Disentangling the roles of free and cytochrome-bound flavins in extracellular electron transport from Shewanella oneidensis MR-1

In addition to its natural environmental significance, the process of extracellular electron transfer (EET) can be exploited at electrode interfaces to enable microbial electrochemical technologies for renewable energy recovery, biofuel production, bioremediation, and wastewater treatment. Success in these applications hinges on a better understanding of the electron transfer pathways that link electrochemically active microbes, such as Shewanella oneidensis MR-1, to electrodes. Here, we report electrochemical measurements that reveal both flavin-dependent and flavin-independent EET pathways from S. oneidensis MR-1. Significantly, differential pulse voltammetry captured the redox signatures of both free and cytochrome-bound flavins simultaneously in one experimental system for the first time, allowing us to compare their relative contributions while studying the impact of cell-removal, flavin addition, different culture conditions, and a mutant disrupted in flavin secretion. Our results indicate that endogenous flavins accelerate EET from Shewanella to high surface area carbon cloth electrodes primarily as cytochrome-bound cofactors, rather than free soluble shuttles, thereby supporting one of the two debated models of Shewanella EET. Our measurements also highlight the extent to which different electrode materials can impact detection of microbial redox signatures. This report motivates additional structural studies to study the binding location and precise interaction of flavins at cytochrome-electrode interfaces.