Effect of Low Concentrations of Arsenic on Microbial Iron Reduction

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In natural environments, arsenite (As(III)) is not only more mobile but is also more toxic than arsenate (As(V)) or organoarsenic compounds. Despite a wealth of knowledge gained in the laboratory or aquatic systems displaying remarkably high concentrations of arsenic, little research has been conducted on the effects of low arsenic inputs to natural aquatic systems. Incubations with sediments from a Georgia river revealed that low ( 10 micro M) but increasing As(V) concentrations spurred increases in microbial iron reduction rates accompanied by the secondary recrystallization of Fe oxides. In this project, the mechanism by which low concentrations of arsenic affect anaerobic respiration on Fe oxides by iron-reducing bacteria will be determined. It is hypothesized that As(V) stimulates the energy generation process in iron-reducing bacteria by substitution of phosphate by arsenate during phosphorylation of ADP. This effect is postulated to result from the expression of phosphate transporters and their regulators by iron-reducing bacteria in phosphate-limiting conditions. Finally, the large impact of submicromolar As(V) concentrations on microbial Fe reduction in natural environments is hypothesized to result from a catalytic cycle whereby As(III) produced by microbial detoxification is rapidly recycled to As(V) via chemical oxidation by Mn(IV) oxides present ubiquitously in natural environments. A series of incubations with pure cultures of metal-reducing Shewanella was designed to investigate in a well-controlled biological system the main molecular and geochemical processes affected by small As(V) concentrations using state-of-the art analytical and molecular techniques. This study will simultaneously provide new information on Shewanella and a model for what may occur in natural environments. This work will also provide novel information on the genes and gene products affected by the addition of low concentrations of As(V) in iron-reducing bacteria. While the influence of mineral oxides on trace metal speciation in aquatic systems is well known, the effect of trace metal concentrations on the transformation of mineral oxides is not well recognized. This work will provide the first evidence that Fe respiration is enhanced by a toxic compound and that the structure and reactivity of Fe oxides is affected by these processes. As Fe oxides play a fundamental role as scavengers of contaminants, this work will have ramifications on the fate of other contaminants in aquaticsystems. Metalloids are among the most toxic inorganic pollutants in aquatic systems but, surprisingly, their presence in water reservoirs receives little attention as their concentration is generally low. This study will demonstrate that even low concentrations of a toxic metal affect these ecosystems by altering biological and geochemical processes.