Solid State Electron Transfer via Bacterial Nanowires: Contributions Toward a Mechanistic Understanding of Geophysical Response of Biostimulated Subsurface

The degradation of organic matter by microorganisms provides a source of electrical potential or so-called 'self potential' (SP) that can be measured by using a voltmeter. During this process electrons are being produced as a waste-product and bacterial cells have to dispose of these to allow for the complete biodegradation of organic matter. Especially in anaerobic microbial communities, exo-cellular electron transfer is the most important driving force behind this process and organisms have developed different, but also similar, ways to transfer electrons to other microorganisms. Recently, it has been postulated that direct electron transfer from cell-to-cell is actually done by 'hard-wired' microorganisms. This shuttling of electrons is most likely done by certain c-type cytochromes that form the functional part of electrically conductive nanowires. In this study we investigated if nanowires can explain the geoelectrical (self potential and spectral induced polarization) signals observed at some biostimulated environments such as DOE sites. The objectives of our project are to: (1) investigate any temporal changes in the geophysical signatures (Self Potential (SP) and Induced Polarization (IP)) associated with nanowires of the bacterium Shewanella oneidensis MR-1, wild type and mtrc/omcA deletion mutant, (2) demonstrate that mutant strains of bacteria that produce nonconductive nanowires do not contribute to geoelectrical responses. We accomplished the following: (1) Provided training to students and a postdoctoral fellow that worked on the project, (2) Conducted several SP & IP measurements correlating the distribution of nanowires and SIP/SP signals in partial fulfillment of object No. 1 and 2. On the following we will report and discuss the results of our last experiment with some emphasis on the source mechanisms of both SP and IP associated with Shewanella oneidensis MR-1, wild type in sand columns