DESCRIPTION (provided by the applicant): One promising strategy for the in situ bioremediation of radioactive contaminants is to stimulate the activity of dissimilatory metal-reducing microorganisms to reductively precipitate uranium and other soluble toxic metals. While Geobacter bacteria are recognized as important agents for the reductive precipitation of soluble U(VI) to insoluble U(IV) in situ, the underlying mechanism is poorly understood. The reduction of U(VI) and other soluble contaminants by these microorganisms is directly dependent on the reduction of Fe(III) oxides, their natural electron acceptor. The recent discovery that Geobacter bacteria employ a novel mechanism for electron transport to Fe(III) oxides via protein nanowires (conductive pili) prompted us to investigate a potential role for these extracellular appendages in U(VI) reduction. Expression of pilus nanowires in the model organism Geobacter sulfurreducens led to a rapid precipitation of U(VI) to a U(IV) mineral along the nanowire length, demonstrating a previously unrecognized role for Geobacter nanowires in electron transfer to U(VI). Our objective in this proposal is to characterize the mechanism of U(VI) reduction mediated by Geobacter's pilus nanowires at the molecular and nano-scale level. As a complex biological and electronic structure of nanoscale dimensions (3-5 nm in diameter), insights into the structure and electronics of Geobacter's nanowires and their contribution to uranium reduction will require innovative approaches that integrate biological, physical and nanotechnological tools. To accomplish this, we bring together a team of researchers with recognized leadership in microbial metal reduction and nanowires, biosensor design, and environmental spectroscopy. Specifically, we propose the following aims: Specific Aim #1: Identify the molecular basis of nanowire-mediated electron transfer. Our working hypothesis, based on strong preliminary data presented in the application, is that the key nanowire components involved in U(VI) reduction can be identified by screening mutants with defects in nanowire functions and studying their effect in U(VI) transformations. Specific Aim #2: Develop nanostructured bioelectronic interfaces that integrate electroactive nanowires and lipid membranes with electrodes. Our working hypothesis, also based upon preliminary data, is that nanowire electrochemical processes can be mimicked and studied using nanostructured bioelectronic interfaces that integrate functional components of the nanowire system with electrodes. These studies are innovative in that they integrate microbiological, physical and nanotechnological tools to elucidate the novel mechanism of uranium reduction by microbial nanowires. At the completion of these studies we will have elucidated the basis of nanowire electron transfer to uranium at the molecular and nanoscale level, to enhance our knowledge of the key biological processes involved in the bioremediation of radionuclides and other soluble toxic metals and provide a basis for improved bioremediation processes.

This work will use nanotechnology-enabled tools to develop a fundamental understanding of radionuclide reduction via Geobacter nanowires. These results will enable design of efficient in situ bioremediation strategies for radionuclides, thereby reducing a widespread and serious threat to the Nation's health.