title: High current densities enable exoelectrogens to outcompete aerobic heterotrophs for substrate.
abstract: In mixed-culture microbial fuel cells (MFCs), exoelectrogens and other microorganisms compete for substrate. It has previously been assumed that substrate losses to other terminal electron acceptors over a fed-batch cycle, such as dissolved oxygen, are constant. However, a constant rate of substrate loss would only explain small increases in coulombic efficiencies (CEs, the fraction of substrate recovered as electrical current) with shorter cycle times, but not the large increases in CE that are usually observed with higher current densities and reduced cycle times. To better understand changes in CEs, COD concentrations were measured over time in fed-batch, single-chamber, air-cathode MFCs at different current densities (external resistances). COD degradation rates were all found to be first-order with respect to COD concentration, even under open circuit conditions with no current generation (first-order rate constant of 0.14 ± 0.01 h(-1) ). The rate of COD removal increased when there was current generation, with the highest rate constant (0.33 ± 0.02 h(-1) ) obtained at the lowest external resistance (100 Ω). Therefore, as the substrate concentration was reduced more quickly due to current generation, the rate of loss of substrate to non-exoelectrogens decreased due to this first-order substrate-concentration dependence. As a result, coulombic efficiencies rapidly increased due to decreased, and not constant, removal rates of substrate by non-exoelectrogens. These results show that higher current densities (lower resistances) redirect a greater percentage of substrate into current generation, enabling large increase in CEs with increased current densities. Biotechnol. Bioeng. 2014;111: 2163-2169. © 2014 Wiley Periodicals, Inc.
authors.affiliation: Department of Civil and Environmental Engineering, 212 Sackett Building, The Pennsylvania State University, University Park, 16802, Pennsylvania.
authors.forename: Lijiao
authors.initials: L
authors.lastname: Ren
authors.forename: Xiaoyuan
authors.initials: X
authors.lastname: Zhang
authors.forename: Weihua
authors.initials: W
authors.lastname: He
authors.forename: Bruce E
authors.initials: BE
authors.lastname: Logan
mesh_terms: D061249:Batch Cell Culture Techniques; D001674:Bioelectric Energy Sources / Q000382:microbiology*; D057919:Biological Oxygen Demand Analysis; D019149:Bioreactors / Q000382:microbiology*; D003470:Culture Media / Q000737:chemistry; D004560:Electricity; D056265:Microbial Interactions; D013997:Time Factors
chemical_list: D003470:Culture Media
keywords: current densities; exoelectrogens; heterotrophs; microbial fuel cell; substrate consumption