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