## 6 PAH Biodegradation Pathway  
As illustrated in the literature, a wide spectrum of microorganisms has been classified, and these microorganisms are known for their high potential in degrading PAHs. These microorganisms include, but not limited to, bacteria, fungus, actinomycetes, protozoa, and algae [69; 87]. Actually, the biodegradation of PAHs has the possibility of being undertaken either under aerobic or anaerobic conditions. However, aerobic conditions are more preferable, due to their documented efficiency [141]. As a result, composting as an aerobic technique has received more attention for treatment for such types of pollution [25; 78; 79; 123; 132; 141].  
Fortunately, it has been documented that a wide variety of bacterial cultures have the potential to biodegradable LMW PAHs directly, using them as the sole carbon and energy source [142; 143; 144; 145]. Otherwise, PAHs (like HMW PAHs) have to proceed through the accumulation of these compounds in the body of microorganisms, and then be decomposed through sequential steps and multiple routes (Figure 2) into a bioavailable form that could be metabolized by microorganisms [141; 146; 123]. Hydroxylation of the aromatic ring via a di- or monooxygenase enzymes or dehydrogenase is the first step in the degradation process, with the formation of a _cis_-dihydro, which gets rearomatized to a diol intermediate by the action of a dehydrogenase. These diol intermediates may then be cleaved by intradiol or extradiol ring-cleaving dioxygenases through either an ortho-cleavage or meta-cleavage pathway, leading to intermediates such as catechols and protocatech acid that are ultimately converted to tricarboxylic acid cycle intermediates, which could be considered as the end of the biodegradation [144; 146; 147; 148; 149]. Bacteria can also degrade PAHs via the cytochrome P450-mediated pathway, with the production of trans-dihydrodiols.  
It should be noted that HMW PAH degradation pathways still need more investigation, as few bacterial isolates were found to be capable of degrading them. Also, their biodegradation in some cases is complicated and passes through different routes, or even proceeds via co-metabolism, like that of benzo[a]pyrene [150; 151; 152].  
Fungal enzymatic activity also has a key role in the biodegradation of PAHs. Lignolytic and non-lignolytic fungi have the capability of oxidizing PAHs utilizing cytochrome P-450 monooxygenase and a lignin-degrading enzyme system for oxidizing aromatic rings. Usually, an oxygen atom is incorporated into the aromatic nucleus, whereas the remaining atom is reduced to water to yield cis-transdihydrodiols. The formed arene oxide, though non-enzymatic, can undergo some rearrangement to form a phenol, which can be further conjugated with glucose, xylose, glucomeric acid, and sulfate. On the other hand, ligninolytic fungi, which are usually known as white rot fungi, have been characterized by their capability to degrade PAHs through ligninolytic and non-ligninolytic culture conditions. Ligninolytic enzymes oxidizes the PAH ring by producing hydroxyl free radicals by the donation of one electron; consequently, PAH-quinones and acids are formed instead of dihydrodiols. [153; 154; 155; 155]. Extracellular enzymes of white rot fungi, which include lacase, LiP, and MnP, have a key role in the degradation of PAHs [156; 157; 158; 159; 153].