Aging enhances the ecological toxicity of polyethylene microplastics to marine medaka larvae (Oryzias melastigma).

Microplastics (MPs), widely distributed in marine environments, pose urgent ecological risks as emerging contaminants. However, current ecological risk assessments are largely based on the commercially produced MPs, which poorly represent in natural settings. Here, we simulated the natural aging of polyethylene MPs (PE-MPs) using ultraviolet radiation (UV) and Fenton reactions, characterized the resultant physicochemical changes, and assessed their ecological toxicity in marine medaka larvae over a 30-day exposure experiment. Short-term aging via UV and Fenton reactions primarily altered the physical properties of PE, including surface morphology, hydrophobicity, and Zeta potential. Both original and aged PE induced intestinal damage and oxidative stress in the larvae, indicating that aged PE retains its toxicity. In contrast, aged PE significantly altered the microbial community structure in the fish intestines: Roseibacillus was significantly enriched in the UV-aged polyethylene (UVPE) exposure group, while Ruegeria and Pseudomonas were enriched in the Fenton-aged polyethylene (FPE) exposure group. Functional predictions of the intestinal microbial communities indicated that exposure to FPE resulted in the upregulation of metabolism-related functions, whereas exposure to UVPE significantly downregulated similar functions. Such upregulation likely reflects microbiome reprogramming under stress rather than improved health. Overall, our study demonstrated that aged PE exhibited pronounced toxicity to marine fish and their larvae by altering gut microbiota, and thus significantly impacting energy metabolism and nutrient absorption, leading to detrimental effects on their growth and development. These findings further underscore the enhanced ecological toxicity effects of aged MPs on marine organisms, highlighting the need for better assessments of the adverse impacts of MPs in marine environments.