Depth-dependent temperature shifts and their impact on zooxanthellae dynamics and physiological stress in Anemonia...

Warming seas represent a major threat: rising baseline temperatures and increasing frequency and intensity of marine heatwaves intensify physiological stress in marine organisms. In symbiotic cnidarians, thermal stress can disrupt the host-symbiont balance by enhancing reactive oxygen species production, potentially leading to oxidative damage and bleaching. Depth can further modulate thermal exposure, as shallow habitats experience greater temperature variability and light intensity than deeper zones. The aim was to assess the effects of depth-dependent temperature shifts in Anemonia sulcata by sampling individuals at 0.2 and 1.5 m depth monthly from June to September 2024, with an additional sampling in November. Zooxanthellae density, chlorophyll a and c concentrations, antioxidant enzymes (catalase, superoxide dismutase, glutathione peroxidase, glutathione reductase), and lipid peroxidation biomarker (malondialdehyde) were quantified. Discrete monthly temperatures did not differ significantly between depths; however, 24-h temperature records in July and August revealed diel peaks up to ∼31.1 °C in shallow habitats, approximately 2 °C higher than in deeper sites. Zooxanthellae density and chlorophyll a and c concentrations increased during the warmest months (August-September), particularly in shallow anemones. Catalase and glutathione peroxidase activities, together with malondialdehyde levels, were elevated in shallow individuals during July-September, whereas superoxide dismutase and glutathione reductase remained comparatively stable. These patterns indicate enhanced photosynthetic activity accompanied by increased oxidative stress in shallow habitats. In conclusion, although A. sulcata appears capable of short-term photoacclimation, the concurrent rise in lipid peroxidation suggests that antioxidant defences may become insufficient under repeated or prolonged warming, potentially preceding bleaching events.