Microbial community composition and carbon cycling processes in cascadian alpine lakes of the Central Caucasus

In August 2023, studies of microbial communities in the water column of a cascade of high-mountain lakes in the Western Caucasus (Lake Klukhorskoye) were conducted. Data on the composition of microbial communities and the biogeochemical activity of microbial processes were obtained and analyzed. The lakes of the cascade (Bolshoye, Maloye and Vostochnoye Klukhorskie) are located at an altitude of 2670–2980 m and are ultra-fresh and oligotrophic, similar in hydrochemical composition, but differing in temperature and water transparency. The value of primary photosynthesis production was 5.3–8.0 μg C l–1 day–1, dark carbon dioxide assimilation 0.5–1.2 μg C l–1 day–1. The values of the total number of microorganisms varied from 70 to 150 thousand cells ml–1. In the oligotrophic waters of the Klukhor Lakes, the bacterial phyla Actinobacteriota, Bacteroidota, and Pseudomonadota, representing mainly aerobic heterotrophic groups, dominated in the microbial community. Archaea constituted less than 1% of the community. Microorganisms that are indicators of turbid and transparent waters of the studied lakes were identified. These are aerobic Gram-positive bacteria of the Ilumatobacteraceae family, psychrophilic bacteria of the Comamonadaceae family – important components of glacial microbiomes, and oligotrophic bacteria of the Sphingomonadaceae family, resistant to solar radiation, including ultraviolet radiation. The microalgae community of the water column of Lake Bolshoe Klukhorskoye included representatives of green microalgae, diatoms Pinnularia, chrysomonads, and cryptophytes. The composition of microalgae in the three lakes differed significantly. The cyanobacteria of the transparent Maloye Klukhorskoye Lake were poorer than those of the Bolshoe Klukhorskoye Lake. In the Bolshoe Klukhorskoye Lake, copepods Centropages sp. and Acanthodiaptomus denticornis, adapted to extreme high-mountain conditions, including low temperatures and high ultraviolet radiation, were found in the water column. The results of the work indicate that the formation of new periglacial lakes and the loss of connection with glaciers during their retreat leads to a change in the structure of plankton communities during the transition from turbid glacial lakes to transparent ones. The composition of microbial communities of water bodies and their biogeochemical productivity can be indicators of large-scale and local climate change.