Changes in microbial communities and associated water and gas geochemistry across a sulfate gradient in coal beds: Powder River Basin, USA

Abstract

Competition between microbial sulfate reduction and methanogenesis drives cycling of fossil carbon and generation of CH4 in sedimentary basins. However, little is understood about the fundamental relationship between subsurface aqueous geochemistry and microbiology that drives these processes. Here we relate elemental and isotopic geochemistry of coal-associated water and gas to the microbial community composition from wells in two different coal beds across CH4 and SO42− gradients (Powder River Basin, Montana, USA). Areas with high CH4 concentrations generally have higher alkalinity and δ13C-DIC values, little to no SO42−, and greater conversion of coal-biodegradable organics to CH4 (based on δ13C-CH4 and δ13C-CO2 values). Wells with SO42− concentrations from 2 to 10 mM had bacterial populations dominated by several different sulfate-reducing bacteria and archaea that were mostly novel and unclassified. In contrast, in wells with SO42− concentrations <1 mM, the sequences were dominated by presumptive syntrophic bacteria as well as archaeal Methanosarcinales and Methanomicrobiales. The presence of sequences indicative of these bacteria in low SO42− methanogenic wells may suggest a syntrophic role in coal biodegradation and/or the generation of methanogenic substrates from intermediate organic compounds. Archaeal sequences were observed in all sampled zones, with an enrichment of sequences indicative of methanogens in low SO42− zones and unclassified sequences in high SO42− zones. However, sequences indicative of Methanomassiliicoccales were enriched in intermediate SO42− zones and suggest tolerance to SO42− and/or alternative metabolisms in the presence of SO42−. Moreover, sequences indicative of methylotrophic methanogens were more prevalent in an intermediate SO42− and CH4 well and results suggest an important role for methylotrophic methanogens in critical zone transitions. The presented results demonstrate in situ changes in bacterial and archaeal population distributions along a SO42− gradient associated with recalcitrant, organic carbon that is biodegraded and converted to CO2 and/or CH4.