What a coral loses when it bleaches

A reef-building coral animal hosts microscopic algae within its tissues. Through photosynthesis, those partners supply much of the energy used by the coral; in return they receive shelter and access to nutrients. Prolonged heat stress can disrupt the partnership, causing algae or their pigments to be lost. The pale skeleton then shows through translucent tissue: bleaching.

Bleached coral is stressed, not necessarily dead. If conditions ease soon enough, some colonies regain partners and recover. Duration, temperature, species, prior exposure, disease and local water quality all shape the result. That variability should not soften the risk: repeated marine heatwaves leave less time to recover, while local protection cannot by itself remove the global heat pressure.