Abstract
Abstract Long-isolated subsurface brine environments (Ma-Ga residence times) may be habitable if they sustainably provide substrates, e.g. through water-rock reaction, that support microbial catabolic energy yields exceeding maintenance costs. The relative inaccessibility and low biomass of such systems has led to limited understanding of microbial taxonomic distribution, metabolism, and survival under abiotic stress exposure in these extreme environments. In this study, taxonomic and metabolic annotations of 95 single cell amplified genomes (SAGs) were obtained for one low biomass (103–104 cells/mL), hypersaline (246 g/L), radiolytically enriched brine obtained from 3.1 km depth in South Africa’s Moab Khotsong mine. The majority of SAGs belonged to three halophilic families (Halomondaceae (58%), Microbacteriaceae (24%), and Idiomarinaceae (8%)) and did not overlap with any family-level identifications from service water or a less saline dolomite aquifer sampled in the same mine. Functional annotation revealed complete metabolic modules for aerobic heterotrophy (organic acids and xenobiotics oxidation), fermentation, denitrification and thiosulfate oxidation, suggesting metabolic support in a microoxic environment. SAGs also contained complete modules for degradation of complex organics, amino acid and nucleotide synthesis, and motility. This work highlights a long-isolated subsurface fluid system with microbial metabolism fueled by radiolytically generated substrates, including O2, and suggests subsurface brines with high radionuclide concentrations as putatively habitable and redox-sustainable environments over long (ka-Ga) timescales.
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