity of representatives of the phylum Proteobacteria, but their fraction in the total microbial community decreased from 43.5% in the lower layer of the oxic zone to 27.4% in the redox zone and remained at the level of 15.0%–16.0% in the sulfide zone(Fig. 4). In the chemocline Proteobacteria were represented by all classes, with dominance of Gammaproteobacteria(13.7% of all Bacteria and 49.8% of all Proteobacteria). At the upper layer of the sulfide zone of the moninolimnion, Deltaproteobacteria were dominant and constituted 47.0% of the Proteobacteria and 7.8% of all Bacteria(Figs.4, 7). Additionally, representatives of order Oceanospirillales, class Gammaproteobacteria were detected in this zone; among them the genera Marinospirillum and Nitrincola were dominant. Detection of representatives of Deltaproteobacteria such as Desulfuromusa(Desulfuromonadales), Desulfonatronum, Desulfonatronovibrio(Desulfovibrionales) and Desulfurivibrio(Desulfobacterales)in both the chemocline and the sulfide zones indicated presence of active processes of the sulfur cycle—the process of sulfate reduction. In the upper layer of the sulfide zone their fraction was only 0.15% and decreased to 0.05% at 5.0 m and again increased to the bottom to 49.9%(Fig. 7).

Fig. 7 Distribution of Proteobacteria along the water column of Lake Doroninskoe during the ice period in March 2013 Relative abundance of selected classes is presented in % of total proteobacteria.

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Cyanobacteria in the upper layer of the mixolimnion formed 6.0% of total microbial community(Fig. 4). The dominant form was the unicellular Prochlorococcus, which is known as an oligotrophic specialist, having the smallest cell size(0.4 to 1.2 μm diameter) and genome((1.7–2.5)×10 ⁶ bp)of all known photoautotrophs(Dufresne et al., 2005; Scanlan et al., 2009), and is high tolerant to light(Kettler et al., 2007). At the upper layer of the oxic mixolimnion their fraction in the community increased sharply, reaching 15.4%. The dominant group of oxygenic phototrophic, chlorophyll a -containing cyanobacteria is represented by phantom genus GpIIa, which included representatives of the cyanobacterial genera Aphanocapsa, Aphanothece, Chlorogloea, Cyanobium, Merismopedia, Microcystis, Paulinella, Prochlorococcus, Synechococcus, and Synechocystis . Cyanobacteria decreased in the redox zone to 11.4% because of the reduced light level(0.001% of the surface intensity). Their fraction increased almost 2-fold up to 16.8% in the anoxic monimolimnion(Fig. 4). This accumulation may be due to deposition and concentration of dead cells.

Bacteroidetes are well known as degraders of polysaccharides and proteins(Thomas et al., 2011), and are used as indicators of these processes in extreme ecosystems(Sorokin et al., 2011). Their content varied significantly at different depths of the lake. Thus, in the upper and lower layers of the mixolimnion they constituted from 4.0% to 34.0% of the total microbial community, respectively(Fig. 4); their abundance increased in the chemocline to 29.9%, and varied in the anoxic monimolimnion from 24.0% in the upper layer to a maximum of 40.1% in the bottom layer. It is interesting to note that their maximal diversity was detected in the chemocline zone, with almost 90% being unclassified Bacteroidetes. This large fraction of uncultivable microorganisms confirms the uniqueness of the Bacteroidetes. Gracilimonas and Sediminibacterium, belonging to the class Sphingobacteria, were dominant at a depth of 2.6 m.

Verrucomicrobia made up 0.9% of total microbial community in the upper layer of the oxic mixolimnion(Fig. 8), and Verrucomicrobiae were the dominant class(67.0%). In the lower layer of the mixolimnion, the redox zone and the upper layer of the monimolimnion their fraction in the community increased, and was 2.1%, 1.2% and 1.2%, respectively. The most diverse and numerical dominant class was the Spartobacteria. In the lower layer of the monimolimnion, Verrucomicrobia constituted only 0.5% of the community, and Opitutae was the dominant class(74.0%).

Fig. 8 Distribution of minor bacterial phyla along the water column of Lake Doroninskoe during the ice period in March 2013 Relative abundance of selected phyla is presented in % of total proteobacteria.

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Previously it was shown that the sulfur cycle in Lake Doroninskoe is carried out by sulfate-reducing, sulfur-oxidizing chemotrophic and phototrophic, anoxygenic phototrophic bacteria and other sulfur loving microorganisms, some of which were isolated in pure cultures(Gorlenko et al., 2010). Microcosm experiments confirmed that the dominant bacterial morphotypes in the brightly pink-colored anaerobic layer of the chemocline were similar to the morphotypes of phototrophic sulfur and non-sulfur purple bacteria detected under natural conditions in the chemocline zone of the lake(Matyugina et al., 2014). During the ice period in the lake chemocline anoxygenic phototrophs of the genus Rhodobaca are dominant, but they do not produce bacteriochlorophyll pigments because of the low light level in this zone; instead they use an aerobic chemotrophic metabolism. Therefore no pink-colored layer was observed.

The main microbiological investigations of Lake Doroninskoe were performed in September during the open water period(Gorlenko et al., 2010). Typical representatives of alkaliphilic chemoorganotrophic aerobic, anoxygenic phototrophic, and alkaliphilic sulfur-oxidizing bacteria were isolated from different layers of the lake(Gorlenko et al., 2010). The first studies of the lake’s microbial community during the longest and the most stable subglacial period were carried out in 2009(Matyugina, unpublished data). Based on 16S rRNA gene sequence analysis 50% of the sequences retrieved from the aerobic zone(from the surface to 3.4 m)belonged to unidentified and uncultured bacteria, 25% to Actinobacteria(Nitriliruptor alkaliphilus), and 25% to the Gammaproteobacteria species Thioalkalimicrobium sp. and Thioalkalimicrobium microaerophilum, known as aerobic chemolithoautotrophs capable of oxidizing inorganic compounds. In the chemocline zone, at a depth of 3.4 m in March 2009, the dominant(45%)organisms were non-sulfur purple bacteria(Rhodobaca barguzinensis, 30%), and an aerobic bacteriochlorophyll a -containing species(Roseinatronobacter monicus, 15%), belonging to the Alphaproteobacteria. Bacteroidetes made up 40%, but all sequences attributed to this group represented yet-uncultured bacteria. The fraction of the Gammaproteobacteria was 15%, and was dominated by Thiocapsa imhoffii(Chromatiaceae). This alkaliphilic purple sulfur bacterium is able to grow photoheterotrophically in the presence of sulfide or thiosulfate with a limited number of organic carbon sources, but photoautotrophic growth requires an increased concentration of CO ₂(Asao et al., 2007).

Important abiotic characteristics of the lake ecosystem in all seasons were described by Borzenko and colleagues(Zamana and Borzenko, 2007, 2009; Borzenko and Zamana, 2008, 2011; Borzenko, 2013). Based on long-term hydrochemical investigations it was found that Lake Doroninskoe belongs to a rare type of saline soda lakes. It is located in an area of sedimentary deposition(Borzenko and Zamana, 2008). It differs from the known world meromictic reservoirs of this type, which are mainly located in zones of volcanic deposits(Grant and Tindall, 1980; Tindall, 1988; Grant et al., 1990; Sorokin et al., 2014). It is known that water of Lake Doroninskoe formed in conditions of evaporative concentration. The salinity of the lake water is at least 27 g/L. The sources of water supply are atmospheric and underground waters where bicarbonates, sulfates and chlorides dominate. During evaporation the accumulation of mineral compounds in the lake water should occur in the same proportions. However, according to the seasonal and annual dynamics of hydrochemical composition of water, the carbonate concentration increases more quickly than chlorides, and sulfates do not accumulate at all. This is possible only if there are additional processes that infl uence the water composition. In this case, an additional source of carbon in carbonates might be organic carbon(Borzenko, 2013), and sulfates are involved in the sulfur cycling with the main process of sulfate reduction(Borzenko and Zaman, 2008). The main processes by which inorganic carbon may be a source of organic carbon in the lake are photo- and chemosynthesis. The total rate of anoxygenic and oxygenic photosynthesis during ice period consists 0.99 mg C/(L×day), thereas chemosysnthesis—0.100 mg/(L×day)(Borzenko et al., 2014b). Thus, the characterization of the chemical composition of waters(Zamana and Borzenko, 2007, 2009; Borzenko and Zamana, 2008, 2011) and the detailed studies of the cultured microbial community of the three main zones during one of the most dynamic periods of open water—September(Gorlenko et al., 2010)have been done previously. These data, combined with metagenomic analyses, during the relatively stable subglacial period allowed an analysis of the microbial community of Lake Doroninskoe, including comparisons with other meromictic lakes. The study of the chemocline zone, where key biogeochemical transformations by the microorganisms such as sulfide oxidation take part, is of special interest. Due to these bacterial activities sulfur is cycled through the chemocline.

The dominant species of microorganisms in the chemocline zone of other meromictic lakes of the world are sulfur-oxidizing chemoautotrophs, anoxygenic photoheterotrophic bacteria, anoxygenic photoautotrophic bacteria, iron-reducing bacteria, iron-oxidizing bacteria, and sulphate-reducing bacteria(Table 1). The dominant role in the chemocline in different climatic zones and different environmental conditions can be performed by anoxygenic phototrophic bacteria and by chemoautotrophs. In the chemocline zone of the studied lakes anoxygenic phototrophic bacteria including purple sulfur bacteria(Gammaproteobacteria) and green sulfur bacteria of the phylum Chlorobiare dominant. This dominance can be explained by a combination of the physiological features of these organisms(Frigaard and Dahl, 2009) and favorable environmental conditions for the fulfillment of their basic ecological role in the sulfur cycle as important anaerobic sulfur oxidizers. Purple non-sulfur bacteria of the Alphaproteobacteria are dominant or co-dominant only in few meromictic lakes: Soap Lake(Washington, USA), Great Salt Lake(Utah, USA) and Lake Doroninskoe(Zabaikalie, Russia). They are predominantly chemoheterotrophs, but have the potential to produce photosynthesis pigment-protein complexes and perform lightmediated electron transport(Imhoff, 2008). In the chemocline of extremely acidic metal-rich stratified pit lakes dominate chemoautothrophic and chemoheterothrophic microorganisms(Falagán et al., 2014). They participate in the transformation of carbon, iron, and sulphur, and use redox transformations of iron, rather than solar radiation. Microbial communities in chemocline include Acidithiobacillus ferrooxidans, which can both oxidize and reduce iron, as well as oxidize reduced sulfur, and others that have more specialized roles in iron/sulfur biogeochemistry(e.g. ferrous ironoxidizing Leptospirillum, sulfur-oxidizing Thiomonas, and sulfate-reducing Desulfomonile). Many different species of Bacteria found in this zone(e.g. Acidocella, Acidobacteriaceae, and Acidithiobacillus)can reduce ferric iron under micro-aerobic and anaerobic conditions(Falagán et al., 2014). The presence of these organisms suggests an increase in the level of physiologically versatile organisms capable of obtaining energy under variable redox conditions in this part of the water column. In each geographic environment special conditions have thus been created, which are refl ected in the dominance of different bacterial groups in the lake chemocline; however, they perform similar ecological functions.

Table 1 Diversity of dominant taxa of bacteria in different meromictic lakes of the world

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Strong stratification of the microbial communities during the ice period in the meromictic Lake Doroninskoe was documented. Metagenomic analysis of the microbial communities of the water column revealed representatives of 11 major Eubacteria phyla: Cyanobacteria, Acidobacteria, Actinobacteria, Proteobacteria, Verrucomicrobia, Tenericutes, Firmicutes, Bacteroidetes, Planctomycetes, Spirochaetes, Deinococcus-Thermus, 5 phantom phyla: TM7, SR1, BRC1, WS3, OD1, and one phylum of Archaea—Euryarchaeota. In the oxic zone of the mixolimnion the dominant phyla were Proteobacteria, Actinobacteria, Bacteroidetes, and Cyanobacteria, in chemocline zone—Bacteroidetes, Proteobacteria, and Cyanobacteria, while in the anoxic monimolimnion Proteobacteria, Actinobacteria, and Bacteroidetes were predominant. In general, the microbial community of meromictic soda Lake Doroninskoe in the contact zone of aerobic and sulfide-containing layers were characterized by a high microbial diversity with dominance of metabolically flexible bacteria with capacity to switch between anoxygenic photosynthesis and aerobic chemotrophic metabolism that belong to the Rhodospirillaceae and the Rhodobacteraceae, class Alphaproteobacteria.