Relationships between the Abundance and Expression of Functional Genes from Vinyl Chloride (VC)-Degrading Bacteria and Geochemical Parameters at VC-Contaminated Sites

Abstract

Bioremediation of vinyl chloride (VC) contamination in groundwater could be mediated by three major bacterial guilds: anaerobic VC-dechlorinators, methanotrophs, and ethene-oxidizing bacteria (etheneotrophs) via metabolic or cometabolic pathways. We collected 95 groundwater samples across 6 chlorinated ethene-contaminated sites and searched for relationships among VC biodegradation gene abundance and expression and site geochemical parameters (e.g., VC concentrations). Functional genes from the three major VC-degrading bacterial guilds were present in 99% and expressed in 59% of the samples. Etheneotroph and methanotroph functional gene abundances ranged from 10 to 10 genes per liter of groundwater among the samples with VC reductive dehalogenase gene (bvcA and vcrA) abundances reaching 10 genes per liter of groundwater. Etheneotroph functional genes (etnC and etnE) and VC reductive dehalogenase genes (bvcA and vcrA) were strongly related to VC concentrations (p < 0.001). Methanotroph functional genes (mmoX and pmoA) were not related to VC concentration (p > 0.05). Samples from sites with bulk VC attenuation rates >0.08 year contained higher levels of etheneotroph and anaerobic VC-dechlorinator functional genes and transcripts than those with bulk VC attenuation rates <0.004 year. We conclude that both etheneotrophs and anaerobic VC-dechlorinators have the potential to simultaneously contribute to VC biodegradation at these sites.

Hydrochemical Conditions for Aerobic/Anaerobic Biodegradation of Chlorinated Ethenes—A Multi-Site Assessment

“…All functional genes together (etnC, etnE, mmoX, pmoA, vcrA and bvcA) were detected in 38 of 49 (78%) samples analysed, indicating that both aerobic oxidation and reductive dechlorination of chloroethenes may take place simultaneously at the same place or in close microenvironments. This finding is consistent with the study of Liang et al [20], which detected functional genes from all three bacterial guilds (ethenotrophs, methanotrophs and reductive dechlorinators) in 99% of groundwater samples collected at six contaminated sites.…”

“…Both sMMO and pMMO are also capable of fortuitous oxidation of chloroethenes. The sMMO have a broader substrate range than pMMO, and are more efficient at degrading chlorinated ethenes [20]. The gene mmoX, which encodes the sMMO α subunit, and pmoA, which encodes the pMMO α subunit, are used as biomarkers of chloroethene cometabolic potential in groundwater [20][21][22].Etheneotrophs can cometabolise VC and DCE when growing on ethene as a primary growth substrate, while several pure etheneotrophic strains, such as Mycobacterium and Nocardiodes, can also utilise VC as their sole carbon and energy source [18].…”

“…The sMMO have a broader substrate range than pMMO, and are more efficient at degrading chlorinated ethenes [20]. The gene mmoX, which encodes the sMMO α subunit, and pmoA, which encodes the pMMO α subunit, are used as biomarkers of chloroethene cometabolic potential in groundwater [20][21][22].Etheneotrophs can cometabolise VC and DCE when growing on ethene as a primary growth substrate, while several pure etheneotrophic strains, such as Mycobacterium and Nocardiodes, can also utilise VC as their sole carbon and energy source [18]. Etheneotrophs, when growing on ethene and VC, express a soluble alkene monooxygenase (AkMO), transforming VC to epoxide chlorooxirane, which is further metabolised to 2-chloro-2-hydroxyethyl-CoM by epoxyalkane:coenzyme M transferase (EaCoMT) [19,23].…”

“…Etheneotrophs, when growing on ethene and VC, express a soluble alkene monooxygenase (AkMO), transforming VC to epoxide chlorooxirane, which is further metabolised to 2-chloro-2-hydroxyethyl-CoM by epoxyalkane:coenzyme M transferase (EaCoMT) [19,23]. The genes etnC and etnE encode the α subunit of AkMO and the EaCoMT, respectively, and serve as emerging biomarkers for ethenotroph-mediated aerobic biodegradation potential, though they do not distinguish between metabolic and cometabolic biodegradation pathways [20,24].Two degradation pathways have been proposed as regards aerobic metabolic degradation of cis-1,2-DCE. Both of them involve degradation through monooxygenase-catalysed epoxidation [25], with the initial step catalysed by cytochrome P450 monooxygenase.…”

“…Both sMMO and pMMO are also capable of fortuitous oxidation of chloroethenes. The sMMO have a broader substrate range than pMMO, and are more efficient at degrading chlorinated ethenes [20]. The gene mmoX, which encodes the sMMO α subunit, and pmoA, which encodes the pMMO α subunit, are used as biomarkers of chloroethene cometabolic potential in groundwater [20][21][22].…”