Award: OCE-1719684

This project was initiated to understand the physiological and ecological mechanisms that reef coral mutualisms cope with environmental change. The close relationship between animal corals and internal photosynthetic symbionts is recognized as a major weakness when exposed to severe marine heat waves; and reasons for mass coral mortality causing reef ecosystem degradation over large geographic scales. Specifically, this research detailed the extent to which the identity of the symbiont contributed to stress sensitivity to 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 over a small geographic area. Once identified, distinct partner combinations were experimented with in the field (reciprocal transplants to different environments) and in laboratory seawater aquaria onshore (artificial thermal stress experiments). Long-term fieldwork of this nature is extremely challenging and while there were some setbacks (e.g. destruction by typhoons; pandemic travel restrictions) the project was an overall success. The intellectual merit of our discoveries include the following: 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 (Figure 1). 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 (Figure 2). 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) (Figure 3). This further emphasizes the need to further study genotype-by-genotype interactions and the importance of this diversity under natural selection. 4. Significant progress was made in codifying symbiont taxonomy critical for use in subsequent scientific correspondence related to this research project and beyond (Figure 4). These supporting works establish that different symbiont species possess different ecological niches. Such fundamental improvements to taxonomy offers new perspectives about the community composition and ecology of these mutualisms and introduces new questions for future research. 5. We assessed the relative importance of autotrophy and heterotrophy among a broad diversity of 8 species of coral living on offshore reefs that are characteristic of contemporary oceanic conditions and from warmer, more acidic, nearshore environments. Using stable isotope analyses, we found that the trophic strategies of colonies living nearshore were distinct from colonies living on offshore reefs. A Bayesian mixing model provided strong support for distinct population and community-wide trophic differences where nearshore colonies consume more zooplankton while symbiotic dinoflagellates provide a larger autotrophic contribution to offshore coral biomass (Figure 5). Our findings demonstrates that reef-building corals rely on diverse nutritional sources in different reef environments to meet their energetic needs and that extensive trophic plasticity represents a critical community-level response that may help corals cope with major environmental changes. 6. Throughout this project we emphasized the importance of working with local Palauan researchers and the local rangers of Koror State. We were accompanied by law enforcement rangers during all field collection trips and develop excellent working partnerships with the local authorities. We gave presentations about our ongoing research to local marine resource managers in Palau and submitted progress reports on an annual basis. Additionally, we worked with local dive operations throughout the course of this project and benefitted from the local knowledge of coral reef ecosystems throughout Palau. We teamed up with Ocean Exploration Inc. to produce a short film titled "Palau Coral: Glimmer of Hope." In Palau, we welcomed environmental journalists and videographer Gaelin Rosenwaks from Global Ocean Exploration, Inc. to gather interviews, capture footage, and document our research expeditions. The documentary showcases our research team and emphasizes significant discoveries from our NSF award. The film gained recognition at the World Ocean Forum, premiered at the 2019 Ocean Film Festival in San Francisco, and has since garnered over 25,000 views on YouTube. Last Modified: 01/20/2024 Submitted by: DustinWKemp