Influence of External Resistance on Electrogenesis, Methanogenesis, and Anode Prokaryotic Communities in Microbial Fuel Cells

The external resistance (R ext ) of microbial fuel cells (MFCs) regulates both the anode availability as an electron acceptor and the electron flux through the circuit. We evaluated the effects ofR ext on MFCs using acetate or glucose. The average current densities (I ) ranged from 40.5 mA/m2 (9,800 Ω) to 284.5 mA/m2 (150 Ω) for acetate-fed MFCs (acetate-fed reactors [ARs]), with a corresponding anode potential (E an ) range of −188 to −4 mV (versus a standard hydrogen electrode [SHE]). For glucose-fed MFCs (glucose-fed reactors [GRs]),I ranged from 40.0 mA/m2 (9,800 Ω) to 273.0 mA/m2 (150 Ω), with a correspondingE an range of −189 to −7 mV. ARs produced higher Coulombic efficiencies and energy efficiencies than GRs over all testedR ext levels because of electron and potential losses from glucose fermentation. Biogas production accounted for 14 to 18% of electron flux in GRs but only 0 to 6% of that in ARs. GRs produced similar levels of methane, regardless of theR ext . However, total methane production in ARs increased asR ext increased, suggesting thatE an might influence the competition for substrates between exoelectrogens and methanogens in ARs. An increase ofR ext to 9,800 Ω significantly changed the anode bacterial communities for both ARs and GRs, while operating at 970 Ω and 150 Ω had little effect.Deltaproteobacteria andBacteroidetes were the major groups found in anode communities in ARs and GRs.Betaproteobacteria andGammaproteobacteria were found only in ARs.Bacilli were abundant only in GRs. The anode-methanogenic communities were dominated byMethanosaetaceae , with significantly lower numbers ofMethanomicrobiales . These results show thatR ext affects not only theE an and current generation but also the anode biofilm community and methanogenesis.