Diversity and structure of prokaryotic communities in the surface layers of bottom sediments in the Kandalaksha Bay of the White Sea

Microbial communities of the bottom sediments of the Kandalaksha Bay in the White Sea are still poorly studied. There is some information in the literature about the rate of some microbiological processes, but very little is known about the microorganisms inhabiting the bottom sediments. In this study, microbial communities of 47 samples of bottom sediments from a depth of 10 cm below the seabed surface in the Kandalaksha Bay were characterized using high-throughput sequencing of variable regions of the 16S rRNA gene. The most numerous phyla were Pseudomonadota and Desulfobacterota, which together accounted for about half of all prokaryotes. It was impossible to identify any taxon at the genus level as dominant. Among chemoorganotrophic organisms, the most represented were the uncultivated Sandaracinaceae (up to 10.8%) and Woeseia (up to 7.5%). Sulfate-reducing bacteria were an important component of the communities even in the investigated surface layers, with the most numerous being uncultivated groups SEEP-SRB1 (up to 7.0%) and Sva0081 (up to 5.9%). The genera Sulfurovum (up to 15.5%) and Thiohalophilus (up to 7.0%), involved in the oxidation of sulfur-containing compounds, were important members of the communities in some of the investigated samples. The genus Nitrosopumilus (up to 6.9% of the total number of prokaryotes), which carries out the oxidation of ammonia to nitrite, occupied the largest proportion of the sequences assigned to the Archaea domain. Many sequences identified as chloroplast 16S rRNA gene regions were found in the investigated samples, indicating that the rates of organic matter accumulation in the upper part of the bottom sediments may be higher than the rates of its decomposition.