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1. Introduction
Polycyclic aromatic hydrocarbons (PAHs) constitute a large and diverse group of priority environmental pollutants and can be formed as products during incomplete combustion of organic matter [1]. PAHs are widespread environmental pollutants commonly found in soil, surface waters, and sediments and their fate in nature is of great environmental concern due to their potential hazards of toxicity, mutagenicity, and carcinogenicity [2,3]. Recent research has shown that chronic exposure to PAHs is associated with cancerous diseases in aquatic animals and enhanced mutagenicity of sediments. Due to their toxicity, carcinogenicity, and ubiquitous distribution, the US Environmental Protection Agency has listed 16 PAHs as priority pollutants [4].
PAHs are natural constituents of fossil fuels and present in relatively high concentrations in petroleum products, so the petroleum industry and transport process are the major pathways which could produce PAHs in the environment [5]. PAHs can enter the environment in many pathways, such as volatilization, photo-oxidation, chemical oxidation, bioaccumulation, adsorption in soil particles and so on [6]. However, research has shown that microbial degradation, with a range of advantages compared to more traditional methods, has been developed as an effective technology for PAH removal [7]. Microbial degradation is a method of bioremediation, which consists of seeding microorganisms in polluted environments to achieve the aim of bioremediation, and it is considered a valuable tool for increasing the rate and extent of biodegradation of pollutants [8]. Nowadays, in order to eliminate PAHs from the environment by bioremediation, many PAH-degradation microorganisms have been isolated. Several articles have focused on the isolation and characterization of strains with the ability to grow using PAHs as their sole carbon and energy source, such as naphthalene, phenanthrene, fluoranthene and pyrene, and various bacteria capable of degrading PAHs were discovered [9,10]. Most of these bacteria belong to the genera Pseudomonas, Rhodococcus, Paenibacillus, Acinetobacter, Bacillus and Mycobacterium [11,12,13,14,15,16]. Recently, different aspects such as PAH metabolism of bacteria, degradation mechanisms, degradation related enzymes of PAHs and related catabolic genes have been researched [17,18,19].
Petroleum components have traditionally been divided into four fractions: saturated hydrocarbons, aromatics, resins, and asphaltenes [20]. Normally, saturated hydrocarbons, such as short-chain saturated hydrocarbons and low-molecular-weight aromatics, are the most susceptible to biodegradation, whereas PAHs in aromatics and resins are less vulnerable to microbial attack...