Experimental studies of charge transport and storage in microbial biofilms*

We discuss results of ac impedance spectroscopy and dc I-V measurements applied to microbial biofilms and demonstrate that the biofilms of \textit{Geobacter sulfurreducens }that grow on the anodes of microbial fuel cells are electronically conductive, with conductivities comparable to conjugated polymers. To investigate the components conferring the conductivity, we studied novel strains of \textit{G. sulfurreducens} and mutants deficient in various outer surface components. Strains producing more current, produced biofilms with higher conductivity, demonstrating that conduction is the mechanism for long-range electron transfer through the biofilms. There was a direct correspondence between biofilm conductivity and the abundance of pili, referred to as microbial nanowires. Electrochemical gating suggested that in contrast to conventional redox activated hopping, charge transport is polaron-like. Biofilms also exhibited supercapacitor behavior. The results are being incorporated into a physical model of electron transfer and storage through the biofilms.