Warming increases the toxicity of plastic additives in the marine copepod Centropages typicus.

Plastic additives are increasingly recognized as emerging chemical contaminants in marine ecosystems, yet their effects under ocean warming remain poorly investigated. Here, we assessed the combined effects of five widely used additives (DBP, BPA, EHDPP, TPP, and TBBPA) and their mixture on the marine copepod Centropages typicus at its baseline temperature (16 °C) and under acute thermal stress (22 °C). All individual additives exhibited clear concentration-dependent lethality, with consistently lower LC₅₀ values at 22 °C, indicating that warming enhances acute toxicity. Joint toxicity showed concentration-dependent additive responses across low and high exposure levels, while warming consistently intensified overall toxic effects. A multi-biomarker analysis revealed significant inhibition of carboxylesterase and acetylcholinesterase activities under sublethal mixture exposure, whereas glutathione-S-transferase activity was primarily modulated by temperature. These responses indicate that chemical and thermal stressors affect distinct physiological pathways, compromising detoxification and neurophysiological functions, with greater effects under warming. Overall, our findings demonstrate that marine heatwaves can exacerbate the toxicity of environmental plastic additives and highlight the need to integrate climate-driven thermal stress into ecological risk assessment of plastic-derived contaminants.