PAH and Degradation: Sources, Formation, Toxicity, and Fungal Metabolism

Running head: PAH AND DEGRADATION
PAH and Degradation
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1PAH AND DEGRADATION
Polycyclic aromatic hydrocarbons (PAH)
There are an abundance of compounds widely spread throughout the environment, among
which organic compounds having a composition of carbon and hydrogen are also prevalent in
nature. Polycyclic aromatic hydrocarbons (PAH) are most noteworthy among these organic
compounds. The carbon and hydrogen atoms are arranged in multiple aromatic rings. Many
chemicals released in nature such as sulphur, nitrogen and oxygen interact with these aromatic
rings of benzene to compose PAH. The characteristic features of PAH include the presence of
non-polar, neutral and hydrophobic nature which bring about specific physical and chemical
properties1. The stability of PAH is attributed to the presence of dense pi-electron clusters with
each of aromatic rings, which make it resistant to nucleophilic attack.
Fig 1: Examples of PAH
1 Zhou, Hui, Chunfei Wu, Jude A. Onwudili, Aihong Meng, Yanguo Zhang, and Paul T. Williams. "Polycyclic
aromatic hydrocarbons (PAH) formation from the pyrolysis of different municipal solid waste fractions." Waste
Management 36 (2015): 136-146.

2PAH AND DEGRADATION
Sources of PAH
Availability of PAH in nature include natural sources such as forest fire, volcanic
eruptions and oil seep; anthropogenic sources include coal tar, garbage, lubricating oil and oil
filter discharges. PAHs are the primary source of environmental pollution; they are generated
mostly through imperfect combustion of organic compounds, also involving natural
carbonization and industrial pyrolysis. Examples of such PAH include anthracene, benzoin [a]
pyrene (Bap) and naphthalene.
Formation of PAH
PAHs are commonly formed through imperfect combustion of organic compounds and
waste products. Unburnt fuel, lubricants and biodegradable fuel and oxygenic combustion are the
major sources of PAH formation 2. The hydrophobic nature and stability of PAH affect their high
molecular weight composition. Hydrophobicity has a reducing effect on biological availability of
PAH.
Toxicity of PAH
Benzo [a] pyrene, present in PAH acts as a threat to environment as well as human
health. Its high solubility in lipids increases its human absorption 3. They have mutagenic,
carcinogenic and teratogenic impacts, degrading the human organ system. Studies have shown
2 Zhou, Hui, Chunfei Wu, Jude A. Onwudili, Aihong Meng, Yanguo Zhang, and Paul T. Williams. "Polycyclic
aromatic hydrocarbons (PAH) formation from the pyrolysis of different municipal solid waste fractions." Waste
Management 36 (2015): 136-146.
3 Kim, Ki-Hyun, Shamin Ara Jahan, Ehsanul Kabir, and Richard JC Brown. "A review of airborne polycyclic
aromatic hydrocarbons (PAHs) and their human health effects." Environment international 60 (2013): 71-80.

3PAH AND DEGRADATION
that PAH are a component of food chains, thereby having potential harmful effects on the soil.
High rate of biomagnification causes greater absorption of PAH into human skin, increasing
toxicity 4. LD50 measures the toxicity level of PAH, indicating that it has a potent carcinogenic
role through formation of reactive epoxides. Fungal enzymes play an important role in degrading
PAH by oxidizing the heterocyclic aromatic rings to generate epoxide and reactive intermediates.
Cytochrome P450 monoxygenase is one such enzyme performing such degradation.
Chemical and physical properties of PAH
K-, bay- and L-regions comprise PAH structure, which is formed through biological and
metabolic reactions of PAH. The simplest of PAH is phenanthrene with steric hindrance between
C4 and C5 and high electron density. Microbial PAH have high molecular weight with high
stability and are resistant to degradation. These are highly hydrophobic and are potentially
harmful to living cells. The molecular size determines the aromatic structure, molecular topology
and ring linkage pattern.
Degradation of PAH
Multiple factors such as chemical, physical properties, concentration, soil diffusion rate
and biological availability affect t the degradation of PAH. A sequential relationship exists
microbial PAH degradation and molecular size, while molecular size shows an inverse relation
with solubility feature. PAHs are used as a source of carbon by aerobic bacteria; Mycobacterium
spp. Oxidize Bap as carbon source. White rot fungi degrade PAH through direct attack on
4 Lee, Hyun Joo, Julien Villaume, David C. Cullen, Byoung Chan Kim, and Man Bock Gu. "Monitoring and
classification of PAH toxicity using an immobilized bioluminescent bacteria." Biosensors and bioelectronics 18, no.
5-6 (2003): 571-577.