Response of methanogen communities to the elevation of cathode potentials in bioelectrochemical reactors amended with magnetite

Electromethanogenesis refers to the process where methanogens utilize current for the reduction of CO 2 to CH 4 . Setting low cathode potentials is essential for this process. In this study, we test if magnetite, an iron oxide mineral widespread in the environment, can facilitate the adaption of methanogen communities to the elevation of cathode potentials in electrochemical reactors. Two-chamber electrochemical reactors were constructed with inoculants obtained from paddy field soil. We elevated cathode potentials stepwise from the initial -0.6 V vs the standard hydrogen electrode (SHE) to -0.5 V and then to -0.4 V over the 130 days acclimation. Only weak current consumption and CH 4 production were observed in the bioreactors without magnetite. But significant current consumption and CH 4 production were recorded in the magnetite bioreactors. The robustness of electro-activity of the magnetite bioreactors was not affected by the elevation of cathode potentials from -0.6 V to -0.4 V. But, the current consumption and CH 4 production were halted in the bioreactors without magnetite when the cathode potentials were elevated to -0.4 V. Methanogens related to Methanospirillum were enriched on the cathode surfaces of magnetite bioreactors at -0.4 V, while Methanosarcina relatively dominated in the bioreactors without magnetite. Methanobacterium also increased in the magnetite bioreactors but stayed off electrodes at -0.4 V. Apparently, the magnetite greatly facilitates the development of biocathodes, and it appears that with the aid of magnetite, Methanospirillum spp. can adapt to the high cathode potentials performing efficient electromethanogenesis. IMPORTANCE Converting CO 2 to CH 4 through bioelectrochemistry is a promising approach to the development of green energy biotechnology. This process however requires low cathode potentials, which takes cost. In this study, we test if magnetite, a conductive iron mineral, can facilitate the adaption of methanogens to the elevation of cathode potentials. In the two-chamber reactors constructed by using inoculants obtained from paddy field soil, biocathodes were firmly developed in the presence of magnetite, whereas only weak activities in CH 4 production and current consumption were observed in the bioreactors without magnetite. The elevation of cathode potentials did not affect the robustness of electro-activity of the magnetite bioreactors over the 130 days acclimation. Methanospirillum were identified as the key methanogens associated with the cathode surfaces during the operation at high potentials. The findings reported in this study shed new light on the adaption of methanogen communities to the elevated cathode potentials in the presence of magnetite.