Selecting Anode-Respiring Bacteria Based on Anode Potential: Phylogenetic, Electrochemical, and Microscopic Characterization

Anode-respiring bacteria (ARB) are able to transfer electrons contained in organic substrates to a solid electrode. The selection of ARB should depend on the anode potential, which determines the amount of energy available for bacterial growth and maintenance. In our study, we investigated how anode potential affected the microbial diversity of the biofilm community. We used a microbial electrolysis cell (MEC) containing four graphite electrodes, each at a different anode potential (E(anode) = -0.15, -0.09, +0.02, and +0.37 V vs SHE). We used wastewater-activated sludge as inoculum, acetate as substrate, and continuous-flow operation. The two electrodes at the lowest potentials showed a faster biofilm growth and produced the highest current densities, reaching up to 10.3 A/m(2) at the saturation of an amperometric curve; the electrode at the highest potential produced a maximum of 0.6 A/m(2). At low anode potentials, clone libraries showed a strong selection (92-99% of total clones) of an ARB that is 97% similar to G. sulfurreducens. At the highest anode potential, the ARB community was diverse. Cyclic voltammograms performed on each electrode suggest that the ARB grown at the lowest potentials carried out extracellular electron transport exclusively by conducting electrons through the extracellular biofilm matrix. This is supported by scanning electron micrographs showing putative bacterial nanowires and copious EPS at the lowest potentials. Non-ARB and ARB using electron shuttles in the diverse community for the highest anode potential may have insulated the ARB using a solid conductive matrix from the anode. Continuous-flow operation and the selective pressure due to low anode potentials selected for G. sulfurreducens, which are known to consume acetate efficiently and use a solid conductive matrix for electron transport.