S148: Studying extracellular electron transfer in microbial fuel cells at the single cell and biofilm level

Electrochemically active biofilms are beneficial to energy harvesting applications as multiple layers of cells coating the electrode has been found to increase the power densities of microbial fuel cells (MFCs).  We have studied Shewanella oneidensis MR-1, a model electrochemically active bacterium often used in MFC experiments, attachment to nano- and macro-scale electrodes under different oxygen conditions.  In macroscopic MFCs using carbon electrodes, wild type MR-1 appears to preferentially form biofilms under air exposure, while strict anaerobic conditions limit cell attachment.  Even though oxygen diffusion to the electrode would result in electron scavenging and limited power production, there is significant current generated under air exposure conditions.  It is hypothesized that MR-1 cells on the liquid-biofilm surface metabolize the dissolved oxygen, preventing oxygen exposure to the cells deeper into the biofilm and closer to the electrode thus allowing for optimal electron transfer from the cells to the electrode.  In addition to macroscopic MFCs, we have studied electron transfer on a nanoelectrode platform to further elucidate the extracellular electron transfer mechanism.  Studies show that MR-1 mainly uses a self-mediated electron transfer mechanism when attaching to nanoscale indium tin oxide/gold nanoelectrodes, while macroscopic experiments using carbon electrodes show both direct and mediated pathways. To gain more understanding of these complexities, we devised methods to utilize controlled culture experiments with MFCs to determine whether precise changes in the cell culture environment alter the extracellular electron transfer pathways used by MR-1.