Identification of Multiple Dehalogenase Genes Involved in Tetrachloroethene-to-Ethene Dechlorination in aDehalococcoides-Dominated Enrichment Culture

Chloroethenes (CEs) are widespread groundwater toxicants that are reductively dechlorinated to nontoxic ethene (ETH) by members ofDehalococcoides. This study established aDehalococcoides-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 ofDehalococcoidesspecies at5.7±1.9×107 copies of 16S rRNA gene/mL. The metagenome of YN3 harbored 18rdhAgenes (designatedYN3rdhA1–18) 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 twordhAs,YN3rdhA6andYN3rdhA12,which were never proved asrdhAfor CEs, showed particularly high transcription upon addition of vinyl chloride (VC), with75±38and16±8.6mRNA 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 genusBacteroides, which might promote CE dechlorination byDehalococcoides.