Fungal Strategies for the Remediation of Polycyclic Aromatic Hydrocarbons 87

and their concentration is elevated in the vicinity of the industries associated with petroleum and gas

production. On the other hand, anthropogenic combustion is the key contributor to PAHs pollution

(Banerjee and Mandal 2020, Ghosal et al. 2016).

PAHs compounds constitute two or more aromatic rings structurally arranged linearly, clusterly

or angularly. Generally, PAH compounds are comprised of carbon-hydrogen atoms; additionally,

nitrogen, oxygen and sulfur atoms can also be involved in forming heterocyclic aromatic compounds.

Broadly PAHs can be categorized into two groups: Low-Molecular-Weight PAHs (LMW-PAHs)

having less than four benzenoid rings whereas High-Molecular-Weight PAHs (HMW-PAHs) with

more than four benzenoid rings. The stability of the PAHs primarily depends on the arrangement of

the aromatic ring; the linear arrangement of aromatic rings (LMW-PAHs) represents the instability

and exhibits fewer recalcitrance characteristics, whereas the angular arrangement of aromatic rings

(HMW-PAHs) is highly stable and exhibits more recalcitrance features of PAHs compounds (Blumer

1976). In addition to this, according to United States Environmental Protection Agency (USEPA),

16 PAHs are classified as priority environmental pollutants and seven PAHs are categorized as

potent human carcinogens, known as carcinogenic PAHs (USEPA 2002).

PAHs are omnipresent environmental contaminants, generated from the partial burning of fossil

fuels. Air mass movement and transboundary deposition play a significant role in the distribution of

PAH in the environment. Through long-range transport, rural areas are also getting affected, which

are usually situated far from the actual origin of the PAHs. Soil and street dust are the primary sinks

for the deposition of atmospheric PAHs. Furthermore, this opens the gateway for PAHs to enter

the aquatic ecosystem. PAHs are preferentially fragmented and assembled in the particle state of

sediments in aquatic environments due to their hydrophobic nature. In this way, PAHs are present

throughout the multi-compartment structure of the ecosystem, paving the path for various exposure

routes to these carcinogens (Hussain et al. 2018). Broadly, PAHs can evolve from two types of

sources, i.e., naturogenic or anthropogenic sources. The naturogenic sources include forest fires,

volcanic eruptions, petroleum spills, bacterial and algal synthesis and decomposition of litter fall

(Abdel-Shafy and Mansour 2016). Various sources of PAHs and their exposure routes to human are

reported by many researchers. The major contributors of PAHs in the ecosphere are anthropogenic

sources such as the combustion of wood gas, fuels, crude oil and industrial wastes (Hussain

et al. 2018). PAHs produced from anthropogenic sources are mainly categorized into pyrogenic,

petrogenic and biological. Pyrogenic mediated PAHs are produced from partial burning of organic

matter subjected to high-temperature ranges from 350 to 1200°C under anaerobic conditions.

Examples of pyrogenic processes include the partial combustion of motors fuels in automobiles

and coal is thermal distilled into coal tar and coke, thermal cracking of oil deposits asphalt creation.

Therefore, pyrogenic PAHs released in the open air are accumulated more in urban regions. The

PAHs generated from the petrogenic process are similar to pyrogenic except regarding petroleum

processing (Nayak et al. 2022).

In this case, bioremediation is nature’s own green machinery that acts consistently as a good

cleanup, cost-effective, energy-efficient and eco-sustainable alternative equipment in comparison to

the physicochemical techniques viz, soil replacement, soil washing and flushing, chemical reduction

and oxidation, incineration and thermal desorption, vitrification, encapsulation, immobilization,

electrokinetic remediation, nonthermal plasma technology, etc. (Mandree et al. 2021, Kuppusamy

et al. 2017). The main objective of the bioremediation approach is based on the mineralization

of these hazardous compounds into non-toxic compounds and can be achieved by employing

bioremediating representatives like plants (phytoremediation), earthworms (vermiremediation),

as well as microorganisms, i.e., bacteria, yeast, fungi and algae. Such microbial agents are well

equipped with their enzymatic systems to bio transform the PAHs compounds into either carbon

dioxide (CO2) or partially degraded non-toxic metabolites, i.e., byproduct where no CO2 is liberated

(Cerniglia 1993). The microbial PAHs metabolism is generally achieved by a variety of mechanisms

based on the enzymatic depository systems.