Photosynthetic response of the Mediterranean zooxanthellate coral Cladocora caespitosa to the natural range of light and temperature.

We investigated photoacclimation in the symbiotic Mediterranean coral Cladocora caespitosa by exposing it to three light levels (30, 80 and 250 micromol m(-2) s(-1)), which are in the range of those recorded for this species. The coral response to a change in both light and temperature was also assessed, by subjecting coral to two treatments corresponding to winter (14 degrees C and 30 micromol m(-2) s(-1)) and summer (23 degrees C and 250 micromol m(-2) s(-1)) conditions, as measured in the Ligurian Sea. Photosynthesis, measured using both respirometry and pulse amplitude modulated (PAM) fluorometry, revealed a linear relationship only at low light levels. At higher irradiance, relative electron transport rate (rETR) approached saturation more slowly than rates of oxygen production. At constant temperature, a change in light did not induce any change in zooxanthellae (zoox) and chlorophyll (Chla+c2) concentrations (mean 3.7 x 10(6) zoox cm(-2) and 14.1 microg cm(-2), respectively); however, chlorophyll concentrations significantly increased under low light and temperature, probably in order to maintain a sufficient level of autotrophy. Maximal gross photosynthesis (Pgmax) as well as the saturation irradiance (Ek) and the respiration rate (R) were, however, significantly higher at 250 micromol m(-2) s(-1) compared to the lower light treatments, independently of temperature conditions. Acclimation to high light appeared to be partly driven by a change in the non-photochemical quenching (NPQ) capacity of the algal cells, and to a maximal rate of photon utilization. Conversely, under low light conditions, coral polyps presented a lower Ek, but also lower respiration rates, which correspond to a decrease in the energy expenditure. This ability to acclimate to different light conditions, might allow C. caespitosa to rapidly regulate its autotrophic rate in the different light conditions encountered in its natural habitats.