Electrochemical fluorescence microscopy reveals insignificant long-range extracellular electron transfer in Shewanella oneidensis anodic processes

Abstract Multiple modes of extracellular electron transfer (EET) have been proposed for the model organism Shewanella oneidensis MR-1, including direct cell attachment to solid electron acceptor, long-range EET through biofilm redox conduction or diffusive electron shuttles. However, the effectiveness of these modes is not fully understood. To address this issue, we used in situ fluorescence microscopy to study a growing electrode biofilm of MR-1 and simultaneously performed electrochemical measurements. We found the current generation to be positively correlated with the electrode coverage by the cells. Meanwhile, biofilm formation appeared to precede the expression of EET-related redox proteins and current generation by bacteria. After the formation of a multilayer biofilm and stabilization of the EET rate, the electrolyte was withdrawn from and then returned to the reactor after the removal of planktonic cells. This operation also reduced the multilayer electrode biofilm to a monolayer. The loss of the bacterial cells was found to have insignificant effect on the generation of bioelectricity and the voltammetric patterns. These observations suggest that, the layer of cells directly associated with the electrode is mainly responsible for the current generation, contribution of the long-range EET is less significant.