Award: OCE-1635695

The central goal of this project was to understand the physiological and ecological mechanisms that symbiotic reef building corals use to cope with environmental change (i.e. global warming). The close relationship between corals and their photosynthetic algae (i.e. the symbionts) is often disrupted when corals are exposed to severe marine heat waves that cause mass coral mortality and degrade reefs. Specifically, this research detailed the extent to which the symbiont identity contributed to coral stress sensitivity or tolerance. The Island nation of Palau provided unique access to a breadth of coral diversity from various reef habitats harboring very different symbiont species in a small geographic area. Corals were manipulated in field experiments (e.g., transplants to different environments/habitats) and onshore experiments (e.g., exposing corals to heat). While there were several setbacks (e.g., sample loss by several typhoons and pandemic travel restrictions) the project was an overall success. Highlights of some of our discoveries and their intellectual merits and impacts are presented below. 1. Corals from different habitats with different symbionts maintain similar energy reserves. While inshore corals better tolerated thermal stress than offshore corals, most colonies from each habitat contained similar amounts of lipids, proteins, and carbohydrates, which was contrary to expectations from other previously published studies. We concluded that the symbiont species, and not necessarily access to greater amounts of nutrient reserves, was the most consequential feature that explained thermal tolerance among colonies at the inshore reef sites. While host genotypic diversity is also important, these findings further underscore the essential role of symbiont identity in the stability of these mutualisms when exposed to physiological stress such as heating. 2. Corals with thermally tolerant symbionts experience little or no metabolic trade-offs. Corals harboring symbionts adapted to high temperatures do not necessarily experience physiological trade-offs. Specifically, nutrient assimilation and carbon translocation is maintained when inshore corals are subjected to high temperatures. This result largely explains why some mutualisms are tolerant to stress. These findings also challenge the dogma that hosting heat-adapted symbionts necessarily has negative consequences to the coral. 3. The diversity, distribution, and high temporal stability of symbiotic algae on a Pacific reef: from the scale of individual colonies to across the host community. Long-term monitoring of diverse corals on the western barrier reef of Palau provided evidence that individual corals typically maintain long-term stable relationships with one dominant symbiont species whose population is genetically homogenous (clonal). This further emphasizes the need to further study genotype-by-genotype interactions among reef corals and their symbionts. 4. Formal characterization and identification of symbiont species. Significant progress was made in codifying symbiont taxonomy critical for use in subsequent scientific correspondence related to this research project and beyond. This supporting work establishes that different symbiont species possess different ecological niches. Such fundamental improvements to taxonomy offer new perspectives about the community composition and ecology of these mutualisms and introduces new questions for future research. 5. Corals living in different thermal environments have markedly different trophic strategies. We assessed the relative importance of autotrophy and heterotrophy among eight coral species on offshore and inshore reefs. Using stable isotope analyses, we found that the trophic strategies of inshore colonies were distinct from colonies living on offshore reefs. We noted distinct population and community-wide trophic differences, where inshore colonies consume more zooplankton while symbiotic dinoflagellates provide greater autotrophy to offshore corals. Our findings demonstrate that reef-building corals rely on diverse nutritional sources in different reef environments to meet their energetic needs, and that extensive trophic plasticity may help corals cope with environmental change. 6. Broader Impacts. Several activities extended the scientific and public dissemination of our project, provided collaboration with local authorities in Palau, and trained future scientists: In collaboration with Ocean Exploration, Inc., we produced a brief public video, called "Palau Coral: Glimmer of Hope," which premiered at the 2019 Ocean Film Festival in San Francisco, and has been viewed over 25,000 times on YouTube. Field work was overseen by local authorities and in collaboration with Palauan researchers and Koror State Fisheries rangers. Several presentations were given to local marine resource managers and private research organizations in Palau. In addition, all of our diving operations were conducted with Palauan captains and dive operators whose local knowledge of the reef ecosystems throughout Palau was invaluable for our research endeavors. Further outreach for this project was achieved by several public science presentations to elementary schools and the general public at the University of Delawares annual Coast Day. This project provided the financial assistance for training four graduate students (three women and one male) who received their Ph.D. degrees from the University of Delaware, the University of Alabama at Birmingham, and Pennsylvania State University. In addition, this project supported the scientific training of numerous undergraduate students at each of these institutions. Last Modified: 08/31/2024 Submitted by: MarkEWarner