Identification of Multiple Dehalogenase Genes Involved in Tetrachloroethene-to-Ethene Dechlorination in a Dehalococcoides -Dominated Enrichment Culture.

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  μ M 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.