Thermally Activated Redox Conduction in Shewanella Oneidensis MR-1 Biofilms

Abstract

Shewanella oneidensis MR-1 can gain energy by performing extracellular electron transport (EET) to external insoluble electron acceptors ranging from natural minerals to synthetic electrode surfaces when poised at proper potentials. Outer membrane c-type multi-heme cytochromes play a critical role in mediating this electron transport. The abundance of these cytochromes on the cell surface has also been hypothesized to facilitate long-distance electron transport along multicellular biofilms, in a manner similar to redox active polymers containing discrete redox moieties. Here we report measurements of electron transport in living S. oneidensis biofilms that bridged interdigitated array (IDA) microelectrodes. Electrochemical gating reveals a peak in the conduction current at the expected formal potential of the cytochrome conduits, consistent with redox conductivity where transport is driven by a multistep electron hopping mechanism through the heme network. The temperature dependency of the biofilm conduction is also consistent with such a thermally activated process, where conductivity increases with increasing temperature. In addition, the measured activation energy (0.315 eV) is consistent with the computationally-determined activation energy of the S. oneidensis Mtr decaheme cytochromes.