MBTs for Chlorinated Ethane Reductive Dechlorination
Introduction
Evaluate microbial populations and functional genes associated with the anaerobic biodegradation of chlorinated ethanes, including 1,1,1-trichloroethane (1,1,1-TCA), 1,1-dichloroethane (1,1-DCA), 1,2-dichloroethane (1,2-DCA), and related compounds. Molecular Biological Tools (MBTs) help determine whether native microbial populations are capable of complete reductive dechlorination, evaluate the need for bioaugmentation, and monitor biological response following amendment application.
Common Functional Gene and Microbial Biomarker Assays for Anaerobic Chlorinated Ethane Reductive Dechlorination
| Molecular Target | Role in Reductive Dechlorination | Typical Application |
|---|---|---|
| Dehalobacter 16S rRNA | Identifies Dehalobacter spp., microorganisms capable of anaerobic reductive dechlorination of chlorinated ethanes and chloroform. | Evaluation of chlorinated ethane biodegradation potential |
| cfrA (1,1,1-Trichloroethane Reductive Dehalogenase) | Dual-functional gene associated with the reductive dechlorination of 1,1,1-trichloroethane (1,1,1-TCA) to 1,1-dichloroethane (1,1-DCA) (and chloroform to DCM). | 1,1,1-TCA biodegradation |
| ctrA (1,1,1-Trichloroethane Reductive Dehalogenase) | Functional gene associated with reductive dechlorination of 1,1,1-trichloroethane (1,1,1-TCA) to 1,1-dichloroethane (1,1-DCA). | 1,1,1-TCA biodegradation |
| dcrA (1,1-Dichloroethane Reductive Dehalogenase) | Functional gene associated with reductive dechlorination of 1,1-dichloroethane (1,1-DCA) to chloroethane. | 1,1-DCA biodegradation |
| dcaA (1,2-Dichloroethane Reductive Dehalogenase) | Functional gene associated with reductive dechlorination of 1,2-dichloroethane (1,2-DCA) to ethene. | 1,2-DCA biodegradation |
Biological Reductive Dechlorination Pathway and Associated Microbial Biomarkers
The figure below illustrates the sequential biological reductive dechlorination pathway for chlorinated ethanes together with representative microorganisms and reductive dehalogenase genes associated with each transformation step.
| Observation | Possible Interpretation |
|---|---|
| Dehalobacter detected | Indicates microorganisms capable of anaerobic reductive dechlorination of chlorinated ethanes are present. |
| cfrA detected | Indicates the microbial community possesses the enzymatic capability to reductively dechlorinate 1,1,1-trichloroethane (1,1,1-TCA). |
| ctrA detected* | Indicates the microbial community possesses the enzymatic capability to reductively dechlorinate 1,1,1-trichloroethane (1,1,1-TCA). |
| dcrA detected | Indicates the microbial community possesses the enzymatic capability to reductively dechlorinate 1,1-dichloroethane (1,1-DCA). |
| dcaA detected | Indicates the microbial community possesses the enzymatic capability to reductively dechlorinate 1,2-dichloroethane (1,2-DCA). |
| 1,1,1-TCA decreases but 1,1-DCA accumulates, and dcrA is absent or at low abundance | May indicate a biological bottleneck where the community can initiate dechlorination but lacks the specific enzymatic capability to reduce 1,1-DCA further to chloroethane. |
| 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. |
*Dual-functional gene associated with the reductive dechlorination of chloroform to DCM and 1,1,1-TCA to 1,1-DCA.
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).
