Various microorganisms are involved in a two-stage process of degradation of PCBs, which happens in aerobic and anaerobic environments. Degrading PCBs is similar to the degradation of biphenyl. However, the chlorines on PCBs prevent them from being utilized as a substrate of biphenyl degradation. Due to high chemical stability, PCBs cannot be used as energy sources. However, due to the chlorination, PCBs can be used as electron acceptors in anaerobic respiration to store energy, which is also the first stage of the degradation pathway, reductive dechlorination. Once the PCBs are dechlorinated to a certain degree, usually lower than five chlorines presenting in the structure and one aromatic ring has no chlorine, they can undergo the biphenyl degradation pathway (BP pathway) to be degraded to accessible carbon or CO in the aerobic environment. BP pathway is a pathway that utilizes series of enzymes (BphA, B, C, D, E, F, G) to convert biphenyl to TCA cycle intermediates (pyruvate and Acyl-CoA) and benzoate. However, there are few microorganisms that can dechlorinate substrate under natural conditions. Even with selective media, the accumulation of PCB dechlorinating microorganisms is still slow, which is one reason for the slow degradation rate. As a result, PCBs usually go through a co-metabolism pathway that involving different microorganism species.
Generally speaking, there are four steps in this process:
# In order for PCBs to enter the cell, they firstly need to be solubilized.
# PCBs are dechlorinated by anaerobic bacteria, then transport the metabolites to aerobic bacteria or fungi through a biofilm.
# The presence of PCBs metabolites triggers the expression of enzymes in BP pathway.
# PCBs are broken down to Acetyl-CoA and then can be utilized or carbon dioxide.
The Figure below shows the complete degradation pathway.