Heat stress disrupts acid-base homeostasis independent of symbiosis in the model cnidarian Exaiptasia diaphana

Heat stress threatens the survival of symbiotic cnidarians by causing their photosymbiosis to break down in a process known as bleaching. The direct effects of temperature on cnidarian host physiology remain difficult to describe because heat stress depresses symbiont performance, leading to host stress and starvation. The symbiotic sea anemone Exaiptasia diaphana provides an opportune system in which to disentangle direct vs. indirect effects of heat stress on the host, since it can survive indefinitely without symbionts. Here, we tested the hypothesis that heat stress directly influences cnidarian physiology by comparing symbiotic and aposymbiotic individuals of a clonal strain of E. diaphana . We exposed anemones to a range of temperatures (ambient, +2°C, +4°C, +6°C) for 15-18 days, then measured their symbiont population densities, autotrophic carbon assimilation and translocation, photosynthesis, respiration, and host intracellular pH (pHi). Anemones with initially high symbiont densities experienced dose-dependent symbiont loss with increasing temperature, resulting in a corresponding decline in host photosynthate accumulation. In contrast, anemones with low initial symbiont densities did not lose symbionts or assimilate less photosynthate as temperature increased, similar to the response of aposymbiotic anemones. Interestingly, pHi decreased in anemones at higher temperatures regardless of symbiont presence, cell density, or photosynthate translocation, indicating that heat stress disrupts cnidarian acid-base homeostasis independent of symbiosis dysfunction, and that acid-base regulation may be a critical point of vulnerability for hosts of this vital mutualism. Summary Statement Warming oceans threaten marine invertebrates. We found that heat disrupts acid-base homeostasis in a model symbiotic sea anemone regardless of symbiont presence or function, highlighting bleaching-independent effects of climate change. ### Competing Interest Statement The authors have declared no competing interest.