Date of Award
Doctor of Philosophy in Environmental Science - (Ph.D.)
Chemistry and Environmental Science
Edgardo Tabion Farinas
1,4-Dioxane (dioxane) has emerged with an escalating concern given its human carcinogenicity and widespread occurrence in groundwater. Bioremediation is promising as an effective and cost-efficient treatment alternative for in situ or ex situ cleanup of dioxane and co-existing pollutants in the field. Soluble di-iron monooxygenases (SDIMOs) are reputed for their essential roles in initiating the cleavage of dioxane and other pollutants. In this doctoral dissertation, molecular foundations for SDIMOs-mediated dioxane biodegradation are untangled to promote the development and implication of site-specific bioremediation and natural attenuation strategies. This dissertation focused on propanotrophic bacteria given their pivotal roles in dioxane metabolism and co-metabolism.
The first part of this dissertation is centered on investigating the distinctive catalytic behaviors between two archetypical dioxane degrading enzymes, propane monooxygenase (PRM) and tetrahydrofuran monooxygenase (THM), belonging to group-6 and group-5 SDIMOs, respectively. They are compared from kinetics, inhibition, and substrate range. Results reveal that PRM is more profitable in environmental conditions such as low dioxane concentration, co-existing chlorinated solvents, and many other pollutants suggesting that PRM may has been long underestimated.
The second section refines the phylogenies of SDIMOs into six groups. The evaluation sequence of this multi-component enzyme family follows the order: group-4 alkene MO &rightarrow group-5 propane/tetrahydrofuran MO &rightarrow group-6 propane MO &rightarrow group-3 methane/butane MO. Their short-chain gaseous hydrocarbon degradation capabilities evolve from unsaturated to saturated compounds and from low C-H bond to high energy. Results allow a robust bioprospecting of SDIMO.
The third part of this dissertation is aimed to untangle downstream dioxane degradation pathways in metabolic degraders via genome the comparison of metabolic and co-metabolic strains. A putative flavin-containing monooxygenase (fmo) gene is cloned and expressed in mc2-155. Unfortunately, no HEAA transformation activity is exhibited by this transformant. Existence of the complete glycolate transformation pathway in all dioxane metabolizers reveals its essential role in dioxane mineralization.
As trace levels of dioxane (<1 mg/L) are widely detected in contaminated sites, the fourth part aims to tackle such biotransformation hindrance by bioaugmentation with a novel dioxane co-metabolizer, Azoarcus sp. DD4. DD4 exhibited formidable adaptability and relatively stable performance on dioxane degradation with the supplement of propane, supporting its feasibility for both in situ and ex situ treatment of dioxane even when its concentration is below 100 µg/L. Pure strain study reveals DD4 can overcome the inhibition of cVOCs and degrade them when supplied with propane.
Last but not the least, a bioremediation treatment train combining the reductive dehalogenation by halorespiring consortium, SDC-9, and cometabolic oxidation by DD4 to address the commingling contamination of TCE and dioxane. SDC-9 can effectively remove TCE, however, lingering with less-chlorinated but toxic metabolites, vinyl chloride (VC) and cis-dichloroethene (cDCE). Subsequent aerobic bioaugmentation with DD4, can concurrently degrade dioxane, VC, and cDCE.
Fei, Li, "1,4-dioxane biodegradation in propanotrophs: molecular foundations and implications for environmental remediation" (2020). Dissertations. 1489.