Molecular Mechanisms of Soluble Fe(III) Reduction by Metal-Reducing Members of the Genus Shewanella

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Iron is the fourth most abundant element in the Earth's crust and plays an essential role in the biogeochemical cycling of many elements. Yet little is known about microbially mediated Fe(III) reduction in anaerobic systems. Such reactions are to a wide variety of environmentally significant processes, including the biogeochemical cycling of Fe, Mn, trace elements, and phosphate; degradation of organic matter; weathering of Fe(III)-containing clays and minerals; and biomineralization of Fe(II)-bearing minerals such as magnetite. This work, which will be carried out by researchers at the Georgia Institute of Technology, strives to determine the molecular mechanism by which metal-reducing members of the genus Shewanella solubilize and subsequently reduce soluble Fe(III). Complementary genetic, biochemical, and in situ voltammetric approaches will be used to clone the Shewanella genes involved in solubilization and subsequent reduction of soluble Fe(III). The soluble Fe(III) reductases expressed from these genes will be analyzed for protein structural characteristics and electron donor oxidation and electron acceptor reduction activities via in situ voltammetry. The purified reductases will also be used as the antigen to generate soluble Fe(III) reductase antibodies for determining the subcellular location of the reductases in Shewanella. Soluble Fe(III) in natural environments may originate from Fe(III)-reducing bacteria which synthesize and excrete Fe(III)-solubilizing compounds. This work will provide the first evidence that Fe(III)-reducing bacteria play a significant role in the production of soluble Fe(III). Results from our proposed study will also demonstrate the importance of the solubilization of solid Fe(III) by Fe(III)-reducing bacteria in iron cycling. Because the bioavailability of Fe(III) depends on the crystallinity of solid Fe(III), the availability of exogenous chelators, or the abundance of active sites onto solid Fe(III), it is possible that Fe(III)-reducing bacteria adapt to their environmental conditions by using multiple Fe(III) reduction pathways. The proposed research will provide novel information on the genes and predicted gene products required to synthesize and excrete endogenous Fe(III)-solubilizing compounds and Fe(III) reductases. This information will be used in subsequent studies to determine the chemical composition of the solubilizing compounds and will provide insights into this possibility by focusing on a poorly studied pathway. Results from our study will help refine diagenetic models in which Fe(III) is assumed to be a reactive solid and may therefore have a significant impact on the fields of biogeochemistry. In terms of broader educational impacts, this study will engage students from two disparate, yet complementary disciplines who will combine their expertise to tackle a complex biogeochemical problem.