Chlorinated Biphenyls | Biodegradation Evaluation

Anaerobic Biodegradation

Commercial formulations of polychlorinated biphenyls (PCBs) were mixtures of 60 to 90 congeners with different degrees or chlorination and positions of chlorine substituents (ortho, meta, or para). Both the number and positions of chlorines impact biodegradation. In general terms, highly chlorinated PCBs are subject to reductive dechlorination while less heavily chlorinated congeners can be co-metabolized aerobically. Thus, while considered persistent in part due to their hydrophobicity, PCBs can potentially be mineralized through a sequence of anaerobic-aerobic biodegradation.

Molecular evidence has conclusively implicated Dehalococcoides in the reductive dechlorination of PCBs. While dechlorination patterns differ between strains, Dehalococcoides strain CBDB1 extensively dechlorinates the main congeners in Aroclor 1260 to tetra- and trichlorobiphenyls.

Submit samples for CENSUS® qPCR to quantify a halorespiring bacteria capable of anaerobic biodegradation of PCBs.

TARGET	CODE	RELEVANCE / DATA INTERPRETATION
Dehalococcoides	DHC	While the range of compounds utilized varies, some Dehalococcoides strains are capable of reductive dechlorination of PCBs.
Dehalobium chlorocoercia DF-1	DECO	Shown to reductively dechlorinate doubly flanked chlorines in PCBs. Interestingly, in a mesocosm study, bioaugmentation with strain DF-1 appeared to also stimulate dechlorination of singly flanked chlorines by a seemingly synergistic effect on the indigenous microbial community.
PCB Reductase	PCBR	Targets the pcbA1, pcbA4, and pcbA5 reductase genes from Dehalococcoides spp. which may serve as biomarkers for the dechlorination of PCBs.
MB Reductase	MBR	Targets the MB reductase gene found in Dehalococcoides spp. which is capable of co-metabolic dehalogenation of PCBs.

Aerobic Biodegradation

There are many examples of aerobic bacteria capable of cometabolic degradation of PCBs during growth on biphenyl. Most isolates have been capable of degrading PCBs with only one or two chlorines, but there are exceptions. Burkholderia xenovorans LB400, for example, is capable of cometabolism of some PCB congeners containing three, four, and five chlorines.

CENSUS®qPCR can be performed to quantify biphenyl dioxygenase genes.

TARGET	CODE	RELEVANCE / DATA INTERPRETATION
Biphenyl Dioxygenase	BPH	Gene encoding the enzyme responsible for initiating aerobic cometabolism of PCBs. The specificity of the biphenyl dioxygenase determines which PCBs are degraded.

Complementary Tools

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 PCB impacted sites, an ISM study could include:

- 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 CENSUS® qPCR quantification of Dehalococcoides and Dehalobium.

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.