In microbial fuel cells and electrolysis cells (MXCs), anode-respiring bacteria (ARB) oxidize organic substrates to produce electrical current. In order to develop an electrical current, ARB must transfer electrons to a solid anode through extracellular electron transfer (EET). ARB use various EET mechanisms to transfer electrons to the anode, including direct contact through outer-membrane proteins, diffusion of soluble electron shuttles, and electron transport through solid components of the extracellular biofilm matrix. In this review, we perform a novel kinetic analysis of each EET mechanism by analyzing the results available in the literature. Our goal is to evaluate how well each EET mechanism can produce a high current density (> 10 A m(-2)) without a large anode potential loss (less than a few hundred millivolts), which are feasibility goals of MXCs. Direct contact of ARB to the anode cannot achieve high current densities due to the limited number of cells that can come in direct contact with the anode. Slow diffusive flux of electron shuttles at commonly observed concentrations limits current generation and results in high potential losses, as has been observed experimentally. Only electron transport through a solid conductive matrix can explain observations of high current densities and low anode potential losses. Thus, a study of the biological components that create a solid conductive matrix is of critical importance for understanding the function of ARB.

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The substrate-utilization rate of anode-respiring bacteria (ARB) directly correlates to the current density, one of the main factors in a microbial electrolysis/fuel cell. This study first evaluates the effects of donor-substrate diffusion and anode potential on the estimation of the half-maximum-rate concentration (K(s)) and the maximum specific substrate-utilization rate (q(max)) of a mixed culture biofilm in a microbial electrolysis cell oxidizing acetate. The intrinsic K(s) value is 119 g COD/m3, substrate diffusion has a significant impact on K(s) estimation, and the effect of the anode potential on K(s) is small. The intrinsic q(max) value is 22.3 g COD/g VS-d for an assumed biomass density of 50,000 g VS/m3 (q(max)X(f) = 1120 kg COD/m3-d). The maximum specific growth rate (micro(max)) is 3.2/d which is substantially faster than for acetate-utilizing methanogens and homoacetogens. Although the anode potential affects q(max), substrate diffusion has a negligible effect. The measured half-saturation anode potential (E(KA)) is very negative, -0.448 V (vs Ag/AgCl), and this low value minimizes anode-potential limitation on the current density and the substrate-utilization rate. Thus, the ARB selected in our biofilm anode were relatively fast growers able to take advantage of their low E(KA) value (-0.448 V).

We demonstrate that the coulombic efficiency (CE) of a microbial electrolytic cell (MEC) fueled with a fermentable substrate, ethanol, depended on the interactions among anode respiring bacteria (ARB) and other groups of micro-organisms, particularly fermenters and methanogens. When we allowed methanogenesis, we obtained a CE of 60%, and 26% of the electrons were lost as methane. The only methanogenic genus detected by quantitative real-time PCR was the hydrogenotrophic genus, Methanobacteriales, which presumably consumed all the hydrogen produced during ethanol fermentation ( approximately 30% of total electrons). We did not detect acetoclastic methanogenic genera, indicating that acetate-oxidizing ARB out-competed acetoclastic methanogens. Current production and methane formation increased in parallel, suggesting a syntrophic interaction between methanogens and acetate-consuming ARB. When we inhibited methanogenesis with 50 mM 2-bromoethane sulfonic acid (BES), the CE increased to 84%, and methane was not produced. With no methanogenesis, the electrons from hydrogen were converted to electrical current, either directly by the ARB or channeled to acetate through homo-acetogenesis. This illustrates the key role of competition among the various H(2) scavengers and that, when the hydrogen-consuming methanogens were present, they out-competed the other groups. These findings also demonstrate the importance of a three-way syntrophic relationship among fermenters, acetate-consuming ARB, and a H(2) consumer during the utilization of a fermentable substrate. To obtain high coulombic efficiencies with fermentable substrates in a mixed population, methanogens must be suppressed to promote new interactions at the anode that ultimately channel the electrons from hydrogen to current.