Factors controlling the co-occurrence of microbial sulfate reduction and methanogenesis in coal bed reservoirs

Sulfate-reducing microorganisms (SRM) and methanogenic archaea have been previously observed in coal bed methane reservoirs, suggesting that the model for separation of these organisms based on sulfate concentration may not apply to such reservoirs. Using a methanogenic consortium enriched from coal, microcosm experiments showed simultaneous activity of methanogens and sulfate reducers at sulfate concentrations ranging from 50 to 1000 μM when coal was the sole substrate. These experiments revealed no apparent correlation between methanogenic potential and sulfate concentration. In other microcosm experiments with varying acetate amendments, concentrations of the phospholipid fatty acids (PLFAs) 14:0, 16:1ω5, 16:1ω7cis, 16:1ω7trans, and cy17:0 correlated strongly with the initial acetate concentration in microcosms with 500 μM sulfate, while i17:0 correlated strongly in microcosms with 200 μM sulfate. A significant portion of the acetate in these experiments went to microbial metabolisms other than dissimilatory sulfate reduction or methanogenesis, suggesting that some of these PLFAs were likely produced by some other unknown acetate-consuming micro-organisms. Copies of the dsrA gene increased at least 10-fold over initial levels in samples without molybdate (MoO42 −) across all experiments, indicating that SRM were active when not inhibited by MoO42 −. In experiments with < 300 μM acetate, copies of the mcrA gene increased over 49 days regardless of sulfate concentration. These results suggest that both SRM and methanogens are active at low acetate concentrations and may compete for available acetate with other acetate-consuming bacteria in coal bed methane reservoirs.