A metagenomic window into the 2-km-deep terrestrial subsurface aquifer revealed multiple pathways of organic matter decomposition

We have sequenced metagenome of the microbial community of a deep subsurface thermal aquifer in the Tomsk Region of the Western Siberia, Russia. Our goal was the recovery of near-complete genomes of the community members to enable accurate reconstruction of metabolism and ecological roles of the microbial majority, including previously unstudied lineages. The water, obtained via a 2.6 km deep borehole 1-R, was anoxic, with a slightly alkaline pH, and a temperature around 45°C. Microbial community, as revealed by 16S rRNA gene profiling over 2 years, mostly consisted of sulfate-reducing Firmicutes and Deltaproteobacteria, and uncultured lineages of the phyla Chlorofexi, Ignavibacteriae and Aminicenantes (OP8). 25 composite genomes with more than 90% completeness were recovered from metagenome and used for metabolic reconstruction. Members of uncultured lineages of Chlorofexi and Ignavibacteriae are likely involved in degradation of carbohydrates by fermentation, and are also capable of aerobic and anaerobic respiration. The Chlorofexi bacterium has the Wood-Ljungdahl pathway of CO2 fixation. The recently identified candidate phylum Riflebacteria accounted for 5%-10% of microbial community. Metabolic reconstruction of a member of Riflebacteria predicted that it is an anaerobe capable to grow on carbohydrates by fermentation or dissimilatory Fe(III) reduction.