Cometabolic vinyl chloride degradation at acidic pH catalyzed by acidophilic methanotrophs isolated from peat bogs in Korea

Abstract

Vinyl chloride (VC) is a carcinogenic chlorinated hydrocarbon often found in soils and groundwater. Reductive dechlorination using Dehalococcoides consortium has been the standard industry protocol for remediation of the sites contaminated with chlorinated ethenes

however, reductive dehalogenation, in general, is known to be less active at acidic pH and especially, reduction of VC at pH<6.0 has not yet been reported.

In this study, the feasibility of utilizing cometabolic degradation of acidophilic methanotroph for removing VC from moderately acidic environments (pH~5.5) was examined. A gammaproteobacterial methanotroph, Methylomonas sp. JS1, was isolated from Sohwangbyungsan peatbog and an alphaproteobacterial methanotroph, Methylocystis sp. MJC1 from Moojechi peatbog. These two strains were examined for their capability to cometabolically degrade $50 \mu M$ VC in presence of 6% $CH_4$ in the headspace. The experiments were performed in presence or absence of copper, as the presence of both mmoX and pmoA genes was confirmed in the genomes of both methanotrophs. The strain MJC1 was capable of oxidizing VC both in presence (pMMO-expressing condition) and absence (sMMO-expressing condition) of copper at the initial pH of 5.0. The strain MJC1 also exhibited limited VC degradation at pH 5.0

however, the cells were apparently incapable of withstanding the pH drop associated with VC co-oxidation. Nevertheless, strain MJC1 was capable of complete degradation of VC when the initial pH was raised to 5.5. The model neutrophilic methanotroph Methylosinus trichosporium strain OB3b was also capable of growth and VC degradation at pH 5.0 both in presence and absence of copper, albeit lack of exponential growth. pMMO-mediated VC degradation was more rapid in strain JS1 and strain OB3b cultures, where CH4 oxidation rates at the pMMO-expressing condition were higher than at the sMMO-expressing condition, while strain MJC1 oxidized both $CH_4$ and VC faster at the sMMO-expressing condition.

Bioaugmenting Sohwangbyungsan peatbog soil slurries with strain JS1, strain MJC1 and strain OB3b at pH 5.0 all resulted in acceleration of VC degradation, due mainly to the shortened lag phase as compared to the unamended slurry sample. Unanticipatedly, addition of the strain MJC1 had greater impact than strain JS1, indicative of the discrepancy between observations from simple axenic culture experiments and complex culture experiments. Another interesting observation, with potential relevance to in situ bioremediation, was the neutralizing effect of methanotroph-mediated CH4 oxidation, observed in both axenic and soil slurry experiments with all three examined strains. These observations altogether suggest the possibility of utilizing methanotrophic degradation to complement reductive dehalogenation for bioremediation of acidic soils contaminated with chlorinated ethenes.