Heatwave conditions increase the toxicity of phthalates in marine organisms.

Climate change- driven marine heatwaves are major risk for marine organisms already facing other anthropogenic hazards, such as chemical contamination in coastal areas. In this study we analyzed the impacts of marine heatwaves and phthalic acid esters (PAEs) pollution as single and combined stressors on development of the sea urchin Arbacia lixula. We tested whether the temperature suggested as optimal for development (24 °C) of this thermophilus species would enhance tolerance to PAEs pollution compared to that showed under ambient temperature (18 °C). Embryo-larval bioassays were conducted in exposures to two temperatures (control: 18 °C, heatwave condition: 24 °C) and ten PAEs concentrations (control: 0 mg L-1; treated: range 0.1-50 mg L-1) in all combinations. Ecotoxicological responses were investigated at three functional levels: i) exposure-response relationships, finding that heatwave exposure increased PAEs- induced toxicity and mortality rates with an EC50 lower by 76 %; ii) morphological, finding combined temperature and PAEs increased abnormality and stunted skeleton growth; iii) biochemical, showing that temperature was the main driver for the modulation of activity of stress response enzymes (alkaline phosphatase, esterase and peroxidase). We show that heatwave conditions negatively impacted sea urchin embryos facing pollution and decreased their tolerance to PAEs. Our results indicate that 24 °C is not the optimal temperature for development of A. lixula from the southwestern Mediterranean and highlight that assays based on just one biological level or single stressor can be misleading to deduce health risks to marine organisms and their thermal optimum, indicating the need for more integrative approaches.