Electron Transport in in Situ and Ex Situ Electroactive Microbial Biofilms

Abstract

Here we report on studies of microbial extracellular electron transport (EET) through Geobacter sulfurreducens biofilms. Electrical conductivity measurements were performed in situ on living biofilms in growth medium, and ex situ on biofilms removed from medium at various states of hydration. Previous in situ conductivity measurements revealed a peak-shaped dependency of G. sulfurreducens biofilm conductivity on gate potential, consistent with EET occurring via a redox-gradient driven, multi-step hopping mechanism (i.e., redox conduction).1-3 The in situ measurements reported here further indicate that conductivity increases with increasing temperature in an Arrhenius manner with an apparent activation energy of 0.13 eV, consistent with hemes of cytochromes acting as the electron transport cofactors.1 In the case of the ex situ measurements reported here, conductivity was shown to have a strong dependence on the biofilm hydration state. When the water content of the sample chamber was maintained at a fixed value, biofilm conductivity decreased by three orders of magnitude (10-1 to 10-4 µS/cm) when the temperature was increased from 12 to 35°C, as biofilms were expected to dehydrate. Alternatively, when the relative humidity of the testing chamber was increased from 25% to 85% at a fixed temperature, biofilm conductivity increased by more than three orders of magnitude, as biofilms were expected to rehydrate. For fully hydrated biofilms immersed in water, we observed an Arrhenius-like temperature dependence with a similar conductivity (1 µS/cm) and activation energy (0.14 eV) to that observed for the in situ measurements (5 µS/cm and 0.13 eV). These results suggest that in the case of the ex situ measurements, biofilm electrical conductivity becomes limited by charge compensating ionic conductivity, which is expected to decrease with decreasing biofilm water content. This conclusion was further corroborated by electrochemical impedance spectroscopy. Polymer films were used as controls (an Os-based redox polymer, a semiconducting benzotriazole polymer, and a conductive polyaniline polymer) to verify our methods and to find a comparable system for G. sulfurreducens biofilms. The G. sulfurreducens biofilms films behaved most similarly to the Os-based polymer, a known redox conductor. The results presented here indicate that G. sulfurreducens biofilms are a redox active material with tunable conductivity based on the hydration state of the film. Additionally, preliminary results suggest that the conductivity of the biofilm can be altered electrochemically and this property is currently under further study.