Coexistence of methanogenesis and sulfate reduction in a sulfate-adapted enrichment culture from an oil reservoir.

Oil reservoirs are complex ecosystems where microorganisms play a vital role in hydrocarbon degradation, mostly with methanogenesis as the terminal electron-accepting process. Especially in offshore oil reservoirs, sulfate-containing seawater is injected during the oil production to maintain the reservoir pressure, resulting in increased sulfate reduction in the reservoir. Methanogenesis and sulfate reduction are typically thought to be mutually exclusive because sulfate reducers outcompete methanogens thermodynamically for hydrogen and acetate. However, coexistence is evident in environments like landfills, estuaries, marine, and polluted river sediments. Here, we studied methanogenesis in an adapted sulfate-reducing microbial community, enriched from an oil reservoir, using incubation experiments with metabolic inhibitors to assess microbial activity and degradation potential. The observed methane production rate accounted for 156.9 µM/a with a parallel carbon dioxide production rate of 67 mM/a and a sulfate reduction rate of 20 mM/a, suggesting a coexistence of sulfate reduction and methanogenesis even in a well-mixed system. The microbial community was predominantly composed of potential sulfate-reducing bacteria including the genera