Molecular and physiological acclimatization of the Mediterranean zooxanthellate coral Balanophyllia europaea at a natural CO2 vent.

Ocean acidification (OA) is a major threat to marine ecosystems, especially for calcifying organisms. This study investigated the molecular and physiological responses of the zooxanthellate coral Balanophyllia europaea along a natural CO2 gradient (Islet of Bottaro, Panarea - Aeolian Islands). This pH gradient is a natural laboratory to study the adaptive potential of marine organisms such as corals, to the acidification conditions expected in future climate scenarios. Through photo-physiological and gene expression analyses, the impact of OA on the photosynthetic performance of zooxanthellae, and on the regulation of genes related to biomineralization, was assessed. The results show that acclimatization to OA occurs through an increase in the total photosynthetic capacity of symbionts, which provides energy for tissue growth, as indicated by a higher host protein content at low pH. However, the energy invested to reach sexual maturity compromises skeletal properties, reducing net calcification and increasing porosity. Growing in a more porous skeleton implies a greater incorporation of organic matrix proteins. Consistently, a higher expression of genes encoding organic matrix proteins was observed at low pH. This study highlights the adaptive potential of B. europaea and suggests strategies for coral resilience to ocean acidification.