Quinone-mediated extracellular electron transfer processes in ex situ biomethanation reactors

Redox mediators are used in a wide diversity of systems including biological ones. We investigated the effect of adding an artificial quinone (2,7-AQDS) as external redox molecule to an anaerobic digester system dominated by hydrogenotrophic methanogens. When oxidized AQDS was present, the methanogens diverted electrons from H2 to reduce AQDS instead of CO2. The AQDS reduction process was accompanied by a temporary CH4 inhibition, which was re-established several days after the full reduction of AQDS to AH2QDS. The presence of AQDS furthermore resulted in a community shift from Methanobacterium as the dominant methanogen to a more diverse community of methanogens. Protein expression profiles showed a shift in cofactor preference of the adapted community, as a potential response mechanism to AQDS inhibition. AH2QDS was only used as electron donor to a limited extent. Stable isotope incorporation experiments here indicated that the acetogen Acetoanaerobium used AH2QDS to reduce CO2 into acetate.