Outward and Inward Electron Transfer Occur through Distinct Interfacial Electron Transfer Mechanisms for Anaerobic Shewanella oneidensis

Microbial electrosynthesis (MES) is an emerging biotechnology with the potential to mitigate climate change by recycling carbon dioxide into organic compounds using sustainably generated electricity as the energy source. MES utilizes bacteria as catalysts to transfer electrons from a cathode to a target electron acceptor, such as CO2. For MES to decrease net carbon emissions on a meaningful scale, the rate of inward electron transfer (ET) from the cathode to bacteria must increase. In this work, we investigate this challenge using Shewanella oneidensis, a model metal-reducing bacterium with an extensively studied extracellular electron transfer pathway. The primary ET mechanism for S. oneidensis is the Mtr pathway, a cytochrome c-rich pathway that connects the respiratory electron transport chain to the outer surface of the cell. While the Mtr pathway in S. oneidensis is bidirectional, the magnitude of outward ET (its native respiratory direction) consistently exceeds inward ET, the direction required for MES applications. Here, we present the first direct comparison of ET for both directions using potentiometric techniques. Through chronoamperometry and turnover cyclic voltammetry, we demonstrate that the ET mechanism is direction-dependent. Specifically, we observed distinct roles for supplemental riboflavin, where inward ET from a cathode was dependent on supplemental riboflavin, but outward ET to an anode was not.