Microbial community of the water column of the Selenga River-Lake Baikal biogeochemical barrier

The microbial communities of the estuarine zone and the mixing zone of river and lake waters in the Selenga River estuary were studied using the fluorescence in situ hybridization (FISH) method. The microorganisms belonging to the phylogenetic group Gammaproteobacteria were found to predominate in the river estuary, constituting up to 17% of the total bacterial community. Among cultivable microorganisms, organotrophic bacteria were predominant (2040 CFU/ml) in this zone, which results in high rates of microbial production (6.0 μg C/(l day). The microbial community structure changed with distance from the river estuary; representatives of the Alpha-, Beta-, and Gammaproteobacteria were present in equal proportions; psychrotolerant and oligotrophic bacteria were numerous. The rate of heterotrophic carbon dioxide assimilation decreased to 3.8 μg C/(l day). At 5–7 km from the river estuary, where the hydrologic, physical, and chemical conditions are similar to those of lake waters, members of the Betaproteobacteria, which are typical of the open waters of Lake Baikal, are the major representatives of planktonic microorganisms.