Response of zooxanthellae in symbiosis with the Mediterranean corals Cladocora caespitosa and Oculina patagonica to elevated temperatures.

Scleractinian symbiotic corals living in the Ligurian Sea (NW Mediterranean Sea) have experienced warm summers during the last decade, with temperatures rapidly increasing, within a few days, to 3–4C above the mean value of 24C. The effect of elevated temperatures on the photosynthetic efficiency of zooxanthellae in symbiosis with temperate corals has not been well investigated. In this study, the corals, Cladocora caespitosa and Oculina patagonica were collected in the Ligurian Sea (44N, 9E), maintained during 2 weeks at the mean summer temperature of 24C and then exposed during 48 h to temperatures of 24 (control), 27, 29 and 32C. Chlorophyll (chl) fluorescence parameters [Fv/Fm, electron transport rate (ETR), non-photochemical quenching (NPQ)] were measured using pulse amplitude modulated (PAM) fluorimetry before, during the thermal increase, and after 1 and 7 days of recovery (corals maintained at 24C). Zooxanthellae showed a broad tolerance to temperature increase, since their density remained unchanged and there was no significant reduction in their maximum quantum yield (Fv/Fm) or ETR up to 29C. This temperature corresponded to a 5C increase compared to the mean summer temperature (24C) in the Ligurian Sea. At 32C, there was a significant decrease in chl contents for both corals. This decrease was due to a reduction in the chl/zooxanthellae content. For C. caespitosa, there was also a decrease in ETRmax, not associated with a change in Fv/Fm or in the non-photochemical quenching (NPQ); for O. patagonica, both ETRmax and Fv/Fm significantly decreased, and NPQmax showed a significant increase. Damages to the photosystem II appeared to be reversible in both corals, since Fv/Fm values returned to normal after 1 day at 24C. Zooxanthellae in symbiosis with the Mediterranean corals investigated can therefore be considered as resistant to short-term increases in temperature, even well above the maximum temperatures experienced by these corals in summer.