Study of Stratification of Prokaryotic Microbial Communities in Sediments of Velikaya Salma Strait and Kandalaksha Bay, the White Sea.

The stratification of prokaryotic microbial communities in shallow and deep-water sediments was investigated in two locations of the White Sea: Velikaya Salma Strait and the deep-water part of Kandalaksha Bay. Taxonomic diversity was analyzed using 16S rRNA gene-based profiling across horizontal sediment layers down to a depth of 50 cm. A total of 55 samples were collected from 15 stations: 43 from 11 shallow-water stations (including 4 in "gas cap" areas) at 2-50 cm depths and 12 from 4 deep-water stations at 2-30 cm. CH₄ concentrations were measured for all stations, while total organic carbon (TOC) content was determined for selected sites. In Velikaya Salma Strait, the upper sediment layers were dominated by microorganisms involved in labile organic matter degradation and sulfate reduction, including Woeseia, Sandaracinaceae, Actinomarinales, SEEP-SRB1, and Sva0081. Deeper layers exhibited a shift toward taxa capable of degrading complex and recalcitrant substrates, such as Desulfatiglans, Hyphomicrobiaceae, and Mycobacterium, alongside uncultured microorganisms belonging to groups like SG8-4, WCHB1-81, Aerophobales, S085, JS1, and Anaerolineaceae. Notably, JS1 made up to half of the microorganisms in deeper layers, highlighting their ecological significance. In "gas cap"-associated sediments no pronounced stratification was observed with more homogeneous microbial community composition across all horizons. Similarly, in the deep-water part of Kandalaksha Bay, the microbial community showed minimal vertical differentiation. The dominant taxa in deep-water sediments included SEEP-SRB1, Sva0081, Sandaracinaceae, Anaerolineaceae, Woeseia, and PHOS-HE36. This study highlights the variability in microbial community structure across sediment depths and environmental conditions in Kandalaksha Bay.