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.
메탄산화균은 염화화합물로 인해 오염된 토양과 지하수의 생물복원과 관련하여 오랜 시간 연구되어 왔다. Dehalococcoides spp.를 이용한 환원적 dechlorination은 실용화되고 있는 대표적 생물복원방법이지만, 대부분 pH 6.0 이상의 환경조건에서 염화화합물 분해가 일어난다. 그 중에서도 염화비닐 (VC)은 pH 6.0 이상에서만 dechlorination 이 진행되며, 그 이하의 pH일 경우 완충과정이 필요하다. 국내의 토양이 대부분 산성임을 감안하여, 산성환경에서의 염화화합물 분해제거의 필요성이 강조되고 있다.
호산성 메탄산화균의 공대사적 VC 제거가 산성환경에서 이루어졌을 때 그 적용효과를 기대할 수 있다고 가정하여, Methylomonas strain JS1과 Methylocystis strain MJC1은 각각 국내 산성 이탄습지인 오대산의 소황병산늪과 정족산의 무제치늪에서 채취한 흙에서 메탄을 이용해 농축배양한 뒤, MPN법과 고체배지를 이용해 분리하였다. 호산성인 두 미생물 그리고 호중성 Methylosinus trichosporium OB3b를 포함하여 pH 이외에 구리의 유무를 달리하여 VC 제거실험을 회분조작으로 진행하였다. JS1과 OB3b의 경우, pH 5.0에서 시작한 구리가 없는 조건과 $10 \mu M$ 구리 조건에서 $50 \mu M$의 VC를 전부 제거하였다. MJC1의 경우, 5.5에서 시작하여 5.0-5.5 사이 그리고 구리가 없는 조건에서 VC를 전부 제거하였다.
소황병산늪 슬러리와 회분조작을 진행한 결과, OB3b를 첨가한 샘플과 MJC1를 첨가한 샘플의 경우, 100시간 내에 대부분의 VC를 분해하였다. VC 분해 활성 구간에 대하여 JS1, MJC1 그리고 OB3b를 첨가한 각각의 샘플에서 소황병산늪 슬러리만을 이용한 샘플보다 높은 VC 분해속도가 관찰되었다.