Response of Mediterranean coralline algae to ocean acidification and elevated temperature
Résumé
The effects of elevated partial pressure of CO2 (pCO(2)) and temperature, alone and in combination, on survival, calcification and dissolution were investigated in the crustose coralline alga Lithophyllum cabiochae. Algae were maintained in aquaria during 1 year at near-ambient conditions of irradiance, at ambient or elevated temperature (+3 degrees C) and at ambient [ca. 400 parts per million (ppm)] or elevated pCO(2) (ca. 700 ppm). Algal necroses appeared at the end of summer under elevated temperature first at 700 ppm (60% of the thallus surface) and then at 400 ppm (30%). The death of algae was observed only under elevated temperature and was two- to threefold higher under elevated pCO(2). During the first month of the experiment, net calcification was significantly reduced under elevated pCO(2). At the end of the summer period, net calcification decreased by 50% when both temperature and pCO(2) were elevated while no effect was found under elevated temperature and elevated pCO(2) alone. In autumn and winter, net calcification in healthy algae increased with increasing temperature, independently of the pCO(2) level, while necroses and death in the algal population caused a net dissolution at elevated temperature and pCO(2). The dissolution of dead algal thalli was affected by elevated pCO(2), being two- to fourfold higher than under ambient pCO(2). These results suggest that net dissolution is likely to exceed net calcification in L. cabiochae by the end of this century. This could have major consequences in terms of biodiversity and biogeochemistry in coralligenous communities dominated by these algae.