Microorganisms in the Sediments of Lake Baikal, the Deepest and Oldest Lake in the World

The review summarizes the results of recent studies of microbial communities of the Lake Baikal sediments obtained using diverse techniques. In the sediments of the areas of stable sedimentation metabarcoding revealed predominance of members of the phyla Alpha- and Gammaproteobacteria (including Betaproteobacteriales), Bacteroidetes, Acidobacteria, Verrucomicrobia, and Thaumarchaeota, which are also common in other freshwater lakes. In the areas of discharge of gas-bearing mineralized fluids, the structure of microbial communities varied depending on the presence of electron acceptors and intensity and component composition of gas-bearing fluids responsible for microbial migration from the deep zone to the upper sediment layers and vice versa. Methanogenic archaea detected in Baikal sediments belonged to the groups capable of all four known catabolic pathways of methanogenesis: hydrogenotrophic, acetoclastic, methylotrophic, and hydrogen-dependent methylotrophic ones. Predominant members of the Baikal archaeal community, hydrogenotrophic methanogens of the family Methanoregulaceae (genera Methanoregula and Methano-sphaerula, as well as uncultured lineages), hydrogen-dependent methylotrophic archaea of the order Methanomassiliicoccales, and acetoclastic methanogens of the family Methanosaetaceae (genus Methanothrix (Methanosaeta)), were the same as in methanogenic communities of other freshwater lakes. Experimental evidence was obtained for anaerobic methane oxidation (AOM) via the nitrate- and nitrite-dependent pathways by archaea of the ANME-2d subcluster and bacteria of the phylum NC10. Structures of the 16S rRNA genes, mcrA, and pmoA exhibited high identity to those of the known freshwater organisms performing this process. Diversity of microbial communities at the sites of natural oil seepage differed at the order and family levels, as well as by the presence of alkane hydroxylases in the genes of the cultured species.