title: Syntrophic interactions among anode respiring bacteria (ARB) and Non-ARB in a biofilm anode: electron balances.
abstract: 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.
authors.affiliation: Center for Environmental Biotechnology, The Biodesign Institute at Arizona State University, Tempe, USA. pparame1@mainex1.asu.edu
authors.forename: Prathap
authors.initials: P
authors.lastname: Parameswaran
authors.forename: César I
authors.initials: CI
authors.lastname: Torres
authors.forename: Hyung-Sool
authors.initials: HS
authors.lastname: Lee
authors.forename: Rosa
authors.initials: R
authors.lastname: Krajmalnik-Brown
authors.forename: Bruce E
authors.initials: BE
authors.lastname: Rittmann
mesh_terms: D000085:Acetates / Q000378:metabolism; D001419:Bacteria / Q000254:growth & development* / Q000378:metabolism*; D001674:Bioelectric Energy Sources / Q000382:microbiology*; D018441:Biofilms / Q000254:growth & development*; D004560:Electricity; D004566:Electrodes / Q000382:microbiology*; D000431:Ethanol / Q000378:metabolism; D005285:Fermentation; D006859:Hydrogen / Q000378:metabolism; D008697:Methane / Q000378:metabolism
chemical_list: D000085:Acetates; D000431:Ethanol; D006859:Hydrogen; D008697:Methane