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

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-CH₄and 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.