Deep subsurface rock-hosted chemolithotrophic bacterial communities exhibited differential CO2 assimilation and bioconversion potential under varying oxygen level.

Deep continental subsurface hosts diverse microbial ecosystems that are primarily driven by chemolithoautotrophy. In this study, we investigated deep continental igneous rock-hosted bacterial populations enriched under microoxic (ME) and anoxic (AE) conditions. Metataxonomic, metagenomics and metabolomics approaches, along with physiological analyses, were performed to elucidate community composition, CO2 utilization and possible bioconversion potential of subsurface rock enrichment cultures under chemolithoautotrophic conditions. Following prolonged incubation, ME enrichments resulted in higher microbial growth with greater species diversity than the AE cultures. Ralstonia and unclassified Comamonadaceae were predominant in both the enrichment conditions. On the other hand, Cellulomonas, Phenylobacterium, Deinococcus, Desulfurispora, etc. were relatively abundant in ME, and Solimonas, Curvibacter, Caulobacter, Novosphingobium, Anaeromyxobacter, unclassified Clostridia, etc. were abundant in AE communities. CO2/H2 utilization and organic acids production were greater in ME enrichments. Shotgun metagenomics and predictive metabolic profiling revealed CBB cycle as the predominant carbon fixation pathway in ME, whereas WL pathway was prominent in AE. Genes for hydrogen, sulfur, and nitrogen metabolisms were observed in both the enrichment cultures. HRLC-MS based untargeted metabolomics indicated the presence of valuable metabolites (organic acids, osmolytes, lipids/amides) in rock cultures, reflecting the potential of deep subsurface microorganisms for CO2 utilization and possible bioconversion to valuable biomolecules.