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 in fish has been extensively studied, the role of polymer additives remains insufficiently understood. This study investigated the combined effects of polystyrene (PS) microplastics and the brominated flame retardant hexabromocyclododecane (HBCD) on the health of rainbow trout (Oncorhynchus mykiss) following six weeks of dietary exposure. Fish were exposed to PS particles with varying HBCD content (0.0 mg/g - PS-HBCDfree, 0.2 mg/g - PS-HBCDlow, 1.0 mg/g - PS-HBCDhigh) or to HBCD alone (HBCDonly). The combination of PS and HBCD induced the most pronounced biological responses at molecular, 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 in antioxidant defence. Histopathology confirmed liver dystrophy and renal lesions, while elevated vitellogenin expression suggested endocrine disruption. Although HBCD accumulation was confirmed only in the 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.