Assessing human pressure on the development of two Mediterranean sea urchin species: combined effects of global warming and pollution

Background – Global warming represents major threat for marine organ-isms already facing chemical contamination in coastal areas. This study examines the combined effects of thermal stress and exposure to three en-vironmentally relevant pollutants (gadolinium, vanadium, phthalates) on the embryonic development of two ecologically relevant Mediterranean sea urchin species: Paracentrotus lividus, a temperate species predicted to be negatively affected by ocean warming, and Arbacia lixula, a subtrop-ical species expected to benefit from rising temperatures, thus representing contrasting biological responses to climate-induced thermal shifts. Methods - Embryos were exposed to several treatments of three tempera-tures (18°C, 21°C, 24°C) and different concentrations of three pollutants (from environmentally relevant to cytotoxic). The single and combined ef-fects to thermal stress and pollutants were tested at three functional lev-els: i) exposure–response relationships, ii) morphological and biominer-alization, iii) cellular/molecular. Results - With respect to developmental progression, elevated tempera-tures at near-future projections accelerated development and biominerali-zation and reduced the negative effects of pollutants, while extreme warm-ing at present-day marine heatwave conditions breached the thermotoler-ance threshold of both species. At the molecular level, we found a relevant increase of apoptotic processes. Our results suggest a fascinating double side effect: while a mild temperature increase reduced the negative effects of pollutants on development, combined heatwave conditions and pollu-tion resulted in a lower proportion of embryos reaching advanced larval stages. Conclusion - Stress resilience will become a pivotal mechanism for spe-cies survival in a changing climate and polluted ocean, even for species that may “like it hot” like A. lixula. This study highlights the unpredicta-bility of the combined effects of global warming and pollution, even on species considered to be favored by global warming, underscoring the ur-gent need to consider interactive stressors in forecasting the resilience of marine species to future conditions.