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Top1. Introduction
Fossil fuels (crude oil, coal, and gas) are the major source of energy production required for industrial and household purposes. Introduction of advanced technologies to the oil industries increases their production capacity. Rapidly growing shale oil industries also allow to raise the volume and productivity of conventional oil reserves (Mirzoev et al., 2020). According to the survey of the World Energy Outlook (WEO) 2019, the production of crude oil in the US grows from 6 mb/d (million per day) in 2018 to a high point of 11 mb/d in 2035 (IEA, 2019). Petroleum hydrocarbons (PHCs) are organic chemicals and are released in the form of oil, gasoline, diesel and a variety of solvents throughout the environment. PHCs such as n-alkanes, alkenes, cycloalkanes, polycyclic aromatic hydrocarbons, biphenyl and naphthen-aromatics are toxic in nature and recorded as precedence pollutants in the US Environmental Protection Agency (EPA) (Bojes & Pope, 2007; Keith & Telliard, 1979). The release of volatile hydrocarbons and spills of oil into the soil and ground water may lead to unintentional penetration of these chemicals into the environment, which causes severe health problems and environmental threats (Al-Mutairi et al., 2008).
Biodegradation is the process by which PHCs are broken down into smaller compounds by enzymes produced via living microbial organisms (Lei et al., 2005). To date, several studies focused on different biodegradation mechanisms. Biodegradation is a biochemical process that arbitrates through microorganisms. Energy-producing chemical reactions are started through microorganisms which are responsible for the breakdown of chemical bonds of the organic contaminants. This energy-producing reaction is an oxidation-reduction reaction where the organic chemical gets oxidized by an electron acceptor which in itself gets reduced. In the case of aerobic conditions, oxygen acts as an electron acceptor whereas in anaerobic conditions, inorganic anions like nitrate, sulfate, and iron act as an electron acceptor (Eskander & Saleh, 2017). Persons and Govers stated that biodegradation rate depends on the rate of uptake of chemicals through the microbial cell wall and the rate of enzymatic transformation (Parsons & Govers, 1990). Xue et al., describe that the adhesion between petroleum degrading bacteria and the petroleum hydrocarbon is a vital step for biodegradation. They also investigated that in presence of bio-surfactant producing bacteria, biodegradation of hydrophobic petroleum hydrocarbons is enhanced (Shi et al., 2019). Degner et al. discuss the effect of enzyme-substrate reaction on biodegradation. They also reported that the volume and radius of a particular compound determined steric fit to the active site of an enzyme and influence the biodegradation process (Degner et al., 1991). Davies and Hughes stated that n-alkanes are easily biodegradable as compared to other cyclic hydrocarbons (Davies & Hughes. 1968). Lead better and Foster proposed the concept of co-oxidation for unsubstituted cycloalkanes and condensed aromatic hydrocarbons (Leadbetter & Foster, 1959; Lead better & Foster, 1960).
Biodegradation half-life (t1/2) is the most widely used parameter for designing bioremediation efforts and predicting environmental fate. To date, out of 1, 45,297 estimated compounds (pre-registered by the EU REACH legislation), biodegradation study of only 4,214 compounds has been done (ECHA, 2017). Performing biodegradation study for the rest of the compounds is very much time consuming, costly and difficult process. Rational strategies are always acceptable in this regard to predict the behavior of chemicals employing available resources. European regulation on Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) also prefer such alternative approaches that are less expensive, less time consuming and avoid animal testing (Mansouri et al., 2013; Cvetnic et al., 2017). Quantitative structure-property/activity relationship (QSPR/QSAR) analysis assumes that the strength of activity or property exerted by the chemicals is directly related to their structural features (Gramatica, 2020). The predictive QSPR modeling paradigm can investigate the chemical features of the petroleum hydrocarbons (PHCs) responsible for their biodegradation without any time - consuming laborious experimental analysis.