Effects of Aged Polymethylmethacrylate Microplastics on Physiological Biochemistry and Intestinal-Sediment Microbial Communities of Urechis unicinctus (von Drasche, 1881)

Microplastics (MPs) widely distribute in marine environments and generate severe hazards to marine biota, yet little is known regarding the toxic impacts of aged MPs on benthic invertebrates and their surrounding sediment microbiota. This work aimed to fill this research gap by exploring the comprehensive toxicity of aged polymethylmethacrylate microplastics (PMMA-MPs) toward Urechis unicinctus and its sediment habitat. A 21-day laboratory exposure trial was performed with two PMMA-MP treatments: environmentally realistic low concentration (10 μg/L) and extreme high concentration (1000 μg/L). Aged PMMA-MPs accumulated in the body wall and intestine of U. unicinctus, with markedly higher body wall MPs loads in the high-dose group (p < 0.05). High-concentration PMMA-MPs significantly raised intestinal total protein and triggered intense oxidative stress (p < 0.01), and the Integrated Biomarker Response value rose dose-dependently. Sediment microbial β-diversity was obviously reshaped (p < 0.05), with enriched pathogens including Klebsiella and Enterobacteriaceae under high MP exposure, while the worm’s intestinal α-diversity and core microbiome remained stable, showing host adaptability. This study reveals multi-level toxic outcomes of aged PMMA-MPs on benthic worms and their sediment microhabitat, offering fundamental data to evaluate the ecological risks of weathered MPs in marine benthic systems.