Award: IOS-1258065

The goals of this project were to investigate the symbiosis ecology and physiological response of various algae that live within reef-building corals. These symbiotic algae are critical for the health and growth of reef corals and are the reason for the existence of tropical coral reefs. In particular, our work sought to determine the importance of thermally tolerant symbiont species. Given that the coral?s symbiont has a significant effect on coral colony physiology, large-scale changes in symbiont distributions may represent a substantial response to ocean warming. One algal species, in particular, called Symbiodinium trenchii, has a high thermal tolerance while living in corals in the Caribbean and the Indo-Pacific. The project was based, in part, on the discovery that Symbiodinium trenchii recently invaded the Caribbean Sea. This widespread symbiont has developed associations with several key Atlantic reef-building corals that may be poorly optimized, or mal-adapted, for this symbiont. Our experiments were designed to address questions about the health costs vs. benefits of S. trenchii spreading to other coral communities around the world, including its possible expansion throughout the Indo-Pacific, as oceans continue to warm. Some of our experiments were designed to examine the functional trade-offs that might exist among corals hosting thermally tolerant vs. thermally sensitive symbionts. Do corals with thermally tolerant symbionts grow at reduced rates because these symbionts may not supply the necessary nutrients for maximal growth? Could this affect the reef-building function of corals in the decades to come, assuming that these tolerant symbionts spread to more and more corals as oceans continue to warm? The coral reefs surrounding the island nation of Palau provide an excellent opportunity to test these questions. In particular, corals living on offshore barrier reefs tend to harbor thermally weaker symbionts, while the same coral species living inshore near the Rock Islands of Koror State harbor the temperature-tolerant S. trenchii. We also investigated the genetics of S. trenchii populations and found that they are genetically homogeneous over thousands of kilometers and can travel/disperse long distances. Thus, this alga has the potential to possibly undergo rapid geographic expansion in timeframes equivalent to the current pace of climate change. We confirmed that this symbiont significantly raises the thermal stress tolerance of a coral, although the magnitude of this tolerance varies from species to species of coral. Moreover, we found that S. trenchii in Pacific corals does not alter the host?s calcification and growth rate. Our findings are essential especially when placed in the context of how thermally tolerant symbionts like Symbiodinium trenchii can contribute to the rapid ecological response of reef corals to the current pace of climate change. However, our findings raised additional questions. For example, we do not know whether temperature, light, pH, or nutrient availability or a combination of these factors affect the stability of thermally tolerant symbionts in coral colonies over time frames of months, years and decades. Also, we found that many corals with S. trenchii in the inshore habitats of Palau obtained more nutrients from feeding and this additional source of energy could also be contributing to the heat tolerance of the coral. Lastly, corals from warm water bays could represent genetically unique populations that enable them to endure warmer temperatures than other members of the same species living on the offshore barrier reefs of Palau. The broader impacts of this project included numerous presentations to the scientific community and general public. Many educational outreach programs were initiated and conducted at local, national, and international venues. Public lectures and presentations were given in Palau as well as at the annual Coast Day festival in Lewes, DE. Further dissemination of project results was through lecture material in a course on the ecology and evolution of coral reefs taught at the University of Delaware, a study-abroad program taught on Little Cayman Island, and via scientific conferences and journals. This project involved the training of 3 Ph.D. graduate students as well as two postdoctoral fellows across two U.S. universities. Last Modified: 07/02/2018 Submitted by: Mark E Warner