Inhibiting inositol transport disrupts metabolite profiles and mimics heat stress in a model cnidarian-Symbiodiniaceae symbiosis.

The nutrient exchange between corals and their symbiotic microalgae (Symbiodiniaceae) is vital for coral survival. Disruptions in this mutualistic relationship, often due to stress-induced dysbiosis, contribute significantly to coral mortality and reef decline globally. Dysbiosis is associated with substantial shifts in various metabolites, notably a rise in inositol, a sugar alcohol, though its role in coral-algae interactions remains unclear. Using a cnidarian model, we identify Symbiodiniaceae as the main source of inositol, with myo- and scyllo-inositol being the dominant forms under normal conditions. During heat stress, scyllo-inositol levels increase by 1.8 times in symbiotic hosts, and up to 26 times in cultured Symbiodiniaceae (Breviolum minutum). Meanwhile, myo-inositol decreases in host tissues but doubles within Symbiodiniaceae, indicating altered nutrient-sharing or stress signalling. In contrast, no changes are observed in aposymbiotic cnidarians (without Symbiodiniaceae). Additionally, inhibiting inositol production and transport in symbiotic tissues disrupts metabolite profiles, mimicking effects seen under heat stress, suggesting that inositol transport is crucial for maintaining metabolic balance and nutrient exchange. These findings reveal that disruptions in inositol dynamics play a critical role in stress responses, offering insights into dysbiosis mechanisms driving coral reef crises.