Polychlorinated biphenyls (PCBs) were widely used in electrical equipment, transformers, and capacitors due to their excellent insulating properties and flame retardancy. However, PCB production was banned in the United States in 1979 for most uses since they are persistent organic pollutants with severe health risks.
Commercial PCBs were a mixture of congeners, and differences between the compounds affect their biodegradation. Highly chlorinated PCBs are more likely to be degraded through anaerobic reductive dechlorination by halorespiring bacteria such as Dehalococcoides and Dehalobium. Less chlorinated PCBs are likely to be degraded through aerobic cometabolism during growth on biphenyl.
For more information on the molecular biological tools that can be used to assess the biodegradation of chlorinated biphenyls, click the section of interest in the dropdown menu below. For guidance tailored to your current needs, contact our project success team at 865-573-8188 or [email protected].
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. |
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. |
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:
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.
| REFERENCE |
|---|
| Adrian L, Dudková V, Demnerová K, Bedard DL. “Dehalococcoides” sp. Strain CBDB1 Extensively Dechlorinates the Commercial Polychlorinated Biphenyl Mixture Aroclor 1260. Applied and Environmental Microbiology. 2009;75. https://doi.org/10.1128/AEM.00102-09. |
| Correa PA, Lin L, Just CL, Hu D, Hornbuckle KC, Schnoor JL, Van Aken B. The effects of individual PCB congeners on the soil bacterial community structure and the abundance of biphenyl dioxygenase genes. Environment International. 2010;36:901-906. https://doi.org/10.1016/j.envint.2009.07.015. |
| Ewald JM, Humes SV, Martinez A, Schnoor JL, Mattes TE. Growth of Dehalococcoides spp. and increased abundance of reductive dehalogenase genes in anaerobic PCB-contaminated sediment microcosms. Environmental Science and Pollution Research. 2020;27:8846–8858. https://doi.org/10.1007/s11356-019-05571-7. |
| Furukawa K, Suenaga H, Goto M. Biphenyl Dioxygenases: Functional Versatilities and Directed Evolution. Journal of Bacteriology 2004;186:5189-5196. https://doi.org/10.1128/jb.186.16.5189-5196.2004. |
| May HD, Sowers, KR. “Dehalobium chlorocoercia” DF-1—from Discovery to Application. In: Adrian, L., Löffler, F. (eds) Organohalide-Respiring Bacteria. Springer, Berlin, Heidelberg. 2016.https://doi.org/10.1007/978-3-662-49875-0_24. |
| Xu G, Zhao S, Chen C, Zhao X, Ramaswamy R, He J. Dehalogenation of Polybrominated Diphenyl Ethers and Polychlorinated Biphenyls Catalyzed by a Reductive Dehalogenase in Dehalococcoides mccartyi Strain MB. Environmental Science & Technology. 2022;56:4039-4049. https://doi.org/10.1021/acs.est.1c05170. |