Nomenjanahary Alexia Raharinirina

• Coral polyps of the order Scleractinia are tiny anemone-like invertebrates interconnected through a common gastrovascular system. Scleractinian corals accrete a carbonate exoskeleton and act as primary builders of limestone structures called coral reefs in the shallow, well-lit and nutrient-poor waters of the tropics. A myriad of other organisms benefits from reef structures, making coral reefs one of the most diverse and productive ecosystems on Earth. The most puzzling aspect about corals is that they thrive in nutrient-poor waters of the tropics. The reason is inherent to a symbiotic association that they form with unicellular photoautotrophs known as zooxanthellae, located in membrane-bound vacuoles, the symbiosomes, in the corals' endodermal cells. Corals host millions of zooxanthellae algae and benefit from the carbohydrates produced from algal photosynthesis. As a result, corals do not rely exclusively on external nutrient sources. Increasing sea surface temperature induces a breakdown of the coral-algae association, causing the whitening of the corals due to a loss of zooxanthellae cells or zooxanthellae pigments, a process called bleaching. Consequently, in a warming world, the future of corals and the rich ecosystem they contribute to create is a matter of great concern. The work presented in this thesis investigates the acclimation capacity of corals under global warming and proposes potential mechanisms for explaining symbiont shuffling. These new model theories can be tested with laboratory experiments thus contributing to the development of strategies for the preservation and restoration of coral reef ecosystems.