CENSUS – Chlorinated Ethenes

Characterization of sites impacted by chlorinated solvents such as tetrachloroethene (PCE) and trichloroethene (TCE) initially emphasizes determination of contaminants of concern (COCs) and evaluation of site geologic and hydrogeologic conditions. Following this first phase, additional site characterization often focuses on evaluation of attenuation mechanisms including biodegradation to ultimately lead to an effective corrective action. Chemical and geochemical data including the concentrations of contaminants, daughter products, and terminal electron acceptors (dissolved oxygen, nitrate, iron, sulfate, etc.) obtained during site characterization provide the first two lines of evidence to evaluate the feasibility of bioremediation as a remedial alternative. While providing valuable information, both are somewhat indirect approaches to assess biological activity. The most direct avenue to evaluate biodegradation as a treatment mechanism is to directly quantify the microorganisms or biological processes responsible for biodegradation of the contaminants of concern.  Following is a breakdown of the CENSUS targets available for evaluating biodegradation of chlorinated ethenes.

CENSUS Targets for Reductive Dechlorination (Analysis of Dehalococcoides, Dehalobacter, Dehalogenimonas and other bacteria capable of reductive dechlorination)

Under anaerobic conditions, PCE can be sequentially dehalogenated through TCE, cis-dichloroethene (cis-DCE), and vinyl chloride (VC) to ethene via microbially mediated reductive dechlorination. Because ethene is an innocuous end product, reductive dechlorination is an attractive treatment mechanism for PCE/TCE-impacted sites.

The following table describes the individual CENSUS targets, their importance in evaluating reductive dechlorination as a treatment mechanism, and provides guidelines for integrating CENSUS results into routine groundwater monitoring for common corrective actions.

Target

MI Code 

  Relevance / Data Interpretation

Dehalococcoides DHC Only known group of bacteria capable of complete dechlorination of PCE and/or TCE to ethene. Absence of Dehalococcoides suggests dechlorination of DCE and VC is improbable and accumulation of daughter products is likely.  The presence of Dehalococcoides even in low copy numbers indicates the potential for complete reductive dechlorination.  Higher copy numbers and the presence of daughter products suggest that dechlorination may be occurring.
Dehalococcoides Functional Genes

 

 

TCE

VCR

BVC

 

Functional genes encoding reductive dehalogenases for TCE and VC.  Presence of TCE reductase indicates the ability to reduce TCE to DCE and VC.  Presence of VC reductases (VCR and BVC) indicates the potential for reductive dechlorination of VC to ethene.  Absence of VC reductases suggests that VC may accumulate.
Additional Functional Genes

 

 

 

 

 

Dehalobacter

PCE1

 

 

PCE2

 

 

DHB

Targets the pceA reductase genes for the sequential reductive dechlorination of PCE to cis-DCE by Sulfurospirillum spp. In mixed cultures, evidence increasingly suggests that partial dechlorinators like Sulfurospirillum and Geobacter may be responsible for the majority of reductive dechlorination of PCE to cis-DCE with Dehalococcoides functioning as cis-DCE and vinyl chloride reducing specialists.

Targets the pceA reductase genes responsible for the sequential reductive dechlorination of PCE to cis-DCE by Geobacter spp.

 

Capable of dechlorination of PCE and TCE to cis-DCE but best known for utilizing chlorinated ethanes including TCA and DCA isomers, which are common co-contaminants at PCE/TCE-impacted sites.

Desulfuromonas DSM Capable of dechlorination of PCE and TCE to cis-DCE using acetate as an electron donor.
Desulfitobacterium DSB Capable of dechlorination of PCE and TCE to cis-DCE.
 

Dehalogenimonas 

 

 

Dehalogenimonas Vinyl Chloride Reductase

 

 

Trans-1,2 Dichloroethene reductive dehalogenase

 

DHG

 

 

 

CER

 

 

TDR

Dehalogenomonas spp. are best known for dichloroelimination of chlorinated alkanes including 1,2-DCA and 1,1,2-TCA. However, the Dehalogenimonas WBC-2 culture and Dehalogenimonas strain GP have been shown to be capable of reductive dechlorination of trans-1,2-dichloroethene and vinyl chloride, respectively.

 

Targets the vinyl chloride reductase gene from Dehalogenimonas strain GP, the only known organism other than Dehalococcoides capable of vinyl chloride reduction.

 

TDR targets the gene for Trans-1,2 Dichloroethene reductive dehalogenase (TdrA) from Dehalogenimonas sp. WBC-2. The TdrA gene is involved in the dechlorination of tDCE to Vinyl Chloride.

Total bacteria EBAC Index of total bacterial biomass.  Domain level.
Methanogens MGN Methanogens utilize hydrogen and carbon dioxide to produce methane.  Compete with dechlorinating bacteria for available hydrogen.
Sulfate Reducing Bacteria APS The assay targets a gene involved in sulfate reduction.  As with methanogens, SRBs can compete with dechlorinating bacteria for available hydrogen.

 

CENSUS Targets for Cometabolism of Chlorinated Ethenes (Methanotrophs and other bacteria capable of co-metabolism)

Under aerobic conditions, several different types of bacteria including methane-oxidizing bacteria (methanotrophs), ammonia-oxidizing bacteria, and some toluene/phenol-utilizing bacteria can cometabolize or co-oxidize trichloroethene (TCE), dichloroethene, and vinyl chloride (VC). In general, cometabolism of chlorinated ethenes is mediated by monooxygenase enzymes with “relaxed” specificity that oxidize a primary (growth supporting) substrate and co-oxidize the chlorinated compound. In the presence of methane, for example, methanotrophs produce methane monooxygenases which oxidize methane to methanol and can also co-oxidize TCE.

The following table describes individual CENSUS targets and their importance in evaluating aerobic cometabolism as a treatment mechanism, and provides guidelines for integrating CENSUS results into routine groundwater monitoring for common corrective actions.

Target

MI Code 

  Relevance / Data Interpretation

Methanotrophs MOB Targets two types of methane oxidizing bacteria (methanotrophs).  Indicates the potential for cometabolic oxidation of TCE.
Soluble Methane Monooxygenase sMMO Targets the soluble methane monooxygenase gene.  Soluble methane monooxygenases are generally believed to support faster cometabolism of TCE.
Propane Monooxygenase PPO Propane can be added as a primary substrate to promote cometabolic oxidation of TCE.
Ring Hydroxylating Toluene Monooxygenase RDEG Applicable at mixed waste sites where BTEX and TCE are co-contaminants.  When expressed, toluene monooxygenases are capable of cometabolism of TCE.
Ring Hydroxylating Toluene Monooxygenase RMO Applicable at mixed waste sites where BTEX and TCE are co-contaminants.  When expressed, toluene monooxygenases are capable of cometabolism of TCE.
Butane Monooxygenase BTM Targets the butane monooxygenase gene.
Ethene Monooxygenase ETN Enumerates key functional genes (etnC and etnE) involved in ethene utilization and vinyl chloride (co)metabolism.
RD-button
census-cometabolism_of_chlorinated_ethenes_button