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 of Rext on MFCs using acetate or glucose. The average current densities (I) ranged from 40.5 mA/m(2) (9,800 Omega) to 284.5 mA/m(2) (150 Omega) 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/m(2) (9,800 Omega) to 273.0 mA/m(2) (150 Omega), with a corresponding Ean range of -189 to -7 mV. ARs produced higher Coulombic efficiencies and energy efficiencies than GRs over all tested R-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 the R-ext. However, total methane production in ARs increased as R-ext increased, suggesting that E-an might influence the competition for substrates between exoelectrogens and methanogens in ARs. An increase of R-ext to 9,800 Omega significantly changed the anode bacterial communities for both ARs and GRs, while operating at 970 Omega and 150 Omega had little effect. Deltaproteobacteria and Bacteroidetes were the major groups found in anode communities in ARs and GRs. Betaproteobacteria and Gammaproteobacteria were found only in ARs. Bacilli were abundant only in GRs. The anode-methanogenic communities were dominated by Methanosaetaceae, with significantly lower numbers of Methanomicrobiales. These results show that R-ext affects not only the E-an and current generation but also the anode biofilm community and methanogenesis.