Particles, chemicals or both? Assessing the drivers of the multidimensional toxicity of car tire rubber microplastic on early life stages of Atlantic cod.

Despite the growing number of ecotoxicity studies on microplastics, few have successfully distinguished between the effects of particles and leached chemicals on organisms. This study assessed the role of car tire rubber (CTR) as both a particle contaminant and a chemical carrier. Early life stages (ELS) of Atlantic cod (Gadus morhua) were exposed to three CTR treatments tested at 1, 10, and 100 mg CTR L⁻¹ : (1) pre-leached CTR particles (CTR-PART), (2) chemicals leached from CTR (CTR-LEACH), and (3) CTR particles with both leached and unleached chemicals (CTR-BOTH). The chemical composition of both particles and leachates was analysed using GC-MS and pyrolysis GC-MS. Investigated endpoints included hatching success, mortality, developmental alterations, and gene expression. While low and medium exposure concentrations had minimal effects compared to control treatments, high CTR-LEACH and CTR-BOTH exposures significantly reduced hatching success and increased the larvae mortality, while CTR-PART did not have any effect. Developmental alterations occurred at high concentrations of all treatments, with severity ranked as CTR-BOTH > CTR-LEACH > CTR-PART. The expression of stress-related genes (mt, gst3a, cat, sod) followed the same pattern. These findings suggest that leached chemicals drive CTR toxicity in cod ELS, while physical particles enhancing their effects.