Involvement of Proton Transfer for Carbon Dioxide Reduction Coupled with Extracellular Electron Uptake in Shewanella oneidensis MR-1

Microbial biosynthesis of hydrocarbon from CO2 reduction driven by electron uptake process from the cathodic electrode has gained intensive attention in terms of potential industrial application. However, a lack of a model system for detailed studies on the mechanism of the CO2 reduction hinders the improvement in efficiency for microbial electrosynthesis. Here, we examined the mechanism of microbial CO2 reduction at the cathode by a well-described microbe for extracellular electron uptake, Shewanella oneidensis MR-1, capable of reducing gaseous CO2 to produce formic acid. Using whole-cell electrochemical assay, we observed stable cathodic current production at -0.65 V vs Ag/AgCl KCl sat. associated with the introduction of CO2. The observed cathodic current was enhanced by the addition of 4 mu M riboflavin, which specifically accelerates the electron uptake process of MR-1 by the interaction to its outer-membrane c-type cytochromes. The significant impact of an uncoupler agent and a mutant strain of MR-1 lacking sole F-type ATPase suggested the importance of proton import to the cytoplasm for the cathodic CO2 reduction. The present data suggest that MR-1 potentially serves as a model system for microbial electrosynthesis from CO2.