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

Oil reservoirs are complex ecosystems where microorganisms play a vital role in hydrocarbon degradation mostly with methanogenesis as terminal electron accepting process. Especially in off-shore oil reservoirs, sulfate-containing seawater is injected by the oil industry to maintain the reservoir pressure resulting in sulfate reduction in the reservoir. Methanogenesis and sulfate reduction are typically thought to be mutually exclusive because sulfate reducers outcompete methanogens for hydrogen and acetate. 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 potentialThe microbial community was predominantly composed of sulfate-reducing bacteria including the genera Desulfotignum, Desulfospira, and Geoalkalibacter. In addition, fermentative bacteria such as Mesotoga and Petrotoga were abundant and the genera Methanofastidiosales and Methanolobus were most prevalent among the archaea. These study results suggest that even with sulfate-adapted communities and high sulfate concentrations, methanogenesis can occur in minor amounts.