One-year experiment on the physiological response of the Mediterranean crustose coralline alga, Lithophyllum cabiochae, to elevated pCO(2) and temperature

The response of respiration, photosynthesis, and calcification to elevated pCO(2) and temperature was investigated in isolation and in combination in the Mediterranean 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°C), and at ambient (ca. 400 μatm) or elevated pCO(2) (ca. 700 μatm). Respiration, photosynthesis, and net calcification showed a strong seasonal pattern following the seasonal variations of temperature and irradiance, with higher rates in summer than in winter. Respiration was unaffected by pCO(2) but showed a general trend of increase at elevated temperature at all seasons, except in summer under elevated pCO(2). Conversely, photosynthesis was strongly affected by pCO(2) with a decline under elevated pCO(2) in summer, autumn, and winter. In particular, photosynthetic efficiency was reduced under elevated pCO(2). Net calcification showed different responses depending on the season. In summer, net calcification increased with rising temperature under ambient pCO(2) but decreased with rising temperature under elevated pCO(2). Surprisingly, the highest rates in summer were found under elevated pCO(2) and ambient temperature. In autumn, winter, and spring, net calcification exhibited a positive or no response at elevated temperature but was unaffected by pCO(2). The rate of calcification of L. cabiochae was thus maintained or even enhanced under increased pCO(2). However, there is likely a trade-off with other physiological processes. For example, photosynthesis declines in response to increased pCO(2) under ambient irradiance. The present study reports only on the physiological response of healthy specimens to ocean warming and acidification, however, these environmental changes may affect the vulnerability of coralline algae to other stresses such as pathogens and necroses that can cause major dissolution, which would have critical consequence for the sustainability of coralligenous habitats and the budgets of carbon and calcium carbonate in coastal Mediterranean ecosystems.