Adaptive potential of a Mediterranean zooxanthellate coral to ocean acidification: insights from a naturally acidified habitat

Ocean acidification caused by shifts in carbonate chemistry resulting from increased atmospheric CO2 concentration is threatening marine calcifying organisms, including corals. Here, we assessed the photo-physiological and molecular mechanisms underling acclimatization mechanisms to low pH/high pCO2 conditions in the Mediterranean zooxanthellate coral Balanophyllia europaea at a CO2 vent off Panarea Island (Italy). In agreement with previous findings, we demonstrated a higher chlorophyll a content per unit of coral host surface, due to higher dinoflagellate endosymbiont densities at low pH sites. This translated in a higher content of photosynthates sustained by an increase in the total photosynthetic capacity of the symbionts. This likely enhanced carbohydrate translocation to coral host, providing nutritional benefits, and increasing energy reserves to sustain tissue growth. The presence of a higher-biomass coral phenotype was confirmed by an increased host protein content at low pH. Our molecular findings, support the idea of a rearrangements of the transcriptional levels of biomineralization-related genes in the host as well as of photosynthetic components in the algal counterpart. Overall, our findings show further details of the acclimatization mechanisms of this coral-dinoflagellate mutualism under ocean acidification, as predicted by end-of-century scenarios, complementing previous studies on the skeletal properties of B. europaea at low pH/high pCO2 conditions.