Award: OCE-1833201

Carbon dioxide and greenhouse gas emissions from human activities are causing ocean temperatures to rise, affecting a number of marine ecosystems and organisms. Coral reefs are particularly vulnerable to ocean warming, as sustained periods of high seawater temperatures, termed marine heatwaves, can result in coral bleaching. When seawater temperatures are warmer than usual, corals expel the algae that live in their tissues, revealing the white skeleton beneath their tissue. If seawater temperatures remain high for too long, corals can starve and die, as they rely on energy provided from the algae to survive. Determining the temperatures at which corals will undergo bleaching is complex, as it is likely to vary as a function of the duration and intensity of marine heatwaves. Additionally, marine heatwaves often can last for weeks to months, complicating the logitics and cost of experimentally replicating marine heatwaves under lab conditions. The principal objective of this project was to test the validity of using short-term heat stress experiments, lasting less than a day, to determine the relative tolerance of corals to higher temperatures, termed thermal tolerance. The project also aimed to develop a low-cost, portable experimental system capable of running short-term heat stress experiments in a variety of remote field locations. Using corals from the northern Gulf of Aqaba, in the Red Sea, we found a strong similarity in the response of corals to short-term heat stress (lasting 7 hours) compared to the same corals exposed to longer-term heat stress (lasting 11 days). Corals from both short and longer experiments displayed similar thermal tolerances, with molecular analyses indicating that both the coral and their algal symbionts produced a fast and strong responses to heat stress in order to cope with these elevated temperatures. The results further confirm previous findings that corals from the northern Gulf of Aqaba are able to tolerate particularly high temperatures, providing hope that naturally heat tolerant coral populations exist that may be able to survive future ocean warming. Our research also investigated the reliability of short-term heat stress experiments using the CBASS as an approach to differentiate between coral thermal tolerances across large spatial scales. We conducted identical heat stress experiments on corals across study sites spanning the entire length of the Red Sea, from Eilat, Israel, to Djibouti in the Gulf of Aden. We found that corals generally increased in their ability to tolerate warmer seawater when they originated from sites with higher average summertime seawater temperatures, indicating that warmer reefs may produce more tolerant corals. However, we also found that corals from the warmest study site had one of the lowest heat tolerances of all the study sites, indicating that seawater temperatuers at some of the warmest reefs may already be exceeding levels that corals can tolerate. Overall, this project has highlighted considerable promise in using the CBASS to conduct short-term heat stress experiments as a low-cost, rapid approach to determine the capacity for various corals to tolerate increasing seawater temperatures. As such, using this approach can accelerate our ability to identify and targert naturally heat tolerant corals future research and conservation. Last Modified: 11/30/2021 Submitted by: Daniel Barshis