Diversity and Structure of Fungal and Heterotrophic Bacterial Communities in Surface Benthic Sediments of the Kara Sea.

Fungal and prokaryotic communities in unique ecosystems of Arctic seas are important to study for understanding global biogeochemical cycles and developing approaches to bioremediation of the ecosystems. High-throughput sequencing of the variable regions ITS1/ITS2 (in fungal genomes) and V3-V4 of the 16S rRNA gene (in bacterial genomes) was performed to study the species composition and taxonomic structure of fungal and heterotrophic bacterial communities in surface benthic sediments of the Kara Sea from depths of 16-417 m. The fungal biome was dominated by operational taxonomic units (OTUs) of Ascomycota (more than 50% of ITS reads in each of the 12 samples), which was followed by the division Basidiomycota (10-20%). Сhytridiomycota accounted for no more than 2% of ITS reads. No significant differences in the mycobiome structure of Kara Sea benthic sediments were found depending on the sampling depth. OTUs of Sordariomycetes and Eurotiomycetes (Aspergillaceae) prevailed. Basidiomycetes were represented mainly by yeasts of the families Filobasidiaceae, Malasseziaceae, Sporidiobolaceae, and Tremellaceae. According to fluorescence microscopy, the total number of fungal propagules ranged 207-546 thousand spores and mycelial fragments per 1 g benthic sediment and was minimum at greater depths. At all of the stations examined, sediments were found to contain numerous aerobic heterotrophic bacteria belonging to various families, orders, and classes of the phyla Pseudomonadota, Actinomycetota, Bacteroidota, Verrucomicrobiota, Gemmatimonadota, Myxococcota, and Acidobacteriota. Interestingly, nucleotide sequences characteristic of strict anaerobes were detected in oxidized surface benthic sediments of the Kara Sea. The set included sulfate-reducing bacteria of the phylum Thermodesulfobacteriota and chemoheterotrophic bacteria of Anaerolineae (phylum Chloroflexota). The data obtained in the study significantly expand our knowledge of the diversity of fungi and bacteria, which are key heterotrophic organisms that destroy organic matter in benthic sediments of Arctic seas.