MBTs for Anaerobic Chlorinated Ethene Reductive Dechlorination
Introduction
Anaerobic reductive dechlorination is the primary biological process responsible for the degradation of chlorinated ethenes, including perchloroethene (PCE), trichloroethene (TCE), dichloroethene (DCE) isomers, and vinyl chloride (VC). Under anaerobic conditions, specialized microorganisms use chlorinated solvents as electron acceptors, sequentially removing chlorine atoms to ultimately produce non-toxic ethene.
Molecular Biological Tools (MBTs) provide direct evidence of a site's biological potential by identifying and quantifying microorganisms and reductive dehalogenase genes responsible for each step of the dechlorination pathway. These analyses help determine whether complete dechlorination is likely to occur, evaluate the effectiveness of electron donor additions and bioaugmentation, and identify biological limitations that may prevent contaminants from degrading beyond intermediate compounds such as cis-DCE or vinyl chloride.
The table below summarizes commonly available microbial targets and functional gene assays used to evaluate anaerobic chlorinated ethene reductive dechlorination.
Common Functional Gene and Microbial Biomarker Assays for Anaerobic Chlorinated Ethene Reductive Dechlorination
| Molecular Target | Role in Reductive Dechlorination | Typical Application |
|---|---|---|
| Dehalococcoides 16S rRNA | Identifies Dehalococcoides spp., the only known microorganisms capable of complete reductive dechlorination of PCE through vinyl chloride to ethene. | Complete chlorinated ethene biodegradation |
| vcrA (Vinyl Chloride Reductase A) | Encodes the enzyme responsible for reducing vinyl chloride and cis-DCE to ethene. | Complete dechlorination to ethene |
| bvcA (BAV1 Vinyl Chloride Reductase A) | Alternative reductive dehalogenase that converts cis-DCE and vinyl chloride to ethene. | Complete dechlorination to ethene |
| tceA (Trichloroethene Reductase A) | Catalyzes reduction of TCE to cis-DCE and may also reduce vinyl chloride under certain conditions. | TCE degradation |
| pceA (PCE Reductase) | Initiates reductive dechlorination of PCE and TCE to DCE isomers. | Early-stage dechlorination |
| tdrA (trans-DCE Reductase) | Catalyzes reduction of trans-DCE to ethene. | trans-DCE degradation |
| Dehalobacter 16S rRNA | Detects Dehalobacter spp., organisms capable of reductive dechlorination of highly chlorinated ethenes. | PCE and TCE degradation |
| Desulfuromonas 16S rRNA | Identifies microorganisms capable of reducing PCE and TCE under anaerobic conditions. | Early-stage dechlorination |
| Desulfitobacterium 16S rRNA | Detects organisms capable of reducing PCE and TCE to DCE isomers. | Early-stage dechlorination |
| Geobacter 16S rRNA | Identifies Geobacter spp., which can reductively dechlorinate PCE and contribute to favorable biogeochemical conditions. | PCE degradation and site geochemistry |
| Geobacter pceA | Detects the Geobacter reductive dehalogenase associated with PCE reduction and enhanced DNAPL dissolution. | Source-zone treatment |
| Sulfurospirillum 16S rRNA | Detects microorganisms capable of reducing PCE to cis-DCE. | Early-stage dechlorination |
| Dehalogenimonas 16S rRNA | Identifies organisms involved in degradation of trans-DCE and vinyl chloride. | Later-stage dechlorination |
| cerA (Vinyl Chloride Reductase) | Encodes the enzyme responsible for conversion of vinyl chloride to ethene by Dehalogenimonas. | Vinyl chloride degradation |
| mbrA (Reductive Dehalogenase) | Functional gene associated with reduction of PCE and TCE to DCE isomers. | PCE/TCE degradation |
Biological Reductive Dechlorination Pathway and Associated Microbial Biomarkers
The figure below illustrates the sequential biological reductive dechlorination pathway for chlorinated ethenes together with representative microorganisms and reductive dehalogenase genes associated with each transformation step.
