Contrasting Effects of Elevated Salinity on Thermotolerance of Red Sea Stylophora pistillata and Indonesian Montipora digitata

Coral reefs are increasingly threatened by ocean warming. Interestingly, corals from hypersaline environments, such as the Gulf of Aqaba, show high thermotolerance. To test whether salinity modulates coral thermal tolerance, we exposed Stylophora pistillata from the hypersaline Red Sea and Montipora digitata from Indonesia to a simulated heatwave (1°C rise every four days) lasting 39 days under ambient (35 PSU) and elevated (42 PSU) salinity. Both species initially appeared healthy at 42 PSU under control temperatures, although half of the M. digitata microcolonies developed tissue necrosis and died in the last week of the experiment. S. pistillata exhibited 100% survival under all treatments. At elevated salinity, S. pistillata showed a reduced decline in photosystem II yield between 32.1–33.8°C (9 versus 3.4% decline at 35 and 42 PSU, respectively). Similarly, loss of chlorophyll a fluorescence in S. pistillata was alleviated by hypersalinity at 34.1°C (94.8 versus 67.6% loss at 35 and 42 PSU, respectively). However, symbiont loss in S. pistillata was not modulated by elevated salinity, with a 68% decline at 34.1°C. In contrast, M. digitata exhibited accelerated mortality under hypersalinity during the heatwave, with no survival at 32.2°C and 33.3°C at 42 and 35 PSU, respectively. Finally, photosystem II in M. digitata was inhibited at 42 PSU only (6.8–18.9% decline at ≥ 31.3°C). This suggests that contrary to S. pistillata , hypersaline conditions exacerbated thermal stress in M. digitata . Our results suggest a limited role for hypersalinity in mediating coral thermotolerance. Rather, the evolutionary history of coral populations may be a more important determining factor.