Interactive effects of increased temperature and gadolinium pollution in Paracentrotus lividus sea urchin embryos: a climate change perspective

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. In this study, the combined effects of thermal stress and exposure to gadolinium (Gd), a metal used as a contrasting agent in medical imaging which enters the aquatic environment, were investigated in the embryos and larvae of the sea urchin Paracentrotus lividus. Embryos were exposed to six treatments of three temperatures (18 °C, 21 °C, 24 °C) and two Gd concentrations (control: 0 μM; treated: 20 μM). With respect to developmental progression, increased temperature accelerated development and achievement of the larval stage, while Gd-exposed embryos at the control temperature (18 °C) showed a general delay in development at 24 h post-fertilization (hpf), and a stunting effect and impaired skeleton growth at 48 hpf. Elevated temperatures at near-future projections (+3 °C, 21 °C) reduced the negative effects of Gd on development with a lower percentage of abnormality and improved skeleton growth. Combined extreme warming at present-day marine heatwave conditions (+6 °C, 24 °C) and Gd treatment resulted in a lower proportion of embryos reaching the advanced larval stages compared to the 21 °C + Gd. At the molecular level, western blot analysis showed that Gd was the main driver for the induction of heat shock protein (HSP60, HSP70) expression. At 48 hpf, temperature increase was the main driver for activation of additional cellular stress response strategies such as autophagy and apoptosis. Combined treatments showed the induction of HSP60 at 24 hpf and autophagic and apoptotic processes at 48 hpf. Treatments having low levels of HSPs expression showed high levels of apoptosis, and vice versa, clearly demonstrating the antagonistic effects of HSPs expression and apoptosis. Detection of fragmented DNA in apoptotic nuclei showed selective apoptosis, likely in extremely damaged cells. Our results indicate that the negative effects of Gd-exposure on P. lividus 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.