Theory of Redox Conduction and the Measurement of Electron Transport Rates Through Electrochemically Active Biofilms

In this chapter, a proposed mechanism for long-distance electron transport that occurs over distances that can exceed 20 μm within electrochemically active biofilms comprised of Geobacter sulfurreducens wild-type strain DL-1 is described. According to this mechanism, referred to as redox conduction, long-distance electron transport results from sequential short-distance electron transfer reactions ("electron hops") between adjacent redox cofactors distributed throughout the biofilm that act as electron transport conduits. The general approach to investigate the mechanism of electron transport through a material is to place the material between two electrodes and measure the dependency of the rate of electron transport through the material from one electrode to the other, in the form of electrical current, on the potentials applied to the electrodes. Materials utilizing different mechanisms of electron transport exhibit different current-potential dependencies. Derivation of the idealized current-potential dependency for a G. sulfurreducens biofilm based on redox conduction is described here, which has been successfully applied to fit experimental results. General methodology is also described for performing biofilm electron transport rate measurements in the laboratory. The goal of this chapter is to describe redox conduction and experimental methods to enable researchers to perform electron transport rate measurements for their own types of biofilms.