From Consumer Plastics to Aquatic Toxicants: Oxidative and Endocrine Effects of Microplastics on Fathead Minnow ( Pimephales promelas ) Larvae

Secondary microplastics (MPs) from single-use consumer products are rapidly becoming one of the most persistent and widespread pollutants of our time. Freshwater systems act as early sinks for MPs where fragmentation and additive leaching expose aquatic life to a broad spectrum of toxic compounds. This study examined the effects of cryomilled MPs from red and green consumer plastic cups, which exhibited a heterogeneous size distribution dominated by particles between 25 and 50 μm, with most particles measuring < 75 μm, on fathead minnow (Pimephales promelas) larvae (< 48 h old) exposed to 0.01-10 mg/L for 96 h. All doses significantly reduced catalase (CAT) activity by 80%-90% and ATP by 35%-50%. At the highest dose, glutathione (GSH) was reduced (77% red MPs; 35% green MPs) and glutathione peroxidase (GPX) activity increased (300% red MPs; 195% green MPs). There were also increases in thiobarbituric acid reactive substances (TBARS) (58% red MPs; 195% green MPs) and superoxide dismutase (SOD) (79% green MPs). Thyroid-stimulating hormone (TSH), thyroxine (T4), and triiodothyronine (T3) were elevated by 64%, 53%, and 228% with red MPs and 118%, 75%, and 223% with green MPs respectively. Red MPs induced stronger oxidative effects, while green MPs produced distinct antioxidant and endocrine responses, highlighting composition-dependent toxicity. Collectively, these results suggest that consumer MPs produce oxidative and endocrine stress but in different mechanisms, emphasizing the need to evaluate real-world plastics in freshwater ecotoxicology.