Electroactive biofilms are capable of extracellular electron transfer to insoluble metal oxides and electrodes; such biofilms are relevant to biogeochemistry, bioremediation, and bioelectricity production. We investigated the extracellular electron transfer mechanisms in Shewanella loihica PV-4 viable biofilms grown at indium tin oxide (ITO) and graphite electrodes in potentiostat-controlled electrochemical cells poised at 0.2 V vs. Ag/AgCl. Chronoamperometry and confocal microscopy showed higher biofilm growth at graphite compared to the ITO electrode. Cyclic voltammetry, differential pulse voltammetry, along with fluorescence spectroscopy showed that direct electron transfer through outer membrane c type cytochromes (Omcs) prevailed at the biofilm/ITO interface, while biofilms formed at graphite electrode reduced the electrode also via secreted redox mediators, such as flavins and quinones. The biofilm age does not affect the prevalent transfer mechanism at ITO electrodes. On the other hand, secreted redox mediators accumulated at biofilm/graphite interface, thus increasing mediated electron transfer as the biofilm grows over five days. Our results showed that the electrode material determined the prevalent electron transfer mechanism and the dynamic of secreted redox mediators in S. loihica PV-4 biofilms. These observations have implications for the optimization of biofilm-based electrochemical systems, such as biosensors and microbial fuel cells
