RODOLFO-METALPA R. 2007 - Responses of two mediterranean corals, Cladocora caespitosa and Oculina patagonica. PhD these. University of AIx-Marseille II.

ABSTRACT

This thesis explores the physiology of two symbiotic Mediterranean corals, Cladocora caespitosa and Oculina patagonica to changes in their environmental parameters. The effect of temperature was investigated both under the normal seasonal range as well as in the perspective of climate change on the two coral species. The effects of irradiance, food supply, and pCO2 level were only investigated with C. caespitosa taken as a model. Temperature was one of the main environmental factors affecting corals’metabolism. Increased concentrations of zooxanthellae and chlorophyll were observed at low temperatures and high food supply, allowing an increase in the photosynthetic efficiency during unfavorable conditions. Conversely, photosynthetic and growth rates were maximal under summer conditions, at temperatures ranging from 18 to 23°C. Higher temperatures (from 24 to 28°C) negatively affected the photosynthesis, growth and symbiosis in both corals. While C. caespitosa resisted 5 weeks at temperatures equal or above 24°C before decreasing its metabolic functions and dying, O. patagonica reduced its symbiont density and growth rates as soon as temperature increased, but showed a low mortality rate. These results, together with the observations of mass mortalities of C. caespitosa during the last summers suggest that this coral is living near its upper thermal limits. We also demonstrated, using C. caespitosa, that food supply was the second most important parameter for these two temperate corals. Conversely, a 8-fold increase in the light level did not change zooxanthellae and chlorophyll concentrations, or growth rates, suggesting that this factor is not important for the coral metabolism. Finally, a doubling in the pCO2 level, as expected by the end of the century, did not either affect C. caespitosa metabolism, leading temperature as the most important parameter.

Key-words: Mediterranean Sea / Temperate corals / Climate change / Cladocora caespitosa / Oculina patagonica / Coral mortality / Temperature / Light / Heterotrophy / Growth / Photosynthesis / Asexual reproduction / Zooxanthellae.