Antioxidant supplementation under microplastic stress: Protective or pro-oxidant effects in amphibians

Microplastics (MPs) are emerging contaminants that disrupt development, physiology, and redox homeostasis in aquatic organisms. However, their detailed impacts on amphibians and the potential for antioxidant-based mitigation remain poorly resolved. This study investigated whether dietary vitamin C can mitigate MP-induced stress in Glandirana rugosa under chronic exposure to environmentally relevant MP levels. Tadpoles were exposed to MPs, vitamin C, or their combination, and their developmental rate, growth, survival, corticosterone, and antioxidant enzyme activity were quantified across multiple ontogenetic stages. The results showed that MPs exposure delayed metamorphosis, reduced survival, and altered oxidative enzyme profiles, indicating sustained redox disruption. Vitamin C supplementation improved early growth and partially alleviated developmental delays under MP-induced stress, while combined exposure consistently elevated catalase activity and produced a persistent oxidative imbalance, suggesting a pro-oxidant shift rather than a protective effect. However, vitamin C alone did not alter physiological indicators, demonstrating that its effects were conditional on MPs exposure. Overall, this study reveals that antioxidants can interact synergistically with MPs to intensify oxidative stress, challenging the assumption that dietary antioxidants provide universal protection. These findings underscore the need to evaluate antioxidant-pollutant interactions within realistic exposure scenarios and highlight important implications for amphibian health and environmental risk assessment.