Award: OCE-1358699

Reef-building corals are symbioses between animals and photosynthetic algae. These partnerships can break down due to high-temperature stress in a process called coral "bleaching", which is now threatening reefs worldwide due to climate change. This project developed and tested a new genetic technique to assess the symbiotic status of corals in an effort to help understand if (and why) corals become more susceptible to heat-induced bleaching in certain environmental conditions. Specifically, this project tested the hypothesis that changes in the ratio of algal symbiont cells to coral host cells affect the thermal tolerance of reef-building corals, with corals that have higher ratios (too many symbionts) being more susceptible to bleaching. The project: (1) established a program in the Florida Keys to monitor changes in symbiont:host cell ratios in the field, how they vary between species, and how they respond to seasonal changes; and (2) used laboratory experiments to test whether corals raised under different environmental conditions (including elevated nutrients and carbon dioxide) increase their symbiont:host cell ratios and become more thermally sensitive as a result. These investigations showed that the symbiont:host cell ratio was a cost-effective way of rapidly obtaining quantitative information on both the abundance and identity of different algal symbionts from just a tiny DNA sample. More importantly, symbiont:host cell ratios are a useful indicator of the "symbiotic health" of corals and (although they are subject to considerable variability across species and can be severely disrupted in the event of bleaching) represent a useful way of monitoring symbiont changes in corals over time. This approach has now become a standard tool in our lab and is beginning to be adopted by other labs. During the course of this award we also expanded the use of the technique into the study of how corals could be manipulated to contain different algal symbionts in order to increase their thermal tolerance, which has some implications for how scientists might be able to increase the thermal tolerance of corals by manipulating their algal symbiont communities. These elements of the project led to the PI serving on a National Academy of Sciences committee at the end of the award (2018-19) to review interventions to increase the persistence and resilience of coral reefs, and guide managers in how they might be employed. This project contributed to the training of one postdoctoral scientist, four PhD students, and four Master's students, and provided research experience to 13 undergraduates. The research being done in the laboratory was also featured in mass media, such as shows like Netflix's "Liquid Science", and the BBC's science and technology show "Click", in which the PI discussed the threats facing coral reefs and how research on coral-algal symbiosis might help coral reefs survive the coming decades. Last Modified: 08/02/2019 Submitted by: Andrew C Baker