Microbial Respiration and Precipitation of Arsenic

This dissertation explores the biogeochemistry, physiology, molecular biology, and environmental impact of dissimilatory arsenate reducing bacteria. A new species, Desulfotomaculum auripigmentum strain OREX-4, was found to precipitate amorphous arsenic trisulfide (AS2S3)both intraand extracellularly. Intracellular precipitation of AS2S3nucleates precipitation in the bulk milieu, and results from the reduction of arsenate and sulfate during respiration. Strain OREX-4differs from the previously described arsenate respiring isolates strains MIT-13 (Geospirillum arsenophilus) and SES-3 (Geospirillum barnesii): strain OREX-4 grows on different substrates, and falls within the gram-positive group of the Bacteria whereas MIT-13 and SES-3 fall together in the epsilon subdivision of the Proteobacteria. Strain OREX-4 prefers arsenate to sulfate, and can grow on the arsenate-bearing mineral scorodite. Dissimilatory arsenate reductase, the enzyme responsible for growth on arsenate, was characterized and purified from strain 5E5-3. This enzyme is membrane-bound, unlike the arsenate reductases of Escherichia coli and Staphylococcus aureus, which are cytoplasmic and used for reductive detoxification of arsenate but not for growth. Enzyme activity in washed cell suspensions exhibited a Vrnax for arsenate reduction of approximately 0.1 Jimol/min/mg and an apparent Krn of 200 JiM arsenate. Evidence for the presence of a b-type cytochrome in cells grown on arsenate was found. NADH can serve as an electron donor for the dissimilatory arsenate reductase, and Fe-S clusters appear to be prosthetic groups in the enzyme. These studies suggest a novel mechanism of arsenate reduction. The ability of bacteria indiginous to arsenic-polluted soil to reduce arsenate was also examined. Microbial arsenate reduction was stimulated in the presence of organic carbon and microaerophilic conditions. Nitrate did not inhibit arsenate reduction, yet the formation of Fe(OHh significantly retarded arsenate reduction as long as iron(III) remained oxidized. These results contrast with the behavior of arsenate-reducing consortia in previous studies. It is now apparent that dissimilatory arsenate reduction is widespread and readily performed by diverse bacteria from site to site. The ability of microbes to moblize arsenic through reduction is cause for concern. Thesis Supervisor: Fran~ois M. M. Morel Title: Professor of Civil and Environmental Engineering