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Title: New dehalococcoides species dechlorinate chloroethenes with unusual metabolic pathways
Authors: CHENG DAN
Keywords: chloroethenes, bioremediation, reductive dechlorination, Dehalococcoides, isolate, vcrA gene
Issue Date: 2-Sep-2010
Citation: CHENG DAN (2010-09-02). New dehalococcoides species dechlorinate chloroethenes with unusual metabolic pathways. ScholarBank@NUS Repository.
Abstract: Chlorinated organic solvents are pervasive groundwater and soil contaminants due to their extensive usage (as solvents, detergents or degreasers), improper disposal and accidental spills. Under anaerobic conditions, chloroethenes such as tetrachloroethene (PCE) and trichloroethene (TCE) can be reductively dechlorinated to the less chlorinated ethenes, cis-1,2-dichloroethene (cis-DCE) by a variety of dechlorinators, and to vinyl chloride (VC) or ethene only by Dehalococcoides species. Although the generation of cis-DCE are much more commonly observed than its isomer, trans-1,2-dichloroethene (trans-DCE), the accumulation of trans-DCE at contaminated sites poses a serious problem due to its recalcitrant nature. Currently, there is no information available on the Dehalococcoides isolates that generate trans-DCE as the main end product. Furthermore, the available isolates Dehalococcoides sp. strains BAV1 and FL2 that are able to dechlorinate trans-DCE to ethene cannot metabolically detoxify TCE or PCE to ethene. Therefore isolates that could detoxify TCE and trans-DCE completely to ethene still remain elusive and complete detoxification of PCE remains a challenging task at chloroethene-contaminated sites. The main purpose of this study is to elucidate mechanisms involved in the generation and detoxification of trans-DCE in PCE/TCE-contaminated sites. Another objective is to achieve complete detoxification of PCE to ethene for efficient bioremediation. The enrichment process of several microcosm studies demonstrated that microorganisms within Cornell subgroup of Dehalococcoides could generate more trans-DCE than cis-DCE and terminate the reductive dechlorination of PCE or TCE at DCEs for the first time. Pure culture Dehalococcoides sp. strain MB was isolated from environmental sediments. It reductively dechlorinates PCE to trans-DCE and cis-DCE at a ratio of 7.3 (± 0.4) : 1. Although strain MB shares 100% 16S rRNA gene sequence identity with the first isolate of the same genus, Dehalococcoides ethenogenes strain 195, these two strains possess different dechlorinating pathways. Microarray analysis revealed that 10 out of 19 putative reductive dehalogenase (RDase) genes present in strain 195 were also detected in strain MB. Transcriptional analysis of RDase genes in strain MB grown with PCE shows that one RDase gene, designated mbrA, exhibited 10-fold up-regulation, higher than the rest of RDase genes. The highly expressed RDase gene, mbrA gene may serve as an important biomarker for evaluating, predicting, and elucidating the biological production of trans-DCE in the chloroethene-contaminated sites. Subsequently, another strictly anaerobic bacterium, designated as Dehalococcoides sp. strain 11a, was isolated in defined medium. Strain 11a rapidly and consistently dechlorinated TCE, 1,1-DCE, trans-DCE, cis-DCE, VC, and 1,2-dichloroethane metabolically to ethene with an average dechlorination rate of 53.1, 22.5, 21.6, 24.8, 86.5, and 16.7 µmol L-1 day-1 respectively. The complete detoxification of PCE to ethene for the contaminated groundwater could be achieved with a co-culture of strain 11a and a PCE-dechlorinating isolate Sulfurospirillum multivorans. Although strain 11a shares 100% 16S rRNA gene sequence identity with the first VC-dechlorinating isolate Dehalococcoides sp. strain BAV1, strain 11a showed broader substrate range than strain BAV1. To summarize, the successful cultivation of strain MB indicates that biotic processes could contribute significantly to the generation of trans-DCE in chloroethene-contaminated sites, while the isolation of strain 11a enhances our understanding of the evolution of this unusual microbial group - genus of Dehalococcoides. This study also provides a promising cost-effective bioremediation solution to the chloroethene-contaminated sites.
Appears in Collections:Ph.D Theses (Open)

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