Trade-off strategy under stress: Growth, reproduction, and antioxidant defense in Daphnia magna exposed to 6PPD-Q.

The occurrence of tire-derived chemical N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q) has raised concerns about its environmental toxicity. However, the toxicity of 6PPD-Q on aquatic invertebrates remains insufficiently characterized. In this study, a model organism, Daphnia magna, was exposed to 0, 0.2, 2, and 20 μg/L 6PPD-Q for 21 days to assess the potential effects of 6PPD-Q on its growth, reproduction, and antioxidant defense. The results showed that long-term exposure to 6PPD-Q led to a pronounced reduction in the body length of D. magna, which was associated with disturbance of the ecdysteroid signaling pathway. 6PPD-Q was proven to be an antagonist of 20-Hydroxyecdysone (20-HE) through a dual-luciferase reporter assay, competitively binding to the heterodimer composed of ecdysone receptor (EcR) and ultraspiracle (USP), and ultimately interfering with the expression of downstream genes. 6PPD-Q markedly delayed the time to first brood and diminished the number of neonates per adult, the average number of neonates per time as well as spawning times. The declined expressions of reproduction-related genes vtg1, vtg2, and vmo1, regulated by an elevated juvenile hormone (JH) level, might account for the reduced fecundity of D. magna. In addition, 6PPD-Q activated the antioxidant defense of D. magna, enabling it to remove excessive ROS and prevent lipid oxidative damage. Collectively, a trade-off between growth, reproduction, and antioxidant defense was suggested in D. magna under 6PPD-Q stress. Our findings provide valuable information for the risk assessment of 6PPD-Q to the freshwater ecosystem.