Aquatic ecosystems are pivotal resources that nurture diverse life forms apart from providing different ecosystem services. Global pollution, directly and indirectly, depletes the quality and standards of these resources and hampers the animals residing there. The incomplete combustion of all sorts of organic substances found in nature produces and release an emerging group of contaminants known as polycyclic aromatic hydrocarbons (PAHs). There are over a hundred different kinds of PAHs known and 16 amongst them are regarded as priority pollutants including phenanthrene (PHE). PHE is abundantly found in the aquatic environment and poses a higher risk to animals. It causes a vast array of toxicities in aquatic animals including genotoxicity, cardiotoxicity, transgenerational toxicity, neurotoxicity, developmental toxicity, and potentially induces oxidative stress and behavioral alterations. However, many areas of PHE toxicity in aquatic organisms are yet to be properly understood and management measures are yet to be initiated.
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In recent times, the worldwide developmental drive continuously increases our dependence on fossil fuels as a primary source of energy. Fossil fuels and all other organic materials upon incomplete burning produce and release a complex cluster of emerging pollutants in the environment known as polycyclic aromatic hydrocarbons (PAHs) (Llamas et al., 2017; Sun et al., 2012). Moreover, in the modern era, human activities have been multiplying the waste and effluents generation in the environment containing different PAHs. All sorts of contaminants including PAHs ultimately find their way into the aquatic ecosystem and thereby deplete the quality and standards of the aquatic habitats directly or indirectly (ATSDR, 1995). PAHs are environmentally concerned pollutants with hundreds different kinds and are found in nature with unique chemical structures, prevalence and physicochemical properties (Zelinkova and Wenzl, 2015). Amongst them, 16 compounds have been identified as priority pollutants by USEPA (ATSDR, 2005) because of their persistent presence, potential toxic properties and probable human exposure (Keith, 2015; Zelinkova and Wenzl, 2015). PAHs are widespread in all the three compartments of the environment such as air, water, and soil (Korsh et al., 2015; Maliszewska-Kordybach et al., 2009). Due to their persistent nature in the environment, the bottom sediments of aquatic ecosystems are also contaminated (Perra et al., 2011). A substantial amount of PAHs are also detected in foods that are subjected to drying, grilling, frying, roasting and baking (Mottier et al., 2000). PAHs remain in the environment as mixtures and some of them are recognized as probable carcinogens (IARC, 2010). These compounds can potentially react with other pollutants such as ozone, sulfur dioxide and nitrogen oxides yielding potentially more mutagenic and carcinogenic diones, sulfonic acids and nitro- and dinitro- PAHs respectively (Wild and Jones et al., 1995). Due to their increasing abundance in the environment and toxic potential, PAHs are being regarded as emerging pollutants of concern. The database of Norman Network provides a list of 1036 emerging pollutants which includes most of the PAHs and PAH metabolites including phenanthrene (PHE) (Norman-Network, 2016).
PHE is one of the 16 priority PAH compounds. It is widely abundant in PAH mixtures and is often the most dominant PAH in aquatic ecosystems (Bhagat et al., 2016; Khan et al., 2013; Machado et al., 2014; Sun et al., 2015a). WHO (1983) describes PHE as a major contributor to the total PAH content of the environment. In recent years, the presence of PHE in surface water has been detected worldwide (Callen et al., 2013; Qiao et al., 2006) and their levels are increasing due to source multiplication (Bigus et al., 2014; Lorgeoux et al., 2016). PHE is found to be abundantly higher around urban catchment areas and oil extraction sites (Loughery et al., 2018). Moreover, it has been reported to be toxic to many aquatic test animals (Bhagat et al., 2016; Brette et al., 2017; Gauthier et al., 2016; Khan et al., 2013; Machado et al., 2014; Morais et al., 2014; Nam et al., 2015; Prosser et al., 2011; Torreiro-Melo et al., 2015; Wu et al., 2015; Zhang et al., 2013). However, many aspects of the toxic properties of PHE and its metabolites are unexplored. The toxicological effects of PHE on many target animals such as amphibians are very scarce. Therefore, it is an area of concern among aquatic ecotoxicologists to understand and monitor the potential threats emanating from increasing concentrations of PAH in general which is crucial for the proper preservation of nature and natural resources.