Role of protein nanowires in metal cycling and mineralization

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Geobacter bacteria are recognized as important agents in the cycling of iron and manganese. These bacteria "breath" iron and manganese oxides, a process that requires the cells to produce hair-like filaments (pili) to bind the minerals and discharge onto them electrons generated in their metabolism. The surface of these biological "nanowires" is decorated with pockets that could trap many other metals, particularly those that are positively charged (cationic metals). This suggests that the range of metals that Geobacter bacteria can cycle and mineralize is far greater than currently acknowledged. This project will investigate the metal spectrum that Geobacter cells can bind and mineralized with their conductive pili. This research will advance NSF's Mission "to promote the progress of science" by studying a novel form of microbial energy transduction that has global implications in nature. Not only are the metals targeted in these studies naturally abundant, they are also byproducts of many industrial activities and accumulate at toxic levels as highly mobile forms, which are rapidly introduced into the food chain and increase the risk of exposure. Investigating biological mechanisms for their immobilization will provide the fundamental knowledge needed to develop technologies for biomining and bioremediation, thus addressing a national need. The research is also intertwined with educational efforts directed at training young professionals at the interface of biology and geology, but with deep understanding of physics, chemistry, and engineering. Efforts are also aimed at training future educators and stirring their motivation to engage in outreach projects that promote science communication and inclusion.This project will focus on Geobacter bacteria, the only microorganisms described to date that use protein nanowires as electronic conduits between the cell and extracellular metal electron acceptors. Each pilus fiber is an assembly of the same peptide subunit (the pilin) and exposes on its surface many carboxyl side chain ligands, which could bind cationic metals and position them optimally for their reduction. The investigator will test this in a series of experiments that evaluate the effect of piliation, pilus conductivity, and charge of the putative metal traps to bind and reductively precipitate trivalent cobalt (Co3+), divalent cadmium (Cd2+) and monovalent silver (Ag+) metal cations. These cationic metals are found in environments where iron reduction by Geobacter spp. is an active process, a process mediated by protein nanowires, and mineralized concomitantly to the reduction of iron oxides. This suggests that the Geobacter pili also mediate the reductive precipitation of the soluble metal cations, which one can assess in biological assays coupled to microscopic examination of the cells and bulk X-ray Absorption Near Edge Structure (XANES) spectroscopic analyses. The researcher will also collect the LIII-edge extended X-ray absorption fine structure (EXAFS) spectra from the pili-associated mineral to model the atomic coordination about the metal. This will allow to characterize the mineral phase and identify the nanowire ligands that are responsible for metal mineralization.