Coral bleaching is usually a slow decline: corals expel their symbiotic algae, lose their main food source and fade, yet can survive for months and recover if the water cools. During recent marine heatwaves, however, scientists have watched coral tissue disintegrate and slough off the skeleton within hours to days, a rapid and irreversible form of mortality documented on the Great Barrier Reef and across the Caribbean and eastern tropical Pacific.
A team at King Abdullah University of Science and Technology set out to explain the difference. Using microsensors to monitor the thin boundary layer of water directly against the tissue of acutely heat-stressed corals, the researchers found that oxygen concentrations collapsed just before tissue breakdown began. Heat appears to impair algal photosynthesis, cutting oxygen supply exactly when the coral's own demand rises, creating an internal oxygen crisis.
The effect appeared in corals from both the Red Sea and Australia, suggesting a general mechanism rather than a regional quirk. Intriguingly, stronger water flow compressed the stagnant boundary layer and delayed mortality in the experiments, published in Global Change Biology. The findings imply that temperature alone is a blunt predictor of reef fate, and that oxygen conditions and local hydrodynamics could help managers identify which reefs are most vulnerable to severe heat events.