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Researchers from the Faculty of Biology of Moscow State University, as part of a team of authors, conducted a study of microbial communities of bottom sediments of the Kandalaksha Bay of the White Sea. It was possible to identify a pronounced stratification of microbial communities of bottom sediments, which is explained by the ability of microorganisms to use either labile, easily digestible organic compounds (upper layers of sediments), or, under conditions of lack of oxygen, decompose complex polymer substrates (layers of bottom sediments at a depth of 30-50 cm). Among the latter group, many microorganisms have been found that are unknown in laboratory cultures and presumably possess hydrolytic enzymes valuable for biotechnology. The results of the study, supported by the grant of the Ministry of Education and Science No. 075-15-2021-1396 within the framework of the national project "Science and Universities", are published in the journal Microbiology.

In the Kandalaksha Bay of the White Sea, organic matter is synthesized by phytoplankton and macroalgae, and also comes from river drains from the continent, causing the further formation of complex trophic chains, including diverse marine fauna. However, part of the organic matter falls to the bottom of the bay, where complex biomolecules are decomposed by bacteria and archaea and the return of biogenic elements to their biogeochemical cycles. Organotrophic microorganisms thus play an important role in maintaining balance in marine ecosystems. Despite the fact that various microbiological studies have been conducted in the Kandalaksha Bay for a long time, the composition of microbial communities of bottom sediments there remained unexplored. The purpose of the study of microbial communities in the Kandalaksha Bay was to determine the composition of microbial communities at different points of the bay and at different depths from the sediment surface. Sampling was carried out in June-July 2022 during marine expeditions on scientific vessels using a 3-meter gravity core. Samples were taken from the cores corresponding to depths of 2, 10, 30 and 50 cm below the seabed. A total of 55 samples were selected. DNA was isolated from all samples and polymerase chain reaction was performed with primers specific to variable regions of the 16S ribosomal RNA (rRNA) gene. In the 80s of the twentieth century, this gene was chosen as a marker to determine the place of a bacterium or archaea in the general system of prokaryotes. Since then, the application of 16S rRNA gene analysis to natural samples has changed our understanding of the diversity of prokaryotes, revealing many new microorganisms that were previously uncultivated in the laboratory.

The nucleotide sequences of amplification products obtained from the White Sea samples were determined by high—throughput sequencing (NGS - new generation sequencing), and the corresponding microorganisms were identified by comparison with databases. As a result, horizontal and vertical profiles of microbial communities of the bottom sediments of the White Sea were obtained. It turned out that according to the sequencing results, all samples can be divided into two groups: the main part of the samples had pronounced stratification, and the communities of the lower sediments differed significantly from the upper ones. However, a small part of the samples were taken in basins where a large amount of organic matter had accumulated, which was actively decomposing. There was no stratification in these samples, and the composition of microorganisms in them in all layers coincided with the composition of communities in the upper layers of stratified samples (2 and 10 cm). Thus, it became clear that both along the entire thickness of the samples from the basins and in the upper layers of the remaining samples, there is an active decomposition of readily available organic matter.

The main dominant groups of microorganisms in these samples were bacteria of the genus Woesia, as well as uncultivated organotrophic bacteria of the family Sandaracinaceae and the order Actinomarinales. Among the sulfate reducers found in the upper sediment horizons, the most numerous groups were SEEP-SRB1 and Sva0081, uncultivated microorganisms widespread in marine sediments. In the lower layers of stratified sediments (30 and 50 cm), the proportion of microorganisms potentially involved in the degradation of complex compounds — representatives of the family Hyphomicrobiaceae, genera Mycobacterium and Desulfatiglans - increased. The proportion of prokaryotes with unknown metabolism, such as JS1, SG8-4, WCHB1-81, S085, common in marine sediments around the world has also increased. "We were able to identify target groups of microorganisms possessing hydrolytic enzymes, with the help of which complex biopolymers buried in anaerobic conditions are decomposed," explained Elizaveta Bonch—Osmolovskaya, Head of the Department of Microbiology at the Faculty of Biology of Moscow State University. – These are polymers of macroalgae — agar and alginate, polymer of invertebrate animal shells — chitin and lignin entering the seabed with effluents from land. Metagenomic analysis of these same communities will provide information on possible ways to cultivate unknown microorganisms, as well as provide direct access to new hydrolytic enzymes that are very important for modern biotechnology."

Information provided by the press service of the Moscow State University

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