Cytochrome OmcZ is essential for the current generation by Geobacter sulfurreducens under low electrode potential

Abstract

Geobacter sulfurreducens is a model exoelectrogen known for its capability to generate high current density. Microbes genetically related to G. sulfurreducens often dominate the anodic microbial consortia in the microbial electrochemical systems. Electroanalyses had revealed that G. sulfurreducens utilizes at least three electron transfer (ET) pathways. The ET pathway with a mid-point potential of − 0.22V is responsible for the reduction of low-potential electron acceptors. Nonetheless, the identity of the corresponding ET proteins remained unknown. This study compared the voltammograms of wild type G. sulfurreducens to those of a strain deficient in the cytochrome OmcZ (strain ΔomcZ). In bioelectrochemical reactors equipped with Indium tin oxide glasses as the working electrodes, the ΔomcZ strain generated lower current density (28.1±8.2μA/cm2) compared to that of the wild strain (82.9±16.7μA/cm2). For ΔomcZ, the catalytic wave in the low-potential region disappeared from the biofilm voltammogram. Differential pulse voltammetries also demonstrated the loss of a low-potential redox couple. OmcZ is therefore likely to be part of the low-potential pathway. Nevertheless, pathways with higher mid-point potentials remained functional after the deletion of the omcZ gene. OmcZ is hence probably specifically required for the low-potential ET pathway. On the other hand, mutant strain deficient in the omcB gene, which was previously found to be up-regulated under current-generating conditions, exhibited similar voltammetric patterns to the wild strain.