Abstract
Chloroethenes (CEs) are widespread groundwater toxicants that are reductively dechlorinated to nontoxic ethene (ETH) by members of Dehalococcoides. This study established a Dehalococcoides-dominated enrichment culture (designated “YN3”) that dechlorinates tetrachloroethene (PCE) to ETH with high dechlorination activity, that is, complete dechlorination of 800 μM PCE to ETH within 14 days in the presence of Dehalococcoides species at 5.7 ± 1.9 × 107 copies of 16S rRNA gene/mL. The metagenome of YN3 harbored 18 rdhA genes (designated YN3rdhA1–18) encoding the catalytic subunit of reductive dehalogenase (RdhA), four of which were suggested to be involved in PCE-to-ETH dechlorination based on significant increases in their transcription in response to CE addition. The predicted proteins for two of these four genes, YN3RdhA8 and YN3RdhA16, showed 94% and 97% of amino acid similarity with PceA and VcrA, which are well known to dechlorinate PCE to trichloroethene (TCE) and TCE to ETH, respectively. The other two rdhAs, YN3rdhA6 and YN3rdhA12, which were never proved as rdhA for CEs, showed particularly high transcription upon addition of vinyl chloride (VC), with 75 ± 38 and 16 ± 8.6 mRNA copies per gene, respectively, suggesting their possible functions as novel VC-reductive dehalogenases. Moreover, metagenome data indicated the presence of three coexisting bacterial species, including novel species of the genus Bacteroides, which might promote CE dechlorination by Dehalococcoides.
Highlights
Chloroethenes (CEs) such as tetrachloroethene (PCE) and trichloroethene (TCE) have been used extensively in dry cleaning and as degreasing agents
YN3 was tested for dechlorination of PCE at a low concentration (70 μM) and showed stable PCE-to-ETH dechlorination activity
Only two rdhAs encoding reductive dehalogenase (RdhA) that dechlorinate vinyl chloride (VC) to ETH, that is, vcrA and bvcA, have been identified by enzymatic assays [15, 16]. vcrA was originally identified via amino acid sequencing of the partially purified VcrA [15], while bvcA was first suggested as a candidate VC RdhA on the basis of its increased transcription in response to VC [22], and BvcA was recently proven to dechlorinate VC by native gel assay [16]
Summary
Chloroethenes (CEs) such as tetrachloroethene (PCE) and trichloroethene (TCE) have been used extensively in dry cleaning and as degreasing agents. They are commonly detected in groundwater because of improper disposal and accidental spills. Dehalorespiring bacteria, such as Dehalococcoides, which reductively dechlorinate CEs via respiration [1], have been applied as biocatalysts to remedy environments contaminated with CEs [2]. No isolated strain can completely dechlorinate PCE to ETH without accumulation of toxic intermediates in pure culture [3]. Consortia containing non-dehalorespiring bacteria together with Dehalococcoides show faster dechlorination [8, 9], and several possible functions of non-dehalorespirators in dehalorespiring consortia have been suggested [8,9,10,11,12]
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