Bacterial Communities of Microbial Mats of the White Sea Supralittoral and of the Littoral of the Lakes Separated from the Sea

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Conditions of formation and phylogenetic composition were studied for pigmented biofilms and microbial mats of the Kandalaksha Bay (White Sea) supralittoral and of the littoral of the lakes separated from the sea. During the sampling period, salinity was 15 to 26 g/L, except for several desalinated salt marsh sites (4‒11 g/L); the temperature was 9‒12°C. The species composition and structure of benthic phototrophic communities were affected by severe climatic conditions of the area, including low average annual temperature and freezing of the littoral zones. Application of Next-generation sequencing of the 16S rRNA gene V3-V4 regions combined with microscopy of the samples and obtained cultures for analysis of the biodiversity of microbial communities resulted in improved understanding of diversity of both oxygenic and anoxygenic bacteria in the Kandalaksha Bay littoral. No significant differences were revealed in species composition of microbial mats from salt marshes and shallow sites of the lakes of marine origin. Members of the phyla Bacteroidetes (up to 36%) and Proteobacteria (up to 67%) predominated in the mats and biofilms. The share of phototrophic bacteria was 0.3–18%. Oxygenic phototrophs (cyanobacteria Phormidium, Oscillatoria, Spirulina, and Anabaena, as well as diatoms) predominated in the phototrophic community. The species number of anoxygenic phototrophic bacteria did not exceed 5% of all prokaryotes. These were mainly mesophilic marine species of purple sulfur bacteria of the genera Thiorhodococcus and Thiocapsa. No members of the family Ectothiorhodospiraceae, which are typical of microbial mats from southern seas, were found in subpolar microbial mats. Nonsulfur purple bacteria and aerobic anoxygenic phototrophic bacteria were mostly similar to the known marine species. Green sulfur bacteria (salt-water Prosthecochloris and Сhlorobium species) were detected in two coastal mat samples from meromictic lakes. Anoxygenic filamentous phototrophic bacteria, which occurred in almost all benthic phototrophic communities, were represented by new phylotypes.