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Phototrophic bacterial mats from Kiran soda lake (south-eastern Siberia) were studied using integrated approach including analysis of the ion composition of water, pigments composition, bacterial diversity and the vertical distribution of phototrophic microorganisms in the mats. Bacterial diversity was investigated using microscopic examination, 16S rRNA gene Illumina sequencing and culturing methods. The mats were formed as a result of decomposition of sedimented planktonic microorganisms, among which cyanobacteria of the genus Arthrospira predominated. Cyanobacteria were the largest part of phototrophs in the mats, but anoxygenic phototrophs were significant fraction. The prevailing species of the anoxygenic phototrophic bacteria are typical for soda lakes. The mats harbored aerobic anoxygenic phototrophic bacteria, purple sulfur and non-sulfur bacteria, as well as new filamentous phototrophic Chloroflexi. New strains of Thiocapsa sp. Kir-1, Ectothiorhodospira sp. Kir-2 and Kir-4, Thiorhodospira sp. Kir-3 and novel phototrophic Chloroflexi bacterium Kir15-3F were isolated and identified.

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We present the results of a study of mesophilic anoxygenic phototrophic Chloroflexota bacteria from Mechigmen hot spring (the Chukotka Peninsula) and Siberia. According to 16S rRNA phylogenetic analysis, these bacteria belong to Oscillochloris trichoides. However, sequencing the draft genome of the bacterium from the Chukotka and analysis of the average nucleotide identity, as well as in silico DNA-DNA hybridization, reveal that this bacterium belongs to a novel species within the Oscillochloris genus. We, therefore, propose ‘Candidatus Oscillochloris fontis’ as a novel taxon to represent this mesophilic alkaliphilic anaerobic anoxygenic phototrophic bacterium. Spectrophotometry and high-performance liquid chromatography analysis show that the bacterium possesses bacteriochlorophylls c and a, as well as lycopene, β-carotene and γ-carotene. In addition, transmission electron microscopy shows the presence of chlorosomes, polyhydroxyalkanoate- and polyphosphate-like granules. The genome of ‘Ca. Oscillochloris fontis’ and the Siberian strains of Oscillochloris sp. possess the key genes for nitrogenase complex (nifH) and ribulose-1,5-bisphosphate carboxylase/oxygenase (cbbL), as previously described for O. trichoides DG-6. The results presented here, and previously published data, show that Oscillochloris bacteria from different aquatic environments have the potential for CO2 and N2 fixation. Additionally, we describe a new primer system for the detection of RuBisCo form I.

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