Exoelectrogenic behaviors of Shewanella oneidensis MR-1 and cytochrome knock-out mutants on smooth electrodes of different materials

Abstract

The development of anodes supporting a high rate of current generation stands as a crucial and challenging aspect in microbial electrochemical systems. A well-defined electrode surface morphology is essential for comprehending the impact of material intrinsic properties on microbial current generation, offering molecular insights into the respiration of exoelectrogens with electrodes made of diverse materials. We fabricated smooth electrodes (root mean square roughness below 10 nm) using four materials, each representing a distinct class of materials commonly utilized to construct bioanodes. These electrodes were employed to investigate current generation, electrochemical characteristics, and biofilm formation by Shewanella oneidensis MR-1 and mutants lacking cytochromes MtrC (ΔmtrC), MtrA (ΔmtrA), or CymA (ΔcymA). In reactors inoculated with MR-1, the poly(3,4-ethylenedioxythiophene) polystyrene sulfonate electrodes demonstrated a current density of 12.5 ± 1.1 μA cm-2, surpassing the 3.57 ± 0.50 μA cm-2 observed with indium tin oxide electrodes, while current generation levels by gold and glassy carbon electrodes fell in between. This disparity was not attributed to distinct biofilm formations by MR-1, as it formed stable monolayer biofilms on all types of working electrodes. At least five electron transfer pathways were identified in the MR-1 Vammograms, with the activities of these pathways depending on the electrode materials. The effectiveness of both endogenous and exogenous electron mediators varied with the electrode materials involved. Additionally, a phenazine compound exhibited diverse formal potentials when interacting with the biofilm and different types of electrodes. The mutants displayed varying degrees of impaired current generation and biofilm formation capabilities, with ΔmtrA and ΔcymA being the most affected regardless of the electrode used. However, for a specific mutant, the overall current generation, relative contribution by individual pathways, and biofilm formations underwent significant changes based on the electrode materials employed.