Lignite coal burning seam in the remote Altai Mountains harbors a hydrogen-driven thermophilic microbial community

Thermal ecosystems associated with underground coal combustion sites are rare and less studied than geothermal features. Here we analysed microbial communities of near-surface ground layer and bituminous substance in an open quarry heated by subsurface coal fire by metagenomic DNA sequencing. Taxonomic classification revealed dominance of only a few groups of Firmicutes . Near-complete genomes of three most abundant species, ‘ Candidatus Carbobacillus altaicus’ AL32 , Brockia lithotrophica AL31, and Hydrogenibacillus schlegelii AL33, were assembled. According to the genomic data, Ca . Carbobacillus altaicus AL32 is an aerobic heterotroph, while B. lithotrophica AL31 is a chemolithotrophic anaerobe assimilating CO 2 via the Calvin cycle. H. schlegelii AL33 is an aerobe capable of both growth on organic compounds and carrying out CO 2 fixation via the Calvin cycle. Phylogenetic analysis of the large subunit of RuBisCO of B. lithotrophica AL31 and H. schlegelii AL33 showed that it belongs to the type 1-E. All three Firmicutes species can gain energy from aerobic or anaerobic oxidation of molecular hydrogen, produced as a result of underground coal combustion along with other coal gases. We propose that thermophilic Firmicutes , whose spores can spread from their original geothermal habitats over long distances, are the first colonizers of this recently formed thermal ecosystem.