Redox and pH microenvironments within Shewanella oneidensis MR-1 biofilms reveal an electron transfer mechanism

Abstract

The goal of this research was to quantify the variations in redox potential and pH in Shewanella oneidensis MR-1 biofilms respiring on electrodes. We grew S. oneidensis MR-1 on a graphite electrode, which was used to accept electrons for microbial respiration. We modified well-known redox and pH microelectrodes with a built-in reference electrode so that they could operate near polarized surfaces and quantified the redox potential and pH profiles in these biofilms. In addition, we used a ferri-/ferrocyanide redox system in which electrons were only transferred by mediated electron transfer to explain the observed redox potential profiles in biofilms. We found that regardless of the polarization potential of the biofilm electrode, the redox potential decreased toward the bottom of the biofilm. In a fully redox-mediated control system (ferri-/ferrocyanide redox system), the redox potential increased toward the bottom when the electrode was the electron acceptor. The opposite behavior of redox profiles in biofilms and the redox-controlled system is explained by S. oneidensis MR-1 biofilms not being redox-controlled when they respire on electrodes. The lack of a significant variation in pH implies that there is no proton transfer limitation in S. oneidensis MR-1 biofilms and that redox potential profiles are not caused by pH.

Figure 1

Diagram of the dual-chamber flat plate biofilm reactor showing the positions of the microelectrode and the biofilm during redox potential and pH measurements. The reactor was operated anaerobically, and the cells respired on the electrodes. PEM: proton-exchange membrane.

Figure 2

Diagram of (A) redox and (B) pH microelectrodes. RE: Ag/AgCl reference electrode. LIX: liquid ion exchange.

Figure 3

Variation in redox potential in a S. oneidensis MR-1 biofilm respiring on an electrode. Redox potential was measured inside a reactor, while the biofilm electrode was polarized to 0 or +400 mV_Ag/AgCl. The inset shows the data over a smaller range on the y axis to make the variation more noticeable.

Figure 4

Redox potential measurements taken at the redox microelectrode in a ferri-/ferrocyanide solution while the electrode was polarized to +400 mV_Ag/AgCl. The redox profile taken in a S. oneidensis MR-1 biofilm shown in Figure 3 is added for comparison.

Figure 5

Changes in redox potential in S. oneidensis MR-1 biofilm grown on an electrode after the bulk medium (with −500 mV_Ag/AgCl redox potential) was replaced with fresh medium (with +250 mV_Ag/AgCl redox potential). The profiles were measured at the open-circuit condition and after the biofilm electrode was polarized to +300 mV_Ag/AgCl. The potentials at the bottom were −500 and +300 mV_Ag/AgCl. These data points are not shown in the figure for clarity.

Figure 6

Variation in pH in a S. oneidensis MR-1 biofilm respiring on an electrode shows that proton transport was not a limiting factor for electron transfer in our steady-state biofilms, while the biofilm electrode was polarized to 0 and +400 mV_Ag/AgCl.