Abstract
While typically investigated as a microorganism capable of extracellular electron transfer to minerals or anodes, Shewanella oneidensis MR-1 can also facilitate electron flow from a cathode to terminal electron acceptors such as fumarate or oxygen, thereby providing a model systems for a process that has significant environmental and technological implications. This work demonstrates that cathodic electrons enter the electron transport chain of S. oneidensis when oxygen is used as the terminal electron acceptor. The effect of electron transport chain inhibitors suggested that a proton gradient is generated during cathode-oxidation, consistent with the higher cellular ATP levels measured in cathode-respiring cells relative to controls. Cathode oxidation also correlated with an increase in the cellular redox (NADH/FMNH2) pool using a bioluminescent assay. Using a proton uncoupler, generation of NADH/FMNH2 under cathodic conditions was linked to reverse electron flow mediated by the proton pumping NADH oxidase Complex I. A decrease in cathodic electron uptake was observed in various mutant strains including those lacking the extracellular electron transfer components necessary for anodic current generation. While no cell growth was observed under these conditions, here we show that cathode oxidation is linked to cellular energy conservation, resulting in a quantifiable reduction in cellular decay rate. This work highlights a potential mechanism for cell survival and/or persistence in environments where growth and division are severely limited.
