NOT ALL LIKE IT HOT: THE CONTRASTING EFFECTS OF TWO GLOBAL WARMING CONDITIONS ON KEY MEDITERRANEAN SEA URCHIN SPECIES EXPOSED TO POLLUTION

Abstract

Gradual ocean warming and marine heatwaves represent major threats for marine organisms already facing other anthropogenic-derived hazards, such as chemical contamination in coastal areas. Sea urchin larvae are a common model system in marine ecotoxicological studies to assess the impacts of climate change and pollution. Here I report the combined effects of thermal stress and exposure to three different pollutants of the marine environment (gadolinium, vanadium and phthalates) on embryos of two common Mediterranean sea urchin species with predicted opposite responses to warming, the temperate Paracentrotus lividus and the sub-tropical Arbacia lixula. These species are among the most abundant echinoids living in shallow rocky reefs of the southwestern Mediterranean and are ecologically important because their herbivorous grazing impacts macroalgae and leads to the formation of barrens habitats. Embryos were exposed to several treatments of three temperatures (18°C, 21°C, 24°C) and different concentrations of the three pollutants (from environmentally relevant to cytotoxic). We tested the single and combined effects to thermal stress and pollutants at three functional levels: i) exposure–response relationships, ii) morphological, analyzing impacts of treatments on larval phenotypes and morphometric traits of larval growth and biomineralization; iii) biochemical/cellular, investigating the effects on activity of enzyme biomarkers, protein expression and the activation of the cellular stress response. With respect to developmental progression, elevated temperatures at near-future projections (+3°C, 21°C) accelerated development and achievement of the larval stage, while extreme warming at present-day marine heatwave conditions (+6°C, 24°C) breached the thermotolerance threshold of both species with a high proportion of abnormal larvae (30 %). We found a fascinating double side effect of increased temperature combined to pollution: a mild temperature increase (+3°C) reduced the negative effects of pollutants on development with a lower percentage of abnormality and improved skeleton growth, while combined heatwave conditions (+6°C) and pollution resulted in a lower proportion of embryos reaching the advanced larval stages. Our results indicate that the negative effects of pollutants-exposure on P. lividus and A. lixula larval development and biomineralization will be mitigated by a near-future ocean warming, up to a thermotolerance threshold when negative synergistic effects were evident. Our data highlight the use of biomarkers as sensitive tools to detect environmental impacts as well as the need for a better understanding of the interactions between the multiple stressors faced by marine species in coastal environments.