In partial fulfillment of the requirements for the degree of   Doctor of Philosophy in Biology In the School of Biological Sciences   Yael Toporek   Will defend her dissertation   Molecular mechanisms of microbial pathways for environmental contaminant remediation   January 9th, 2023 3:00 pm   https://gatech.zoom.us/j/96188765357?pwd=V0Y1cjhWM0JPc0J4MjEwQzZLRmp3dz09    Thesis Advisor: Thomas DiChristina, Ph.D. School of Biological Sciences Georgia Institute of Technology   Committee Members: Brian Hammer, Ph.D. School of Biological Sciences Georgia Institute of Technology   Frank Stewart, Ph.D. Department of Microbiology and Cell Biology Montana State University   Amit Reddi, Ph.D. School of Chemistry and Biochemistry Georgia Institute of Technology   Martial Taillefert, Ph.D. School of Earth and Atmospheric Sciences Georgia Institute of Technology     This thesis examines the molecular mechanism of alternate strategies for remediation of contaminated environments. Radioiodine, perfluoroalkyl substances (PFAS), and 1,4-dioxane represent emerging contaminants of national concern. For example, microbially-catalyzed reductive methylation of 129IO3- has received recent attention as an alternate strategy for remediation of radioiodine-contaminated environments. This thesis identified enzymes required for IO3- reduction coupled to organic acid oxidation in the facultative anaerobe Shewanella oneidensis: cytoplasmic electron donors are oxidized, and the electrons are transferred through the periplasm via cytochromes of the metal-reducing pathway to extracellular dimethylsulfoxide (DMSO) reductase, which directly reduces IO3- to iodide (I-) as an alternate substrate. Future work aims to investigate the apparent import of I- back to the cytoplasm, where it is putatively methylated and volatilized by a promiscuous thiopurine methyltransferase, presenting a potential strategy for bioremediation of radioiodine.   In the case of PFAS, the industrial surfactant and flame retardant perfluorooctanoic acid (PFOA) has been designated as an emerging contaminant. In the present study, the microbially driven Fenton reaction (MFR) was employed to attempt degradation of PFOA by cycling between aerobic and anaerobic ferric iron (Fe(III))-reducing conditions. Under aerobic conditions, S. oneidensis reduced molecular oxygen (O2) to hydrogen peroxide (H2O2), while under anaerobic conditions, S. oneidensis reduced Fe(III) to Fe(II). During aerobic-to-anaerobic transition periods, Fe(II) and H2O2 interacted chemically via the Fenton reaction to produce contaminant-degrading hydroxyl (HO•) radicals, which in turn interacted with PFOA. PFOA concentrations, however, remained unchanged, which most likely reflects the stability of carbon-fluorine bonds and consequent inability of HO• radicals to oxidatively degrade PFOA.  Finally, the present study aimed to describe the contribution of several genes related to oxidative stress response in S. oneidensis during aerobic respiration and H2O2 stress. In contrast to S. oneidensis anaerobic respiration, aerobic respiration is understudied, and a key contributor to the success of MFR in degrading organic and chlorinated environmental contaminants like 1,4-dioxane. This work describes the contribution of individual genes, particularly catalases and peroxidases, to intracellular H2O2 scavenging rates using the genetically-encoded ratiometric fluorescent sensor HyPer-3 as a reporter.