Electron transfer mechanisms in electrochemically active biofilms

The chemical and electrochemical gradients, or collectively microscale gradients, in biofilms play a critical role in electron transfer processes between cells and a solid electron acceptor. Typically, electron transfer processes have been investigated in the bulk phase, for a biofilm electrode or for an isolated component of a biofilm. Currently the knowledge of microscale gradients in living biofilms respiring on a solid surface is limited. We believe that quantifying the microscale gradients and not bulk conditions, an isolated part of the biofilm, or a single cell, are critical for explaining electron transfer mechanisms. In order to measure microscale gradients, we developed biofilm reactors that would allow us to make in situ microelectrode measurements during electron transfer processes. Additionally, we developed new microelectrodes that could be used above polarized electrodes. We combined the use of microelectrodes with electrochemical techniques in a new way. First, we observed pH and redox potential gradients inside anodic biofilms of Shewanella oneidensis MR-1 and Geobacter sulfurreducens. We introduced the concept of using redox potential measurements to directly measure electron transfer occurring in the soluble phase in electrochemically active biofilms. We found that (1) pH was not always the limiting factor for current production in these biofilms and (2) redox potential could not explain the electron transfer through these biofilms. We followed these redox potential measurements with a new type of measurement where the microelectrode tip was allowed to electrically connect to the biofilm matrix. Here we introduced the concept of the local biofilm potential to measure the electron transfer associated with the biofilm matrix. For G. sulfurreducens biofilms, the local biofilm potential was found to coincide with the open circuit potential of the biofilm electrode, suggesting that under current-producing conditions, the biofilm was always reduced. In cathodic biofilms, we measured oxygen and pH microgradients during electrode polarization. We observed that the negative impact of high-current density on cathodic biofilms limit the practical use of these cathodic biofilms in applications such as sediment microbial fuel cells. Through these microscale gradient measurements, we found that the biofilm reactor configuration could control the microscale gradients in biofilms. Therefore, we proposed that when electrochemically active biofilms are investigated at the fundamental level, quantifying the microscale gradients inside the biofilm for different reactor configurations should be a critical and necessary component of future investigations.