Response of zooxanthellae in symbiosis with the Mediterranean corals Cladocora caespitosa and Oculina patagonica to elevated temperatures
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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 3–4°C above the mean value of 24°C. 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 (44°N, 9°E), maintained during 2 weeks at the mean summer temperature of 24°C and then exposed during 48 h to temperatures of 24 (control), 27, 29 and 32°C. 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 24°C). 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 29°C. This temperature corresponded to a 5°C increase compared to the mean summer temperature (24°C) in the Ligurian Sea. At 32°C, 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 24°C. 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.
KeywordsPhotosynthetic Efficiency Electron Transport Rate Scleractinian Coral Zooxanthella Density Tropical Coral
The work presented here is part of the PhD thesis of R. Rodolfo-Metalpa, supported by the Centre Scientifique de Monaco. Thanks are due to Drs. R. Delfanti, and A. Peirano (Marine Environmental Research Centre ENEA, Spezia, Italy) for their support during sample collection, to Prof. G. Fierro (University of Genoa, Italy) and Dr. D. Rubeo (France). Temperature data were either provided by SOMLIT (Service d’Observation en Milieu Littoral, CNRS, INSU, Villefranche-sur-Mer) or by the Oceanographic Museum in Monaco (thanks to T. Thèvenin). Zooxanthellae genotype of the coral C.caespitosa was gently provided by Dr. Didier Forcioli (University of Nice-Sophia-Antipolis). We also thank C. Matthew Moy (Stantford, USA) for English revision and three anonymous reviewers for an improvement of the manuscript.
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