Developmental toxicity of carboxylated microplastics in zebrafish mediated by mitochondrial dysfunction and inflammatory activation

Carboxylated polystyrene microplastics (PS-COOH), generated during environmental aging and surface oxidation, display increased hydrophilicity and biological reactivity. However, their developmental and mechanistic impacts on aquatic organisms remain unclear. Here, we evaluated the developmental toxicity of environmentally relevant concentrations (0.1-100 μg/L) of PS-COOH in zebrafish (Danio rerio) larvae over 120 hpf. Developmental endpoints, immune responses, and mitochondrial function were systematically assessed, and molecular docking was performed to probe protein-level interactions. PS-COOH exposure induced dose-dependent developmental defects, including reduced tail coiling, bradycardia, and growth inhibition. Inflammatory activation were evidenced by neutrophil depletion in Tg(lyz:DsRed2) zebrafish, elevated immune biomarkers (LYSO, C3), and paradoxical inflammatory gene responses-upregulation of IL-1β and NF-κB alongside downregulation of TNF-α and PPARG. Mitochondrial disruption was characterized by ROS overproduction, ATP/NAD depletion, biphasic SOD and CAT responses, and suppression of key respiratory chain genes (mt-nd1, cox4i1, atp5a1), partially offset by compensatory uqcrc1 upregulation. Notably, the mitochondrial-targeted antioxidant MitoQ significantly mitigated these effects, confirming mitochondrial dysfunction as a central toxicity mechanism. Collectively, our results identify carboxylation-driven toxicity involving mitochondrial-inflammatory crosstalk and underscore the importance of incorporating surface-modified microplastics into ecological risk assessments.