The contribution of polymer additives to microplastic toxicity: A long-term study with Oncorhynchus mykiss exposed to polystyrene microparticles with different hexabromocyclododecane content

Although microplastic toxicity infish has been extensively studied, the role ofpolymer additives remains insufficiently understood. This study investigated the combined effects ofpolystyrene (PS) microplastics and the brominated flame retardant hexabromocyclododecane (HBCD) onthe health ofrainbow trout (Oncorhynchus mykiss) following six weeks ofdietary exposure. Fish were exposed toPSparticles with varying HBCD content (0.0mg/g– PS-HBCDfree, 0.2mg/g– PS-HBCDlow, 1.0mg/g– PS-HBCDhigh) ortoHBCD alone (HBCDonly). The combination ofPSand HBCD induced the most pronounced biological responses atmolecular, biochemical and histological levels. In the liver, oxidative stress and upregulation of pro-inflammatory cytokines (il8, il2), together with increased catalase expression, were observed in PS- and/or PS-HBCD-exposed groups, indicating an imbalance inantioxidant defence. Histopathology confirmed liver dystrophy and renal lesions, while elevated vitellogenin expression suggested endocrine disruption. Although HBCD accumulation was confirmed only inthe HBCDonly group, the polymer-bound form caused comparable physiological alterations, supporting its contribution to combined toxicity. Overall, the study demonstrates that PS microparticles can act as vectors enhancing the bioactivity of embedded additives such as HBCD, resulting in complex multi-organ effects and emphasising the importance of assessing additive–polymer interactions when evaluating the environmental risks of microplastics.