Aging process potentially aggravates microplastic toxicity in aquatic organisms: Evidence from a comprehensive synthesis

Microplastics (MPs; <5 mm) will inevitably encounter aging processes after being released into the environment. However, the effect of aging on MPs toxicity in aquatic environment is still unclear despite that aging plays a critical role in changing MPs characteristics and behavior. Here, we conducted a meta-analysis to assess the effects of aging on MPs biotoxicity in aquatic environment. We found that aging displayed an overall aggravating effect (Hedges' g = -0.595, P < 0.05) on MPs toxicity in aquatic organisms, while the effects varied across different taxa; namely, aging potentially alleviates MPs biotoxicity to hydrophytes (Hedges' g = 0.383, P > 0.05) while significantly exacerbates MPs toxicity to other organisms, such as algae (Hedges' g = -0.784, P < 0.05), zooplanktons (Hedges' g = -0.366, P < 0.05), and fish (Hedges' g = -0.560, P < 0.05). Moreover, the aggravating effects of aging on MPs biotoxicity were closely related to biological traits (e.g., Hedges' g = -0.378 for growth and development, Hedges' g = -0.957 for metabolism, and Hedges' g = 0.054 for immune system). We further found that aging methods, MPs characteristics, and environmental designs were also crucial regulators for the aging impacts on MPs toxicity. Taken together, our findings demonstrated that aging process appears to boost MPs biotoxicity, and there are complex factors determining aging impacts on MPs biotoxicity. Given the persistent release of MPs and the aggravating effects of aging in aquatic environments, the risk posed by MPs should be carefully considered in the future.