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 chlorinated ethenes to ethene. | Evaluation of complete chlorinated ethene biodegradation |
| vcrA (Vinyl Chloride Reductase A) | Functional gene associated with reductive dechlorination of cis-DCE and vinyl chloride to ethene. | Evaluation of complete dechlorination potential |
| bvcA (BAV1 Vinyl Chloride Reductase A) | Alternative reductive dehalogenase associated with conversion of cis-DCE and vinyl chloride to ethene. | Evaluation of complete dechlorination potential |
| tceA (Trichloroethene Reductase A) | Functional gene associated with reductive dechlorination of TCE to cis-DCE and, under some conditions, vinyl chloride. | Evaluation of TCE biodegradation |
| pceA (PCE Reductase) | Functional gene associated with the initial reductive dechlorination of PCE and TCE. | Evaluation of early-stage dechlorination |
| tdrA (trans-DCE Reductase) | Functional gene associated with reductive dechlorination of trans-DCE. | Evaluation of trans-DCE biodegradation |
| Dehalobacter 16S rRNA | Identifies microorganisms capable of reductive dechlorination of highly chlorinated ethenes. | Early-stage dechlorination |
| Desulfuromonas 16S rRNA | Identifies microorganisms associated with reductive dechlorination of PCE and TCE. | Early-stage dechlorination |
| Desulfitobacterium 16S rRNA | Identifies microorganisms capable of reducing PCE and TCE to DCE isomers. | Early-stage dechlorination |
| Geobacter 16S rRNA | Identifies Geobacter spp. associated with PCE reduction and favorable iron-reducing conditions. | PCE degradation and biogeochemical evaluation |
| Geobacter pceA | Functional gene associated with PCE reduction and enhanced DNAPL dissolution. | Source-zone treatment |
| Sulfurospirillum 16S rRNA | Identifies microorganisms capable of reducing PCE to cis-DCE. | Early-stage dechlorination |
| Dehalogenimonas 16S rRNA | Identifies microorganisms associated with degradation of trans-DCE and vinyl chloride. | Late-stage dechlorination |
| cerA (Vinyl Chloride Reductase) | Functional gene associated with vinyl chloride reduction to ethene. | Evaluation of VC biodegradation |
| mbrA (Reductive Dehalogenase) | Functional gene associated with reductive dechlorination of PCE and TCE to DCE isomers. | Evaluation of early-stage dechlorination |
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.
| Observation | Possible Interpretation |
|---|---|
| Dehalococcoides detected | Indicates microorganisms capable of complete reductive dechlorination of chlorinated ethenes to ethene are present. |
| vcrA , bvcA, and/or cerAdetected | Indicates the microbial community possesses the enzymatic capability to convert cis-DCE and/or vinyl chloride to ethene. |
| tceA detected | Indicates the microbial community possesses the enzymatic capability to reductively dechlorinate TCE. |
| pceA detected | Indicates the microbial community possesses the enzymatic capability to initiate reductive dechlorination of PCE. |
| cis-DCE or vinyl chloride accumulates and vcrA/bvcA/cerA are absent or present at low abundance | May indicate incomplete reductive dechlorination due to the absence or insufficient abundance of microorganisms capable of the final dechlorination steps. |
| Low biomarker abundance | May indicate that microbial populations are insufficient or that site conditions (electron donor availability, competing electron acceptors, pH, etc.) are limiting biological activity. |
| Biomarker abundance increases following treatment | Provides evidence that electron donor addition and/or bioaugmentation successfully stimulated the target microbial community. |
| Parent compounds persist with little evidence of daughter products despite favorable geochemistry | May indicate that key microorganisms or functional genes are absent or present at insufficient abundance to support effective reductive dechlorination. |
Note: Detection of a microorganism or functional gene indicates the biological potential for a specific biodegradation pathway but does not, by itself, confirm that biodegradation is actively occurring. MBT results should be interpreted in conjunction with groundwater geochemistry, contaminant concentration trends, daughter product distributions, and other performance monitoring tools such as Compound-Specific Isotope Analysis (CSIA).
