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theses

The soluble oxyanions of tellurium and selenium are the most toxic forms of the two elements. While various stages of the biogeochemical cycles of each have been explored by other researchers, there remain large gaps in knowledge of how microorganisms interact with various tellurium and selenium compounds. What is known is that the sulfur metabolism of microorganisms is often involved in these interactions. In Chapter 2 of this thesis, explores the role of sulfate transporters in the uptake of tellurate in Escherichia coli K-12. Previously it was unknown how tellurate enters the cells. A mutant strain with a deletion of the cysW gene of the CysPUWA sulfate transporter system accumulated less cellular tellurium and exhibited higher resistance to tellurate compared to the wild type strain. Chapter 3 of this thesis begins with the observation that tellurate was significantly more toxic to E. coli K-12 cells grown on LB medium compared to M9 medium. Cystine was demonstrated to be the key media component controlling this difference. The cystine must be transported intracellularly—and therefore reduced to cysteine—in order to enhance the toxicity of tellurate. Tellurate was found to be reactive with cysteine and is reduced to elemental tellurium in that reaction. Oxidation of intracellular thiols increases the resistance of E. coli K-12 to tellurate. These results suggest that the interaction between the two contributes to the toxicity of tellurate. In Chapter 4, a Bacillus species was isolated from seleniferous soils that is capable of solubilizing elemental selenium. This process occurred extracellular and was demonstrated to be mediated by the sulfur metabolites (sulfide, sulfite, and thiosulfate) secreted by the organism. Finally, in Chapter 5, expands on the study of extracellular sulfur metabolite production by microorganisms under non-sulfur-respiring conditions. The dissimilatory metal-reducing bacterium Shewanella oneidensis MR-1 produces a substantially larger quantity of sulfite than the Bacillus species isolated in Chapter 4. This sulfite production was observed under aerobic and anaerobic conditions and was suppressed by amendment with cysteine suggesting that the sulfite is generated from the assimilatory sulfate reduction pathway. The data in Chapter 4 and Chapter 5 suggest that bacteria can produce and export substantial quantities of sulfite that can impact the solubility and speciation of elements in the environment.

ETD_10453

Ph.D.

Includes bibliographical references

doi:10.7282/t3-fv0d-9m67

ETD doctoral

The author owns the copyright to this work.