Abstract
In this work, the first-ever metagenomic study of the microbial community from the deep-underground saline spring located at the Baksan Neutrino Observatory (BNO) (Kabardino-Balkaria, Russia) is presented. Using the metagenomic approach, we obtained 19 metagenome-assembled genomes (MAGs) attributed to the phyla Pseudomonadota (the dominant phyla), Planctomycetota, Myxococcota, Nitrospirota, Gemmatimonadota, Armatimonadota, and Cyanobacteriota. Archaea are generally absent in the metagenome. The microbial community of the Baksan Neutrino Observatory demonstrates a high metabolic diversity, including carbon dioxide-fixing, methane-oxidizing, dinitrogen-fixing, nitrate- and iron-reducing, anammox, nitrifying, and predatory bacteria. Hydrogen, methane, ammonia, and reduced iron compounds, present in the ecosystem, provide energy for primary organic production. The abundance and diversity of bacteria capable of carrying out various stages of the nitrogen cycle suggest that nitrogen compounds are of great significance for microbial community metabolism. On the basis of the Genome Taxonomy Database Toolkit classification of MAGs and comparison to the closest RefSeq genomes, we have identified six new genera, with the proposed names-"Candidatus Jinrbaksania," "Candidatus Neutrinellum," "Candidatus Jinrextremum," "Candidatus Inrsubterrania," "Candidatus Inralta," and "Candidatus Neutrinobacter." Comparative analysis with metagenomes of microbial communities from the deep underground granitic sites and karst caves reveals that the BNO microbial community represents a unique transitional ecosystem on the boundary between the deep anoxic and surface aerobic biosphere.IMPORTANCEThe deep biosphere makes up 12-20% of the Earth's biomass and is poorly studied due to its inaccessibility. To date, only a few metagenomic studies of local deep biospheres have been performed in Russia. The Baksan Neutrino Observatory (BNO) is a deep-underground laboratory, with some abandoned tunnels. One of them hosts a mineral spring saturated with volcanic gases from the peripheral magma chamber of Mount Elbrus. The metagenomic analysis of the biofilm from this mineral spring has revealed the presence of unique microbial community whose composition occupies a transitional position between deep-underground microbial communities and communities of karst caves. We believe that this study of the microbial metagenome of the saline spring of the BNO will make a valuable contribution to understanding the composition and functioning of microbial communities formed at the oxic-anoxic interface.