Synchronic Biotransformation of 1,4-Dioxane and 1,1-Dichloroethylene by a Gram-Negative Propanotroph Azoarcus sp. DD4

Bioremediation of 1,4-dioxane is strongly challenged by the concurrent contamination of chlorinated solvents, particularly 1,1-dichloroethylene (1,1-DCE), a potent inhibitor of archetypic 1,4-dioxane degraders (e.g., Pseudonocardia and Mycobacterium). Vegetative growth and aggregation behavior of these Gram-positive Actinomycetes also hinder their field application. A new Gram-negative bacterium Azoarcus sp. DD4 was isolated from an activated sludge sample. Notably, DD4 can sustain the concurrent co-oxidation of 1,4-dioxane and 1,1-DCE using propane as the primary substrate without observable formation of clumps. Microcosm assays prepared with source zone groundwater samples from a contaminated site indicated DD4 can efficiently remove dioxane, with the concentration decreasing from an initial value of 10.4 mg/L to <0.4 μg/L within 14 days of incubation. Removal of 1,4-dioxane was partially inhibited when an excessive amount of 1,1-DCE (3.28 ± 0.19 mg/L) was artificially spiked into the microcosms but significantly accelerated immediately after the complete depletion of 1,1-DCE. Furthermore, a gene encoding a putative propane monooxygenase was discovered, which may contribute to the oxidation of propane, 1,4-dioxane, and/or 1,1-DCE. Detection of 2-S-glutathionyl acetate and synchronic dechlorination suggest that DD4 detoxifies its primary metabolite, 1,1-DCE epoxide, via conjugation with glutathione. All these findings indicate the suitability of DD4 as a robust inoculum candidate for in situ bioaugmentation to remediate co-contamination by dioxane and 1,1-DCE.