Phenanthrene alters oxidative stress parameters in tadpoles of Euphlyctis cyanophlyctis (Anura, Dicroglossidae) and induces genotoxicity assessed by micronucleus and comet assay

Phenanthrene (PHE), a tricyclic polycyclic aromatic hydrocarbon (PAH), is ubiquitously found in aquatic environments. It is one of the major components in PAH mixtures. It has been identified as one of the 16 priority PAHs for toxicological evaluations. PHE is reported to induce lethal and sub-lethal toxicity in various aquatic indicator organisms. However, no toxicological data of PHE in anuran amphibians could be found. Amphibian larvae (tadpoles) develop in aquatic habitats. Therefore, exposure to PHE could negatively impact their development and fitness in later periods as they move in to the terrestrial habitat following metamorphosis. In the present study, we have analyzed the effects of PHE in Euphlyctis cyanophlyctis tadpoles. PHE induced concentration-dependent lethal effects in the tadpoles. The estimated LC₅₀ values were 16.52, 15.29, 13.69, and 12.28 mg/L at 24, 48, 72, and 96 h of exposure respectively. These LC₅₀ values are significantly higher than the reported environmental concentration of PHE. However, the strong negative correlation (R² = 0.997, p < 0.001) between the LC₅₀ value and exposure time indicates that longer exposure to lower concentration may cause significant lethal effects. Besides, PHE at environmentally relevant concentrations induced significant sub-lethal toxicities. Exposure to sub-lethal concentrations was found to be genotoxic in erythrocyte micronucleus as well as comet assays. Sub-lethal concentrations of PHE significantly increased superoxide dismutase activity and tissue glutathione level as well as induced lipid peroxidation. The present findings clearly indicate that PHE is a potential threat to the early life stages of amphibians. Further investigations are necessary to ascertain the implications of these early effects during adult life stages.