Earthworm – PAHs Interactions with special reference to Gut flora

Polycyclic aromatic hydrocarbons (PAHs) are a large group of organic compounds consisting of two or more fused aromatic rings. PAHs are ubiquitous pollutants and are of significant environmental concern due to their persistence, mutagenicity, and carcinogenicity to living organisms. On the other hand, earthworms are important and abundant organisms in soil accounting for 60-80% of the total soil faunal biomass. Earthworms play an important role in soil fertility through their activities in the decomposition of organic matter, release of nutrients, and aeration of the soil. Earthworms are considered as soil engineers. Earthworms due to their contact with soil serve as sentinel species to evaluate the effects of soil pollution and are considered appropriate model organisms for soil ecotoxicity studies. Although, PAHs have been extensively studied for their effects on terrestrial biota including their remediation, there is no or limited information on their chronic effects such as earthworm reproduction, metabolomics changes a,nd gut flora. Therefore, this study investigated the toxicity of 3 model PAHs, phenanthrene, pyrene and benzo-a-pyrene (B(a)P) alone and in combination on earthworm (E.fetida) survival, reproduction, PAH bioaccumulation, microbial diversity in earthworm gut and soil in addition to changes in earthworm metabolome. Results demonstrated that the test PAHs even at lower concentrations (5-10 mg/kg soil) showed a significant decrease in earthworm reproduction although these concentrations did not affect the survival of parents. Bacterial diversity in earthworm gut was significantly altered with a dose-dependent decrease compared to the respective soils. Interestingly PAH mixtures showed antagonistic effects. Mixed PAHs even at lower concentrations significantly affected several metabolic pathways in E. fetida as compared to individual compounds. Overall, this study provided novel insights into the toxicity of PAHs on earthworms and this information is invaluable for environmental risk assessment of PAHs.