Award: EF-1316055

Many tropical animals in the phylum Cnidaria have a unique symbiotic relationship with single cell microalgae called dinoflagellates. These animals include reef building corals as well as many sea anemones, and their symbiotic algae provide a significant source of food in the form of carbon made from photosynthesis. There is a growing concern that as climate change progresses, sea water temperature and ocean acidification will continue to rise and destabilize these animal-algal symbioses. When compared to reef corals, previous work from natural and experimental systems suggested that sea anemones may not be as susceptible to elevated CO2 alone, however we know far less about the possible interactive effects of high temperature and high CO2. This project investigated how the model sea anemone Exaptasia diaphana and its algal symbionts would react to the combination of high temperature and high CO2 in short (> 4 months) and long-term (11 month) exposures. We used several different combinations of symbiotic algae (Breviolum minutum or Symbiodinium spp.) that naturally occur in this animal and a natural assemblage of animals originally collected from the Florida Keys. Short-term experiments were designed to raise CO2 to levels expected approximately eighty years from now, followed by two periods of increased heating (from 26°C to 29°C for several weeks and then to 33°C for five days). We found marked differences in the physiological response of the anemones which correlated with the type of symbiont. For example, under high CO2 alone, photosynthesis increased in anemones hosting Symbiodinium sp., while there was no change in animals hosting the B. minutum alga. This trend continued as temperature increased. Once we reached the maximum temperature + CO2 conditions, animals with the B. minutum alga displayed the greatest loss in photosynthetic function as well as asexual reproduction. Long-term experiments were designed to provide a sub-lethal temperature exposure (31°C) with elevated CO2 in order to test for possible changes in algal and animal physiology, and asexual reproduction. Photosynthetic activity fell significantly in the original adult anemones under high temperature and CO2, while there was no change photosynthetic activity in the treated first generation asexual offspring (F1) animals. By 84 days of exposure, the original treatment adults ahd lost a significant number of algal cells, while the F1 asexual offspring maintained higher numbers of symbiotic algae that also donated more carbon from photosynthesis to the F1 animals. Despite these positive trends in the F1 anemones, their asexual reproductive output was markedly lower than the adults. Further, second generation asexual offspring from the F1 animals (i.e., F2) grew very slowly and produced far less offspring when compared to the F1 and original adult animals. It appears that the symbiotic algae within the asexual offspring may gain some positive benefits from the long-term treatment, However, our long-term experiments have shown that the positive benefits of high CO2 alone (e.g., elevated photosynthesis and carbon production for the animal) are negated when high CO2 is combined with non-lethal high temperature. Despite treatment conditions triggering an increase in both algal carbon production and "food" for the host anemone, the reduced asexual reproductive fitness in all treatment anemones indicates that the possible benefits of high CO2 availability does not offset the detrimental effects of increased temperature. This project contributed directly to the training of three graduate students and one post doctoral fellow at the University of Delaware, as well as three high school research interns. Several elements of this project were incorporated into new teaching materials in three courses offered to undergraduate and graduate students within the School of Marine Science and Policy (SMSP) at the University of Delaware. In addition, "hands-on" research demonstrations, lectures and dissemination of results were presented to the general public through visits to elementary schools as well as through the public open house festival, Coast Day, and by public tours of the University of Delaware marine campus in the summer periods of the project. Last Modified: 02/05/2020 Submitted by: Mark E Warner