This project was initiated to understand the physiological and ecological mechanisms that reef coral mutualisms cope with environmental change (i.e. global warming). The close relationship between animal corals and internal photosynthetic symbionts is recognized as a major weakness when exposed to severe marine heat waves and the cause of mass coral mortality and reef ecosystem degradation over large geographic scales. Specifically, this research detailed the extent to which the identity of the symbiont contributed to stress sensitivity as well as stress tolerance. Working in the Island nation of Palau provided unique access to a breadth of coral diversity represented by colonies from various reef habitats harboring very different symbiont species in a small geographic area. Once identified, distinct partner combinations were manipulated in field experiments (i.e. reciprocal transplants to different environments/habitats, in situ monitoring) and onshore tank experiments (i.e. artificial thermal stress experiments). Long-term fieldwork of this nature is extremely challenging and while there were several setbacks (e.g. destruction by typhoons; pandemic travel restrictions) the project was an overall success. Highlights of our discoveries (as well as their intellectual merits and impacts) are presented below. Note that this list is not exhaustive. There were many other accomplishments and published discoveries including several manuscripts in preparation, which were omitted for brevity. 1. Nutritional equivalences among corals from different habitats with different symbionts (Figure 1). Building upon recent findings, which documented inshore colonies better tolerated thermal stress than offshore colonies, we found that most colonies from each habitat contained similar lipid, protein, and carbohydrate content contrary to expectations, and therefore concluded that the symbiont species, not necessarily access to greater amounts of nutrient reserves, was the most consequential feature that explained thermal tolerance among colonies. While host genotypic diversity is also important, these findings further underscore the essential role of symbiont identity in the stability of these mutualisms exposed to physiological stress. 2. Corals with thermally tolerant symbionts experience little or no metabolic trade-offs (Figure 2). Colonies harboring symbionts adapted to high temperatures do not necessarily experience physiological trade-offs that compromise the growth and reproductive capabilities of the host. Moreover, the continued maintenance of nutrient assimilation and translocation when subjected to high temperatures largely explains why some mutualisms are tolerant to stress while others are not. These findings dispel the dogma that hosting heat-adapted symbionts has negative consequences and further highlight how these mutualisms may respond on ecological time scales to significant environmental change through partner recombination while maintaining ecosystem productivity. 3. The diversity, distribution, and temporal stability of coral zooxanthellae on a Pacific reef: from the scale of individual colonies to across the host community (Figure 3). Complementary to the major objectives of this project, discoveries from coinciding (opportune) research substantiated and advanced our fundamental understanding of these mutualisms. Our continued monitoring of numerous and diverse tagged corals showed conclusively that individual colonies typically maintain long-term stable relationships comprising one symbiont species whose population is genetically homogenous (clonal). This further emphasizes the need to further study genotype-by-genotype interactions and the importance of this diversity under increased natural selection pressure. 4. Formal characterization and identification of species (Figure 4). Major progress was made in codifying symbiont taxonomy and systematics critical for use in subsequent scientific correspondence related to this research project and beyond. These supporting works establish that different symbiont species possess different ecological niches. Such fundamental improvement to taxonomy offers new perspectives about the community composition and ecology of these mutualisms and reveals and abundance of new questions for future research. 5. Broader Impacts. Over the course of our project there were numerous public outreach activities and presentations at K-12 schools, National Academy workshops, Scientific conferences and university alumni associations. A few major highlights include: -YouTube/vimeo video about our project. Palau Coral: Glimmer of Hope was produced with funds from this grant (https://www.youtube.com/watch?v=o5M2_tXG5Zo&t=28s). and has been viewed 25,000 times on YouTube and was featured at the 2019 International Ocean Film Festival in San Francisco. -Feature (cover) Article, Solar Symbionts, in Coral Magazine (vol.18 (5) Sept/Oct 2021) a beautiful comprehensive review of coral-algal mutualisms written for lay enthusiasts. This magazine is a high quality bi-monthly periodical for marine aquarists with a circulation of thirty-five thousand international subscribers. Four graduate students (3 women and one male) directly involved in this project received their Ph.D. degrees: 1 from Delaware, 1 from U. of Alabama, Birmingham, and two from Penn State. Several other students also benefitted, albeit somewhat indirectly. Numerous undergraduates at all three institutions received scientific training as well. To date, well over a dozen research papers supported by this grant are published and already well cited. Last Modified: 01/12/2024 Submitted by: ToddCLajeunesse