INSTRUCTOR RESOURCE
PCB ACTIVITY
INTRODUCTION:
THE MICROBIOLOGY OF PCB DEGRADATION
It may surprise you to learn that even though PCBs are extremely persistent in the environment
and very recently introduced in the earth ecosystem (human manufacturing – no natural source
exists), it is not difficult to find and isolate bacteria capable of metabolizing them. Studies dating
back to the 1970s have demonstrated degradation in the laboratory in pure cultures, in
microcosms, and in soil. The references that accompany this introduction will help students
explore these early findings in order to address the problem presented to them. I have
annotated them so that you will be better able to guide students when they need help finding
evidence for their lines of reasoning and inquiry.
How do bacteria degrade PCBs?
PCBs are degraded both aerobically and anaerobically, but the mechanisms are of course very
different. This activity focuses mainly on aerobic metabolism but it is unwise not to mention
anaerobic capablities since these are quite important in addressing PCB remediation.
PCBs are biodegraded in 3 known ways:
 Aerobically as growth substrates: Lower-chlorinated congeners can serve as electron
donors (energy sources) for many genera of bacteria.
 Aerobically through co-metabolism: Larger numbers of genera can de-chlorinate
congeners as long as additional compounds are present to help boost energy
production. Common co-metabolites are biphenyl itself, and
 Anaerobically by reductive dehalogenation
As a generalized rule, it is more difficult to find bacteria that degrade highly chlorinated PCBs
than those with lower numbers of chlorine. Anaerobic degraders actually prefer highly
chlorinated compounds but are more difficult to isolate and cultivate. It is also important to note
that dehalogenation results in substitutions of chlorine among existing molecules (and lesschlorinated congeners) but not necessarily reduction in total PCB content in sediment. The rings
stay intact.
The resulting progressive activity of anaerobic dehalogators results in compounds that are less
hydrophobic and more capable of being completely de-chlorinated by aerobic degraders.
Aerobic degraders have preferences for attacking the 2, 3, or the 3, 4 position, which, in the
reaction, is opened up to "release" one of the rings. If any carbon in these positions is
chlorinated, the enzyme cannot catalyze the reaction. By these aerobic pathways, chlorinated
biphenyls are converted into combinations of benzoates and chlorobenzoates, and straightchain compounds that are open to a variety of paths that can convert them into TCA
intermediates such as succinate.
INSTRUCTOR RESOURCE
Many bacteria of the genera Pseudomonas, Vibrio, Aeromonas, Micrococcus, Acinetobacter,
Bacillus, and Streptomyces have been shown to degrade PCBs as long as they are only mone,
di or tri-chlorinated compounds with "open" 2-3 or 3-4 enzymatic attack sites.
In this excellent diagram from Bedard (1986) one can see the range of capabilities among a set
of isolates, when tested with different sets of congeners.
Reminder of numbering
system
The strains H850 and LB400 are in the genera Alcaligenes (Bedard, 1984) and Pseudomonas
(Bopp, 1986), respectively, and are considered "superior" PCB degraders. They have been
extensively studied and used as controls in PCB experiments.
What is cometabolism?
As already discussed, biphenyl is a naturally-occuring compound in nature, and many bacterial
genera and species have enzymes required to break certain bonds of one of these aromatic
rings, thus destabilizing the compound for further degradation.
INSTRUCTOR RESOURCE
Because degradation of biphenyl is one characteristic shared by almost all aerobic PCB
degraders, it is logical to see how addition of biphenyl to a mixture or natural sample could help
enrich for PCB degraders. As research studies show, it acts not only as an enrichment
substrate, but serves as a co-metabolite to enhance the rate of de-chlorination.
How does this happen? The presence of biphenyl appears to enrich for biphenyl-degrading
microbes, which, in the course of this activity, can also attack some chlorinated biphenyl
substrates in the vicinity, although this reaction is not favored. Nonetheless, as long as the
chlorinated compounds have some desired "open" sites, such co-metabolism will occur.
One problem with using biphenyl as a field enrichment compound is the fact that biphenyl itself
is toxic, and not something that should be purposely released into the environment! This has
led to thoughts about alternative compounds that could be used, such as aromatic flavenoids
and terpenes produced naturally by many plant species.
Can PCB contaminated areas be cleaned up through bioremediation?
The phenomenon of co-metabolism has led to attempts at using such stimulating substrates in
bioremediation efforts. Some of the major questions about bioremediation are:
1. Is it possible for bacteria to de-toxify PCB compounds?
Answer: Yes. As discussed here, many studies in the lab show that these metabolic
capacities exist.
2. Can PCB degrading organisms be isolated for the purposes of bioremediation?
Answer: Yes. Many isolates have been obtained that can dechlorinate PCBs, even
highly chlorinated ones.
3. In actual sediment samples in the lab, can PCB degradation (and dechlorination) be observed
and is it significant?
Answer: Yes, this has been well-documented. Anaerobic activity results in conversion of
highly chlorinated compounds to less chlorinated PCBs and less chlorinated PCBs can
be metabolized by many species of aerobic bacteria.
4. In the field, has seeding with these strains of bacteria been demonstrated to be successful?
Answer: NO. Actual field trials of bioremediaiton and bioaugmentation studies has
yielded mixed results. I have included one interesting field trial study for your use with
students, Please see Beede Waste Oil Study, EPA Technical Report prepared by
Vincente Gallardo. You will see that in this study, the experimenters used alternative
cometabolite compounds (not biphenyl) and cultured strains of bacteria to attempt
remediation of a contaminated site.
CONCLUSIONS: I hope that with this short introduction, I have been able to "brief you" on the
current state of our understanding of microbial processes and PCB remediation. I have not done
justice to the wide body of literature that exists, especially what is known about anaerobic
processes. But, all this can be explored if your students are interested. The attached library of
references is a great place to start, and the PBL exercise (if you adopt it) allows your students to
fully delve into it.