Award: OCE-1833215

Continued release of carbon dioxide and other greenhouse gases that are sourced from human activities is causing a rapid rise in atmospheric and ocean temperatures. Coral reefs are especially susceptible to marine heat waves, as elevated water temperature can result in the expulsion and loss of symbiotic algae from reef-building corals, also known as bleaching. Once corals lose these vital symbiotic partners, their growth and reproductive output often declines, and they are often more susceptible to other stressors (e.g., disease) that may lead to even greater mortality and reef loss. However, not all reef corals and their resident algal symbionts show the same degree of thermal sensitivity. Furthermore, determining the thermal response in corals often takes considerable effort by conducting long-term (many days to weeks) chronic heating experiments that are also limited in the number of coral species that may be examined. This project was designed to test the validity of using short-term (acute) heating assays that last only a few hours in order to differentiate thermal stress across a range of coral species. These rapid heating assays were tested in a recently developed, open-source, heating system coined the Coral Bleaching Automated Stress System (i.e., CBASS). These systems are designed to be highly portable and function with low power requirements yet are capable of maintaining verry accurate experimental temperatures to provide rapid controlled heating of small coral fragments. Two species of corals from the Red Sea were sampled at two locations with differing thermal histories that included a warmer protected site and a cooler exposed site that was subjected to greater water mixing and slightly lower temperature profiles. Corals from these locations were tested in acute and slower heating assays in order assess how a range of physiological measurements (i.e., algal photochemistry by chlorophyll a fluorescence, symbiont number, chlorophyll concentration, and oxygen production and respiration) may change under the different heating rates of each experiment. Importantly, we also tested a fast and slow heating rate for the more prolonged chronic heating experiments (0.5 or 1.5 C per day). While we were able to detect fine scale differences in thermal tolerance in one coral species (Porites lobata) that did correlate to the original sites and their respective thermal histories, only the dose response analysis of the chlorophyll fluorescence data confirmed this pattern and only in the faster, more prolonged heating protocol. Meanwhile, no clear differences were noted between sites across acute and chronic heat stress experiments in the other coral species (Acropora hempirchii). Additional laboratory experiments were used to test if differences in thermal thresholds are detectable in short-term acute heating assays with the same CBASS system used in the Red Sea, but using a model symbiotic sea anemone infected with thermally selected and thermally weak algal symbionts originally grown in culture outside of the animal. After exposing animals to a range of temperatures (2836C) for six hours, these experiments verified the utility of using the rapid heating protocol to differentiate thermal sensitivity or tolerance. Anemones containing the more thermally sensitive algae had a greater susceptibility to photo stress with heating, while the anemones harboring more thermally tolerant algal cells displayed a higher thermal threshold across the same tested temperature range. While this work confirmed the utility of using rapid heating assays to help in targeting thermal sensitivity, active chlorophyll fluorescence appears to be in better agreement than other physiological proxies for heat stress such as algal cell loss and loss in total chlorophyll. While this work did confirm the utility in using rapid heating assays to identify thermally sensitive reef corals, results are complicated by the specific corals tested. In some cases, animal-based stress parameters may play a larger role in defining thermal stress. Nevertheless, determining coral thermal sensitivity based on the response of the resident algal symbionts to acute heating has proven to be a valuable tool that requires further validation across a larger taxonomic range of reef corals growing under different conditions. Last Modified: 08/31/2024 Submitted by: MarkEWarner