Bacterial Communities of the Selenga River, Its Tributaries, and the Selenga Shallows in the Summer of 2021

The first data on the diversity and structure of bacterial communities in the Selenga River, its tributaries, and delta waters (Selenga shallows) during the summer high-water period of 2021 were obtained. The chemical indices (alkalinity, mineralization, as well as concentrations of sulfate and chloride ions) in the Russian part of the Selenga River exhibited a tendency to decrease gradually in the downstream direction. As before, the chemical composition of the Selenga River waters was determined by water content and anthropogenic pressures, while the effect of tributary waters was local. At a distance of 1 km from the mouths in the mixing zone of river water and lake water, the chemical composition and taxonomic structure of bacterial communities were close to those observed in the pelagic zone of Lake Baikal. In all analyzed 16S rRNA gene libraries, nine common bacterial phyla were identified, with a predominance of Pseudomonadota (51.71–76.83%) in river communities and Actinomycetota (17.28–66.32%) in the communities of the Selenga shallows. The most significant differences in the taxonomic composition of bacterial communities were noted at the genus level. We suggest that a stable bacterial community exists throughout the Selenga River course, with dominant members representing the bacterial genera Limnohabitans, Pseudarcicella, Rhodoluna, and Polynucleobacter. The same genera also predominated in the small rivers of South Baikal. A comparative analysis of the diversity of tributary communities did not reveal any significant impact on the microbiomes of the Selenga River and Lake Baikal. Only a few riverine bacterial taxa were identified in the littoral and pelagic zones of the lake.