Abstract
1,1,2-trichloroethane (1,1,2-TCA) has become a common groundwater pollutant due to historically extensive utilization, improper disposal, as well as from incomplete dechlorination of 1,1,2,2-tetrachloroethane. Currently, limited information is available on microbial detoxification of 1,1,2-TCA. Desulfitobacterium sp. strain PR, which was isolated from an anaerobic bioreactor maintained to dechlorinate chloroethenes/ethanes, exhibited the capacity to dechlorinate 1,1,1-trichloroethane and chloroform. In this study, the dechlorinating ability of strain PR was further explored. Strain PR showed the capability to dechlorinate 1,1,2-TCA (~1.12 mM) predominantly to 1,2-dichloroethane (1,2-DCA) and chloroethane, and to trace amounts of vinyl chloride and ethene within 20 days. Strain PR coupled growth with dechlorination of 1,1,2-TCA to 1,2-DCA, while no cell growth was observed with dechlorination of 1,2-DCA to chloroethane. Later, through transcriptomic and enzymatic analysis, the reductive dehalogenase CtrA, which was previously reported to be responsible for 1,1,1-trichloroethane and chloroform dechlorination, was identified as the 1,1,2-TCA reductive dehalogenase. Since trichloroethene (TCE) is usually co-contaminated with 1,1,2-TCA, a co-culture containing Dehalococcoides mccartyi strain 11a capable of detoxifying TCE and 1,2-DCA and strain PR was established. Interestingly, this co-culture dechlorinated 1,1,2-TCA and TCE to the non-toxic end-product ethene within 48 days without chloroethane production. This novel pathway avoids production of the carcinogenic intermediate dechlorination product vinyl chloride, providing a more environmentally friendly strategy to treat 1,1,2-TCA.
Highlights
1,1,2-trichloroethane (1,1,2-TCA), molecular formula CHCl2-CH2Cl, has been widespread in the environment by extensive usage as a degreasing agent, through improper storage an
We show that Desulfitobacterium sp. strain PR reductively dechlorinates 1,1,2-TCA to 1,2-DCA and chloroethane via a stepwise reductive dechlorination process
The growth yield of strain PR from dechlorination of 1,1,2-TCA is 3.72×1013 cells per mole chlorine, which is comparable to the growth yields from dechlorination of chloroform or 1,1,1-TCA by strain PR and of chloroethanes by other organohalide respiring bacteria [11, 15]
Summary
1,1,2-trichloroethane (1,1,2-TCA), molecular formula CHCl2-CH2Cl, has been widespread in the environment by extensive usage as a degreasing agent, through improper storage and PLOS ONE | DOI:10.1371/journal.pone.0119507 April 2, 2015. 1,1,2-TCA is very persistent in the environment, with an estimated half-life of 136–360 days in soil and 136–720 days in groundwater [1] It is found in at least 262 out of 1293 sites in the United States on the National Priorities List (NPL) identified by the U.S Environmental Protection Agency [2]. The methanogen Methanobacterium thermoautotrophicum was observed to cometabolically dechlorinate 1 μM of 1,1,2-TCA to vinyl chloride (89%), 1,2-DCA (11%), and a trace amount of ethane at 60°C in a complex medium [6] Both uncharacterized mixed cultures [7, 8] and other reported strains belonging to the Desulfitobacterium, Dehalobacter, and Dehalogenimonas genera [9,10,11,12,13] have been shown to dechlorinate 1,1,2-TCA through dihaloelimination, resulting in the accumulation of vinyl chloride as the end product.
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