Overlap and differences between the inward and outward electron transfer pathways in <i>Shewanella oneidensis</i>

The extracellular electron transport chain of Shewanella oneidensis MR-1 has been well characterized in the context of current generation on an anode. However, work to understand electron uptake from the cathode is less mature and major questions remain regarding the pathway and purpose of electron uptake. To employ this organism as a biocatalyst for microbial electrosynthesis, we must have a clear picture of the path of electrons into the cell to mitigate off-target reactions and find opportunities for pathway improvement. In this work, we confirm that the outer membrane electron conduit MtrCAB is essential for electron uptake, while the inner membrane cytochrome CymA is important but can be partially compensated for by other proteins. Additionally, we show that endogenous flavins are important for electron uptake and their absence cannot be complemented by exogenous flavins. Finally, hydrogenases are not directly involved in electron transfer but may play a role in cell survival during stationary phase on the cathode. Overall, the inward electron transfer pathway largely overlaps with the outward electron transfer pathway although we find differences in the role of flavins, particularly exogenously added riboflavin.