For phenols
Figure6: The catabolic pathway for degradation of phenol and the organization of Dmp operon. (Powlowski J, Shingler V., 1994)
For PCBs, two distinct classes of bacteria have now been identified as being able to degrade PCBs.
Aerobic bacteria which live in oxygenated environments and anaerobic bacteria which live in oxygen free environments such as aquatic sediments. They use different mechanisms, aerobes attack PCBs oxidatively, breaking open the carbon ring and destroying the compounds. Anaerobes, on the other hand, leave the biphenyl rings intact while removing the chlorines.
The evidence in the literature suggests that PCDD/F compounds are subject to biodegradation in the environment as part of the natural chlorine cycle.
Figure7: The catabolic pathway for degradation of biphenyl by aerobic bacteria and the organization of bph gene cluster (Kensuke F., Hidehiko F., 2008) .
For nitrites, degradation pathway are very well known because they belong to the nitrite cycle. It exist different biologic denitrification systems to reduce nitrates concentration in water that use bacteria as Pseudomonas with a denitrification yield of 80%. Bacteria are fixated on a mineral support and feed with acetic acid or ethanol (SNIDE). The major drawback is the production of nitrous and nitric oxide that are greenhouse gas. To make a bacterium able to transforme nitrite into nitrogen we just have to add two enzymes:
- hydroxylamine oxydase from Parococcus denitrificans: nitrite + H2O = hydroxylamine
- hydrazine oxydoreductase from Candidatus Brocadia anammoxidans: hydroxylamine + NH3 + acceptor = N2 + H2O + reduced acceptor
Figure8: Nitrogen cycle.
Unfortunately, when we talk about heavy metals we talk about atoms which cannot just be degraded by a biological pathway. But we can imagine some ways of accumulation of these elements in bacteria that we can remove and treated after like chemical waste.
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