Age-dependent modulation of CYP450 and MXR activity and physiological trade-offs in Daphnia magna exposed to organic UV filters

Organic UV filters are ubiquitous aquatic contaminants, yet their sublethal, age-specific effects on non-target invertebrates remain poorly understood. This study investigated the ontogenetic shifts in detoxification strategies of Daphnia magna exposed to three common UV filters (Avobenzone, Octocrylene, Octinoxate) and their environmentally relevant ternary mixture. By comparing 3-day-old juveniles and 6-day-old subadults, we integrated molecular biomarkers (in vivo ECOD and MXR efflux activity, and CAT/SOD antioxidant enzymes) with whole-organism physiological endpoints (heart rate and locomotion). Results revealed a distinct developmental window of susceptibility. Juveniles exhibited a highly responsive phenotype, characterized by robust MXR capacity and highly inducible CYP450 enzymes and active antioxidant mobilization. However, this active biochemical state was accompanied by pronounced bradycardia under chemical stress, consistent with the physiological cost of detoxification. In contrast, 6-day-old sub-adults displayed ttenuated molecular response pattern, showing lower biochemical inducibility but maintaining stable cardiac function. Alarmingly, Avobenzone acted as a dual-action toxicant in adults, simultaneously inhibiting both CYP metabolism and MXR efflux. Furthermore, the low-dose mixture compromised the adult MXR system without triggering explicit avoidance behaviour. These findings highlight that UV filters act as active modulators of the Daphnia defence network, emphasizing the need to incorporate life-stage specific vulnerabilities and mechanistic biomarkers into future ecological risk assessments.