Importance of organic acids degrading bacteria in a microbial methanation system using a subterranean microbial community

Microbial methanation, which produces CH4 from CO2 and H2 using hydrogenotrophic methanogens, is a useful technology to turn CO2 into a resource. In this study, we developed a microbial methanation system that produces CH4 from H2 and CO2 using subterranean microbial community in anaerobic groundwater collected from a deep aquifer. As controls, the cultures using an isolate strain of hydrogenotrophic methanogens and using two strains of hydrogenotrophic methanogens and fermentative bacteria were also constructed. The microbial community and the strains were added to the liquid medium, respectively. Each culture medium was then amended with H2/CO2 and anaerobically incubated in serum bottles. The results showed continuous CH4 production in three types of cultures. However, the rate of CH4 production in the cultures with the strain of hydrogenotrophic methanogens gradually decreased in continuous cultivation with H2/CO2 exchange. On the other hand, in the culture using the microbial community derived from anaerobic groundwater, the CH4 production rate did not significantly decrease. The analysis of organic acids indicated that the accumulation of formate, acetate, and propionate in the culture using the microbial community was significantly lower than in that with the strain of hydrogenotrophic methanogens. These results suggest that fermentative bacteria in the microbial community efficiently consume organic acids that inhibit the activity of methanogenic archaea and play a role in maintaining the rate of CH4 production. This study demonstrates the effectiveness of the microbial community, including not only hydrogenotrophic methanogens but also anaerobic bacterial community, in establishing the microbial methanation system.