Metabolomics identified distinct molecular-level responses in Daphnia magna after exposure to phenanthrene and its oxygen and nitrogen containing analogs

The prevalence of polycyclic aromatic hydrocarbons and their oxygenated and nitrogen containing analogs in freshwater ecosystems are of concern due to their reported toxicity to several aquatic species including Daphnia magna. This study explored the molecular-level responses of phenanthrene (PHEN), 9,10-phenanthrenequinone (PHQ), and phenanthridine (PN) as little is known about the impacts of these pollutants on the metabolic profile of D. magna. For this purpose, D. magna was exposed to three sub-lethal concentrations of these pollutants for 24 h. To assess molecular-level responses, 52 polar metabolites were extracted from individual adult daphnids, and analyzed using a mass spectrometry-based targeted metabolomics approach. Exposure to PN resulted in the most statistically significant changes to the metabolic profile of D. magna followed by PHQ, and then PHEN exposures. After PN exposure, the biochemical pathway analysis showed that all exposure concentrations shared 21 perturbed metabolic pathways. However, the number of disrupted metabolic pathways increased with increasing exposure concentrations for PHEN and PHQ. The results suggest that PN and PHQ exposures are more disruptive due to the presence of reactive functional groups when compared to PHEN exposure. For the tested concentration ranges, the findings indicate that exposure to PN resulted in non-monotonic disruptions across exposure concentrations. In contrast, exposure to PHEN and PHQ elicited perturbations that were concentration-dependent. Although the reported median effective concentration (EC₅₀) for PN is higher than PHEN and PHQ, our data shows that metabolomics captures molecular-level changes that may not be detected by traditional toxicity metrics.