Ravin, Nikolay V.

AbstractThermal 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 CO2 via the Calvin cycle. H. schlegelii AL33 is an aerobe capable of both growth on organic compounds and carrying out CO2 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.

Summary A novel moderately thermophilic, facultatively anaerobic chemoorganotrophic bacterium strain P3M ‐2 T was isolated from a microbial mat developing on the wooden surface of a chute under the flow of hot water (46° C ) coming out of a 2775‐m‐deep oil exploration well ( T omsk region, R ussia). Strain P3M ‐2 T is a moderate thermophile and facultative anaerobe growing on mono‐, di‐ or polysaccharides by aerobic respiration, fermentation or by reducing diverse electron acceptors [nitrite, F e( III ), As ( V )]. Its closest cultivated relative (90.8% rRNA gene sequence identity) is I gnavibacterium album , the only chemoorganotrophic member of the phylum C hlorobi . New genus and species M elioribacter roseus are proposed for isolate P3M ‐2 T . Together with I . album , the new organism represents the class I gnavibacteria assigned to the phylum Chlorobi . The revealed group includes a variety of uncultured environmental clones, the 16S rRNA gene sequences of some of which have been previously attributed to the candidate division ZB1 . Phylogenetic analysis of M . roseus and I . album based on their 23S rRNA and RecA sequences confirmed that these two organisms could represent an even deeper, phylum‐level lineage. Hence, we propose a new phylum Ignavibacteriae within the Bacteroidetes – C hlorobi group with a sole class I gnavibacteria , two families I gnavibacteriaceae and M elioribacteraceae and two species I . album and M . roseus . This proposal correlates with chemotaxonomic data and phenotypic differences of both organisms from other cultured representatives of C hlorobi . The most essential differences, supported by the analyses of complete genomes of both organisms, are motility, facultatively anaerobic and obligately organotrophic mode of life, the absence of chlorosomes and the apparent inability to grow phototrophically.