Biogeochemical Impact of Thermal Abrasion of Frozen Rocks on the Arctic Coast

The biogeochemical consequences of the destruction of permafrost horizons due to thermal abrasion of coastal cliffs was assessed for the Mamontovyi Khayata outcrop (Bykovsky Peninsula, Northern Yakutia). The study employed an ecosystem-based approach, which included characterizing the state of the rocks at the time of sampling, lithological description of the frozen sediments, evaluation of the organic matter content and the presence of methane, and description of the taxonomic diversity of microbial communities buried in the permafrost horizons. The studied samples consisted of clayey aleurite, which, in addition to rock-forming elements, contained biogenic elements: $${\\text{NH}}_{4}^{ + }$$ , $${\\text{NO}}_{2}^{ - }$$ , $${\\text{NO}}_{3}^{ - }$$ , $${\\text{PO}}_{4}^{{3 - }}$$ , acetate, and other organic compounds in various proportions. Methane was present in all samples, and its concentrations correlated with the organic matter content in the samples. 16S rRNA gene profiling showed that microbial communities buried in the frozen sediments had a diverse species composition and included both eubacteria and archaea. The dominant bacterial phyla in the communities were Chloroflexota, Actinobacteriota, Acidobacteriota, Pseudomonadota, Bacillota, and Actinomycetota. Archaeal communities consisted mainly of methanogens, as well as anaerobic methanotrophs. The dominant methanogen groups represented the genera Methanosarcina and Methanothrix (order Methanosarcinales) and uncultured Rice Cluster II (order Methanomicrobiales); among them, there were both acetoclastic and hydrogenotrophic methanogens. The detected methanotrophic archaea were phylogenetically close to group ANME-2d (Candidatus Methanoperedens). No methanotrophic bacteria were detected in the bacterial communities. Viable microbial cells were identified in all samples. The presence of viable cells, available biogenic elements, and organic substrates suggests the possibility of rapid activation of microorganisms from various physiological groups following the thawing of frozen soils, which could make a significant contribution to the transformation of organic matter and the emission of greenhouse gases into the atmosphere.