Similarities and Differences in the Structure of Microbial Communities of High-Altitude Cascade Lakes

High-altitude lakes are unique, vulnerable ecosystems, good indicators of the global environmental changes. This work presents the results of comprehensive analysis of microbial diversity and geochemical characteristics of the Markinskie Lakes cascade in the Teberda Biospheric Reserve (Western Caucasus, Russia). The physicochemical parameters indicated oligotrophic conditions with low concentrations of nutrients and the major ions. The results of the 16S rRNA gene sequencing showed significant differences between microbial communities of the water column and the upper sediment layers. Aerobic heterotrophs of the phyla Pseudomonadota, Bacteroidota, Actinobacteriota, and Bacillota were predominant in the water, including members of the genera Exiguobacterium and Nanopelagicales, typical inhabitants of freshwater oligotrophic ecosystems. In the sediment, microorganisms were mostly represented by anaerobic taxa: Chloroflexota, Acidobacteriota, Nitrospirota, and the Halobacteriota methanogens. Archaea were revealed only in the sediments; their dominant species was Methanosarcina subterranea. These data show pronounced spatial differentiation of microbial communities. This is the first description of microbial diversity in high-altitude lakes of Western Caucasus, improving our understanding of the structure and functioning of freshwater ecosystems under conditions of climatic and anthropogenic changes.