Chlorinated Ethanes (TCA, DCA)

Under anaerobic conditions, chlorinated ethanes are susceptible to reductive dechlorination by several groups of halorespiring bacteria, most notably Dehalobacter and Dehalogenimonas spp., and functional genes for reductive dehalogenases continue to be identified.  Anaerobic biodegradation of 1,1,2-TCA and 1,2-DCA proceeds via dichloroelimination producing vinyl chloride and ethene, respectively.  Dehalobacter spp. have also been isolated that are capable of sequential reductive dechlorination of 1,1,1-TCA through 1,1-DCA to chloroethane.

QuantArray®-Chlor includes quantification of all Targets listed in the table below including Dehalobacter, Dehalococcoides, additional halorespiring bacteria, functional genes, and competing microorganisms.  Alternatively, CENSUS® qPCR can be performed to quantify a select subset such as Dehalobacter and Dehalogenimonas.

TARGET	CODE	RELEVANCE / DATA INTERPRETATION
Dehalobacter	DHB	Dehalobacter spp. and Dehalobacter-containing cultures have been shown to be responsible for sequential reductive dechlorination of 1,1,1-TCA through 1,1-DCA to chloroethane. Moreover, Dehalobacter spp. perform the dichloroelimination of 1,1,2-TCA and 1,2-DCA to vinyl chloride and ethene, respectively.
Dehalogenimonas	DHG	Dehalogenimonas spp. utilize a broad spectrum of vincinally chlorinated alkanes including 1,1,2,2-TeCA, 1,1,2-TCA, and 1,2-DCA as growth-supporting electron acceptors.
Dehalococcoides	DHC	Dehalococcoides are capable of dichloroelimination of 1,2-DCA. However, perhaps the most important reason to quantify Dehalococcoides at chlorinated ethane sites is to assess the potential for reductive dechlorination of vinyl chloride produced by biodegradation of 1,1,2-TCA by Dehalobacter and Dehalogenimonas spp.
Desulfitobacterium	DSB	The range of electron acceptors and pathways utilized varies considerably between Desulfitobacterium isolates. For example, D. dichloroeliminans strain DCA1 utilizes 1,1,2-TCA and 1,2-DCA producing vinyl chloride and ethene while Desulfitobacterium sp. PR sequentially dechlorinates 1,1,2-TCA to 1,2-DCA which is cometabolized to chloroethane. In addition, strain PR sequentially dechlorinates 1,1,1-TCA to 1,1-DCA and chloroethane and D. hafniense Y51 transforms hexachloro-, pentachloro-, and tetrachloroethanes but not 1,1,2-TCA.
1,2-Dichloroethane Reductive Dehalogenase	DCAR	Targets the 1,2-dichloroethane reductive dehalogenase gene from members of Desulfitobacterium and Dehalobacter , which dechlorinate 1,2-DCA to ethene.
1,1-Dichloroethane Reductive Dehalogenase (DCA)	DCA	Targets the 1,1-dichloroethane reductive dehalogenase gene found in some strains of Dehalobacter.
Chloroform/1,1,1-TCA Reductase (CFR)	CFR	Targets the cfrA gene of Dehalobacter spp. that encodes a reductase enzyme responsible for dechlorination of 1,1,1-TCA.
Functional Genes	TCE, BVC, VCR	As mentioned previously, anaerobic biodegradation of 1,1,2-TCA leads to the production of vinyl chloride. Quantification of vinyl chloride reductase genes (BVC, VCR) along with Dehalococcoides is used to assess whether vinyl chloride will accumulate or be further dechlorinated to ethene.
Total Bacteria	EBAC	Index of total bacterial biomass.
Methanogens	MGN	Methanogens utilize hydrogen and can compete with halorespiring bacteria for available electron donor.
Sulfate Reducing Bacteria	APS	Sulfate reducing bacteria can compete with halorespiring bacteria for available hydrogen.

While less widely studied than cometabolism of TCE and other chlorinated ethenes, some chlorinated ethanes including 1,1,1-TCA and 1,2-DCA are also susceptible to cometabolism/co-oxidation.  Cometabolism of chlorinated ethanes is also mediated by monooxygenase enzymes with “relaxed” specificity that oxidize a primary, growth supporting substrate such as methane and co-oxidize the chlorinated compound.

CENSUS® qPCR can be performed to quantify specific functional genes such as soluble methane monooxygenase.  For sites impacted by both chlorinated ethanes and ethenes, consider QuantArray®-Chlor which includes quantification of soluble methane monooxygenase (sMMO) genes as well as toluene monooxygenase (RMO, RDEG), phenol hydroxylase (PHE), the toluene dioxygenase (TOD) which are capable of cometabolism of TCE.

TARGET	CODE	RELEVANCE / DATA INTERPRETATION
Soluble Methane Monooxygenase	sMMO	Targets the gene encoding soluble methane monooxygenases which can co-oxidize a broad range of chlorinated compounds. In laboratory studies, sMMO has been shown to co-oxidize a number of chlorinated ethanes including 1,1,1-TCA and 1,2-DCA as well as TCE, cis-DCE, and vinyl chloride.
Propane Monooxygenase	PPO	Propane can be added as a primary substrate to promote growth of propane utilizing bacteria capable of cometabolism of TCE and chlorinated ethanes including 1,1,2-TCA.
Butane Monooxygenase	BMO	Like propane, butane can be added as a primary substrate to support cometabolism of chlorinated ethenes and chlorinated ethanes including 1,1,1-TCA.

Next Generation Sequencing (NGS)

Multiple lines of evidence can provide a more complete picture. At complex sites, CENSUS® qPCR or QuantArray® is performed to quantify known contaminant degraders like Dehalobacter, Dehalococcoides and functional genes while next generation sequencing (NGS) is often used for an overall profile of the microbial community composition. Knowing which microorganisms are present and their relative abundances provides insight into the types of microbial processes might be occurring such as fermentation or metals reduction.

In Situ Microcosms (ISMs)

In Situ Microcosms (ISMs) are field deployed microcosm units containing passive samplers that provide the microbial, chemical, and geochemical data for simultaneous, cost-effective evaluation of multiple remediation options.

For remedy selection at chlorinated solvent sites, an ISM study typically includes:

• • An unamended MNA unit to evaluate monitored natural attenuation
  • A BioStim unit amended with an electron donor
  • A BioAug unit amended with a commercial bioaugmentation culture and an electron donor

Each ISM unit contains passive samplers – passive diffusion bags (PDBs) for VOCs analysis of contaminant concentrations, passive geochem samplers for dissolved gases (ethene, ethane, methane) and anions like sulfate, and Bio-Traps® for QuantArray®-Chlor or CENSUS® qPCR quantification of Dehalobacter, Dehalogenimonas, Dehalococcoides, functional genes, and other halorespiring bacteria.

By comparing contaminant concentrations, daughter product formation, geochemical conditions, and concentrations of halorespiring bacteria between the MNA, BioStim, and BioAug units, site managers can evaluate each remediation option at a fraction of the cost of a lab bench treatability study or pilot scale study.

REFERENCE
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