Size- and polymer-dependent effects of polystyrene and polyethylene micro- and nanoplastics in zebrafish (Danio rerio).

Microplastics (MPs) have been extensively investigated, whereas nanoplastics (NPs) are increasingly recognized as emerging contaminants with potentially greater biological concern. To address the current lack of comparative data, we systematically evaluated the biological impacts of both particle size classes and polymer compositions. Zebrafish (Danio rerio), a well-established vertebrate model for toxicological assessment, was used to examine the behavioural and developmental effects of polystyrene (PS) and polyethylene (PE) MPs and NPs. We first performed the standard Fish Embryo Toxicity (FET) test where acute toxicity parameters were observed. Post-hatching (96-120 hpf), heart rate variability, oxygen consumption, and swimming behaviour were measured as indicators of physiological and behavioural responses to the test conditions. We confirmed that NPs induced adverse effects, including impaired hatching (PS NPs at ≥75 mg/L), increased mortality (PE NPs, ∼25% at 10 mg/L and complete mortality at 100 mg/L), and altered locomotor activity. Specifically, PS NPs reduced swimming distance and activity at higher concentrations (≥75 mg/L), while PE NPs impaired swimming performance across concentrations without consistently reducing activity time. In contrast, MPs primarily affected locomotor activity without inducing systemic developmental or physiological toxicity. These effects were associated with particle adhesion to embryonic and larval surfaces, supporting surface adhesion as a potential contributing or initiating interaction in NPs-induced toxicity.