Marine heatwaves outweigh microplastic exposure in driving physiological changes in the coral Astroides calycularis

Global climate change and anthropogenic pollution are major threats to marine ecosystems, with heatwaves and microplastics posing a particular risk to corals. The combined effects of these stressors are still poorly understood, especially for non-symbiotic species like the Mediterranean coral Astroides calycularis. In this study, the combined effects of a simulated heatwave (HW) and microplastics (MPs) on physiological (respiration and excretion rates) and biochemical responses (oxidative stress and energy metabolism biomarkers) of this coral were studied for the first time. Colonies were exposed for four weeks to four conditions: control (26 °C), HW (29 °C), MPs, and a combination of both. Results show that the HW was the primary driver of change, significantly increasing respiration and excretion rates and modulating the antioxidant system. Superoxide dismutase (SOD) activity increased, while catalase (CAT) and glutathione-S-transferases (GSTs) decreased. This indicates a strong metabolic and antioxidant response to thermal stress. In contrast, MPs had only a minor physiological effect on respiration and did not induce a significant biochemical stress response. No significant interaction was detected between the two stressors, suggesting they act independently. Our findings highlight the vulnerability of A. calycularis to thermal stress and underscore the need to study multiple stressors to better understand coral resilience in a changing ocean.