Ocean temperature, but not acidification, causes sea anemone bleaching under a near-future climate scenario

Climate change is causing ocean temperature and partial pressure of carbon dioxide (pCO2) to increase. For sea anemones that have Symbiodiniaceae, high temperatures induce bleaching, whereas rises in pCO2 can enhance photosynthesis and increase host growth and abundance. It is, however, not clear how the interaction of these two stressors impacts sea anemones that provide habitat for anemonefishes. Here, we investigated the bleaching response of the sea anemone Entacmaea quadricolor, under four conditions: (i) current temperature and current pCO2 (control); (ii) future pCO2; (iii) future temperature; and (iv) future temperature and future pCO2. After 16 days of exposure, future temperature, but not pCO2 nor their interaction, significantly reduced the Symbiodiniaceae density and total chlorophyll Symbiodiniaceae cell−1. Colour score was lower in the sea anemones exposed to future temperature than current temperature from day 4 onwards. In contrast, total chlorophyll symbiont cell−1 increased in the future temperature treatments, and light-adapted effective quantum yield remained similar in all treatments. Although pCO2 had no impact within the time frame of our experiment, the predicted future temperature induced bleaching in E. quadricolor. As bleaching events increase in frequency and severity, this will likely impact the abundance of host sea anemones and their symbiotic anemonefishes.