Discovery of a Threshold-Dependent Response in Exaiptasia diaphana to Repeated Hyposalinity Stress

In this study, we present novel evidence to explain how recent increases in tropical cyclone intensity caused by climate change affect coral reefs, which are crucial marine habitats. We determined the impact of hyposalinity fluctuations caused by heavy freshwater precipitation and runoff from tropical cyclones on cnidarian health, expression of a heat shock protein (Hsp70), and tolerance to rising ocean temperatures, a relationship that has not been thoroughly characterized. Using the sea anemone Exaiptasia diaphana as a model organism, we subjected anemones to different levels of hyposalinity fluctuation and subsequently conducted a heat stress assay. The algal symbiont (Symbiodiniaceae) density, growth, and thermotolerance peaked at moderate (25 parts per thousand, or ppt) hyposalinity fluctuations, likely due to the increased expression of Hsp70. However, at severe (20 ppt) fluctuations past the hyposalinity threshold, the negative effects of hypoosmotic stress overwhelmed the Hsp70 response, resulting in increased bleaching and decreased health. Additionally, the frequent-fluctuation group showed decreased Symbiodiniaceae density and increased bleaching, suggesting that higher frequencies of freshwater influx could further worsen coral reef health. Our findings reveal a response akin to hormesis, where moderate hypoosmotic stress enhances thermotolerance through stress response mechanisms, such as Symbiodiniaceae proliferation and Hsp70 overexpression, whereas severe or frequent fluctuations lead to fatigue of these mechanisms. Overall, these results demonstrate the adverse effects of hyposaline conditions resulting from intensifying hurricane events, emphasizing the urgent need to mitigate climate change and restore ecosystems.