Microbial fuel cells (MFCs) use microbes as biocatalysts to transform chemical energy into electricity, and are promising systems for applications such as wastewater treatment. MFC performance can be significantly affected by operational conditions, such as pH, temperature, electrode materials, and oxygen concentration. Oxygen intrusion has been considered detrimental to the process because it acts as a competitor with the anode electrode and consumes electrons that could otherwise be recovered during current production. Oxygen also inhibits some strictly anaerobic microbes and minimally aerotolerant electro-active bacteria.

Shewanella are well-known exoelectrogens that can grow and develop under both anaerobic and aerobic conditions. The aerotolerance property of Shewanella has attracted research interests because it can recover electricity from a given substrate without the need for extremely anaerobic conditions. Studies on how Shewanella functions upon oxygen exposure have shown inconsistent results. Generally, increased oxygen concentrations will decrease or even eliminate current production due to the fact that oxygen is a preferred electron acceptor for Shewanella relative to a solid electrode. However, several researchers observed enhanced current production with oxygen exposure in their MFCs [1-3], which is hypothesized to be a result of promoted growth of Shewanella under aerobic conditions. However, the oxygen effect on per-cell extracellular electron transfer behavior is still unclear since it has been difficult to study the real-time single cell behavior in operating MFCs.  

Here, we use optically accessible bioelectrochemical systems to independently investigate the effect of oxygen on the growth and the per-cell extracellular electron transfer (EET) rates of Shewanella oneidensis MR-1. The optically accessible bioelectrochemical systems allow noninvasive monitoring of cell populations in real-time, so the per-cell EET rate can be calculated. Our experiments confirmed that oxygen exposure impairs per-cell EET rate, though it promotes biomass growth. The overall effect of oxygen on current production is the competition between the increased cell population and decreased per-cell EET rate. 

Additionally, we discovered that after the concentration of dissolved oxygen (DO) in the electrolyte of the MFC was switched from high to low level, the per-cell EET rate can be recovered (Fig. 1 a&b). Thus it is possible to enhance current production by introducing oxygen in the MFC during the initial stage and thus promote biomass growth, and reduce the DO value later to achieve high per-cell EET rate. Our preliminary results showed that by maintaining 1.26 mg/L DO for 8.5 hours and then reducing to 0.40 mg/L DO, a 57% increase in current production was achieved, compared to the MFC operated with DO maintained at 0.42 mg/L (Fig. 1 c&d). However, the current production per cell begins to subside five hours after this change is made.  Future work will evaluate the physiological changes that may occur as a result of oxygen exposure under different operational condition so that we can acquire a better understanding for how to maximize per-cell electron transfer rates in bioelectrochemical systems.

References:

[1]      J. C. Biffinger, R. Ray, B. J. Little, L. A. Fitzgerald, M. Ribbens, S. E. Finkel, and B. R. Ringeisen, Biotechnol. Bioeng., vol. 103, pp. 524–531, 2009.

[2]      M. Rosenbaum, M. A. Cotta, and L. T. Angenent, Biotechnol. Bioeng., vol. 105, pp. 880–888, 2010.

[3]     M. A. TerAvest, M. A. Rosenbaum, N. J. Kotloski, J. A. Gralnick, and L. T. Angenent, Biotechnol. Bioeng., vol. 111, pp. 692–699, 2014.