Trait-based modeling of coral-algae symbiosis in a warming ocean


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. 


This thesis presents a new modeling framework for investigating the adaptive dynamics of coral-algae symbiosis in response to warming. The framework is presented, tested, and discussed over three projects. In the first project, I designed and analyzed a mathematical model, that constitutes the theoretical basis for the other studies. The model showed that the ability of corals to invest energy into the symbiotic relationship decreases with increasing symbiont to coral biomass ratio. It also showed that the coral-algae complex can thrive for a broad range of symbiont to coral biomass ratio when the costs incurred by the corals in the symbiotic relationship are low, but this survival range narrows down with increasing cost of symbiosis. In the second project, I extended the previous model by including the effects of temperature change on the symbiotic relationship between corals algae. I then used this model to investigate the acclimation capacity of corals to global warming in three iconic coral reef regions of the tropics: the Great Barrier Reef, the Caribbean, and South East Asia. The results indicated that corals of the Great Barrier Reef have a higher acclimation capacity than corals of the other regions, although substantial declines in coral biomass are expected by the year 2100 in all regions. The bleak but clear result of this project is that the acclimation capacity of corals will not be sufficient to rescue corals from the destructive effects of global warming. In the third a final project, I further modified the model to include and test mechanisms of symbiont shuffling. This study showed that classical competition theory explains symbiont shuffling when the competitive abilities of different symbionts are proportionally related to their thermal-tolerances. The results also suggest that rapid symbiont shuffling can  occur in the presence of a positive feedback, according to which symbiont growth is tuned according to the symbiotic benefits they provide to corals.


Overall, 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.