Bianca Allegra Parodi, Ioana Stanca, Laia Burgués Palau, Zhen Qin, Elise Laetz

Anthropogenic climate change is primarily driven by carbon dioxide release, which causes a domino effect of warming, acidification, and hypoxia in aquatic habitats. Using a fully-crossed experimental design, we investigated how exposure to this “deadly trio” of environmental stressors affects the sea anemone, Exaiptasia diaphana and its endosymbiotic dinoflagellates. To mimic conditions found on tropical reefs, we cycled hypoxic treatments between high oxygen saturation during the day (100 %) and hypoxia (40 %) at night. We increased ocean warming and acidification treatments to 30 °C and 7.7 pH in accordance with the “worst case” predictions in the IPCC 2023 report. After exposure for two weeks, we measured each anemone’s metabolic rate, thermal tolerance, and heat tolerance plasticity. We also assessed algal density and photosynthetic efficiency, both of which remained consistent across all treatments, indicating negligible stress levels and a healthy symbiosis. Interestingly, metabolic depression was observed in anemones exposed to the deadly trio, which is likely an energy conservation strategy due to its co-occurrence with increased thermal tolerance. Furthermore, we found an interaction between ocean acidification and nightly hypoxia as combined exposure improved heat tolerance plasticity, potentially because increased carbon dioxide enhanced photosynthetic activity. We therefore conclude that E. diaphana will survive short-term exposure to the “worst case” levels of deadly trio stressors. To verify this interpretation, we determined their acute stress tolerance, finding that they can withstand termporary exposure to more than 34 °C seawater, a minimum pH of 6.01 and anoxia for more than 4 days. These conditions greatly exceed worst-case predictions, confirming that this species should survive acute exposure to the deadly trio, potentially due to their symbiosis. These findings highlight the importance of considering all three stressors in climate change ecophysiological studies.