COS 145-4 - Beyond colony morphology: the trade-off between photosynthetic energy gains and losses as driver of biodiversity in zooxanthellate corals

Abstract: Sunlight is the major energy source for zooxanthellate corals, main architects of the hyperdiverse coral reef ecosystems. Light intensity has crucial effects on the primary productivity of symbiotic zooxanthellae and the amount of photosynthetic usable energy supplied to their coral host (PUES). Both high- and low-light levels can limit the energetic output of the zooxanthellae: in high-light environments as a result of increased metabolic maintenance costs while photosynthesis is saturated, and in low-light environments due to the proportional reduction of photosynthetic rates and carbon fixation. Although the paramount importance of light for zooxanthellate corals is widely acknowledged, its influence on coral species distribution and community organization has been largely descriptive and qualitative, usually as a second-order factor. Here, we used a mechanistic model to investigate the relationship of coral biomass allocation inherently linked to colony morphology variation using a “within-colony biomass distribution coefficient”, with the light availability, and the usable energy at the whole-colony level across depth gradients. To test the model, we used previously published datasets that report the depth-distribution of coral genera or growth-form categories. Only genera composed of species with similar morphologies (e.g., Pocillopora = branching, Pavona = massive, Leptoseris = laminar) were used in generic-level analyses. Results indicate that the change in PUES associated with morphological variation explains most of the depth-related variation in generic abundance. The model had higher explanatory power for the vertical distribution of genera dominated by branching and laminar morphologies (~90% - 99%), which tend to dominate shallow and deep environments respectively. The model exhibited slightly less explanatory power (~70%) for the vertical distribution of genera dominated by massive morphologies, potentially associated with greater intra-genus morphological variation regulating light capture and optimizing PUES across the depth gradient. When considering broad morphological categories, the model explained most of the depth-distribution variation of all categories (~85% - 95%). Our findings indicate that changes in biomass allocation inherently linked to coral colony morphology is an essential strategy for regulating PUES at the whole-colony level, which results from the compromise between maximizing the photosynthetic production while minimizing maintenance costs of the symbiotic zooxanthellae. This compromise may be a major driving force in the evolution of colony morphology and depth segregation in zooxanthellate corals. The knowledge of light gradients associated to the topographic relief of colonies and not only depth, is crucial to the understanding of the physiology, ecology and evolution of zooxanthellate corals.