Wildfire ash undermines the physiology of the Mediterranean coral Cladocora caespitosa

Wildfires represent a natural disturbance phenomenon whose frequency and intensity are expected to increase due to climate change-driven heatwaves, droughts, and anthropogenic pressure. This is particularly concerning in fire-susceptible areas, such as the Mediterranean basin, raising concerns about ecosystem functioning and biodiversity. While wildfire impacts have been extensively investigated in terrestrial environments, little is known about their effects on marine organisms. Here, nubbins of the temperate coral Cladocora caespitosa, an endemic key ecological species in the Mediterranean Sea, were subjected to a 93-h exposure to four environmentally relevant wildfire ash concentrations (0.05, 0.5, 2.5, and 5 g L^-1) under controlled laboratory conditions. Coral responses were assessed through oxidative stress biomarkers (activity of antioxidant enzymes SOD, CAT, GR, and LPO levels) and bleaching parameters (chlorophyll a and c2 concentration and Symbiodiniaceae density). A fluctuating response of SOD, a significant decrease in CAT activity, and an increase in GR activity were observed, suggesting oxidative stress overwhelming primary defenses. This interpretation was supported by a significant increase in LPO, indicating oxidative damage to cellular structures, possibly exacerbated by elevated metal concentrations in the ash. Symbiodiniaceae density decreased significantly across all tested ash concentrations, while chlorophyll content per cell increased, potentially reflecting photoacclimation to reduced light penetration caused by water turbidity. Overall, our results could indicate that C. caespitosa is vulnerable to wildfire ash exposure. These findings highlight wildfire ash as an emerging marine stressor, thereby broadening our understanding of wildfire impacts on benthic ecosystems.