APPLIED MICROBIAL AND CELL PHYSIOLOGY Kinetics of consumption of fermentation products by anode-respiring bacteria César I. Torres & Andrew Kato Marcus & Bruce E. Rittmann Received: 4 June 2007 / Revised: 18 August 2007 / Accepted: 6 September 2007 # Springer-Verlag 2007 Abstract We determined the kinetic response of a community of anode-respiring bacteria oxidizing a mixture of the most common fermentation products: acetate, butyrate, propionate, ethanol, and hydrogen. We acclimated the community by performing three consecu- tive batch experiments in a microbial electrolytic cell (MEC) containing a mixture of the fermentation prod- ucts. During the consecutive-batch experiments, the coulombic efficiency and start-up period improved with each step. We used the acclimated biofilm to start continuous experiments in an MEC, in which we controlled the anode potential using a potentiostat. During the continuous experiments, we tested each individual substrate at a range of anode potentials and substrate concentrations. Our results show low current densities for butyrate and hydrogen, but high current densities for propionate, acetate, and ethanol (maximum values are 1.6, 9.0, and 8.2 A/m 2 , respectively). Acetate showed a high coulombic efficiency (86%) compared to ethanol and propionate (49 and 41%, respectively). High methane concentrations inside the MEC during ethanol experiments suggest that methanogenesis is one reason why the coulombic efficiency was lower than that of acetate. Our results provide kinetic parameters, such as the anode overpotential, the maximum current density, and the Monod half-saturation constant, that are needed for model development when using a mixture of fermentation products. When we provided no electron donor, we measured current due to endogenous decay of biomass (~0.07 A/m 2 ) and an open-cell potential (-0.54 V vs Ag/ AgCl) associated with biomass components active in endogenous respiration. Keywords Coulombic efficiency . Fermentation products . Microbial electrolytic cell . Open-cell potential Introduction Anode-respiring bacteria (ARB) are able to transfer electrons to a solid conductive anode as part of their energy-generating respiration. ARB are of special interest for generating electrical current directly from the elec- trons contained in biodegradable organic compounds present in wastes and other forms of biomass. The energy in the electrons can be harvested as electrical power in a microbial fuel cell (MFC) or as hydrogen (H 2 ) in a microbial electrolytic cell (MEC; Liu et al. 2005a). Electricity generation using MFCs has been studied using simple organic compounds, such as acetate, butyrate, and glucose (Chaudhuri and Lovley 2003; Min and Logan 2004; Liu et al. 2005b); complex organic matter, such as wastewater sludge and swine wastes (Dentel et al. 2004; Min et al. 2005); and inorganic electron donors, such as sulfide (Rabaey et al. 2006). Thus, ARB can have the dual benefit of harvesting energy and removing pollutants from water. Fermentation appears to be an important process in an MFC (or MEC) utilizing complex organic substrates. Fermentative bacteria can hydrolyze and ferment complex organic substrates to simple organic acids (e.g., propionate, Appl Microbiol Biotechnol DOI 10.1007/s00253-007-1198-z Electronic supplementary material The online version of this article (doi:10.1007/s00253-007-1198-z) contains supplementary material, which is available to authorized users. C. I. Torres (*) : A. Kato Marcus : B. E. Rittmann Center for Environmental Biotechnology, Biodesign Institute at Arizona State University, Tempe, AZ 85287-5701, USA e-mail: cit@asu.edu