Bacterial Exudates: Characteristics and Role in Selenium Reduction

The goals of this research are to further our understanding of interactions between soluble bacterial exudates and selenium, and to determine how these interactions may affect the environmental behavior of Se. In Chapter 2, I discuss a study that tests whether soluble bacterial exudates can reduce aqueous Se(IV). Exudate solutions were isolated from three different model species of bacteria and tested for reductive activity to added Se(IV). The results show that some bacterial exudates reduce Se(IV), but that those from Shewanella oneidensis can not unless the cells are grown in the presence of aqueous Se(IV). The results also demonstrate that Se(IV) reduction by each of the exudate solutions involves exudate-hosted sulfhydryl binding. Chapter 3 describes potentiometric titration experiments that were used to measure the concentrations of proton-active sites on the exudate molecules produced by several bacterial species as a function of soak time, and the chapter also reports the first measurements of sulfhydryl site concentrations on exudate molecules. The total binding site concentrations of the exudates are primarily controlled by DOC concentration, unlike sulfhydryl site concentrations, suggesting that production of sulfhydryl-site bearing exudates is primarily an active process. The research reported in Chapter 4 uses comparative proteomics to identify candidate proteins that are responsible for the inducible Se(IV) reductase activity of soluble S. oneidensis exudates. Combining the control of S. oneidensis exudates’ inducible activity with knowledge of required characteristics for Se(IV) reduction allows us to narrow the list to six candidate proteins for further investigation. The research in this dissertation demonstrates for the first time the ability of soluble bacterial exudates to reduce Se(IV) and suggests that Se(IV) binding onto sulfhydryl sites controls the reduction reaction. The research characterizes the total concentration of binding sites and the concentration of sulfhydryl sites specifically on exudate molecules, and it identifies six candidate proteins that are responsible for the observed Se(IV) reduction. Overall, the research presented here improves our understanding of Se(IV)-exudate reactions, and could be useful in refining environmental models of selenium behavior and developing bioremediation methods for selenium contamination.