Toxic effects of polyethylene microplastic exposure: Stress and immune responses, and acetylcholinesterase levels in Far Eastern catfish, Silurus asotus, according to different particle sizes.

Polyethylene microplastics (PE-MPs) are widespread in freshwater, and their toxicity may vary among particle-size groups; however, mass-equivalent groups also differ in particle-number concentration. This study compared biomarker responses in Far Eastern catfish, Silurus asotus, exposed to small (S-MPs; nominally 34 μm) and large (L-MPs; nominally 125 μm) PE-MPs at 0-1,600 mg/L for 5 or 10 days. Levels of heat shock protein 90 (HSP90), cortisol, lysozyme, immunoglobulin M (IgM), and acetylcholinesterase (AChE) were measured in gill, intestine, and liver. Exposure to PE-MPs generally increased HSP90 and cortisol levels and reduced lysozyme, IgM, and AChE levels. S-MPs treatments generally elicited significant responses at lower nominal mass concentrations than L-MPs treatments and showed descriptively higher integrated biomarker response version 2 (IBRv2) values at all concentrations. Principal component analysis (PCA) showed discernible but partially overlapping distributions of the two groups. Permutational multivariate analysis of variance (PERMANOVA) identified concentration as the dominant source of variation in the biomarker profile (R2 = 64.0%), with effects of particle-size group (R2 = 6.1%), exposure duration (R2 = 3.3%), and the particle-size group × concentration interaction (R2 = 4.9%). At equal nominal mass concentrations, S-MP treatments were estimated to contain approximately 17 times as many particles as L-MP treatments, and measured size distributions partially overlapped. Thus, responses should be interpreted as particle-size-group-associated under mass-equivalent conditions rather than attributable solely to particle diameter. Because the tested concentrations substantially exceeded reported freshwater abundances, this study provides short-term, high-concentration hazard-characterization data rather than evidence of effects at current environmental exposure levels.