What process causes corals to lose their symbiotic algae during severe heat stress?

Coral bleaching is the process that causes corals to lose their symbiotic algae during severe heat stress.

Most reef-building corals live with microscopic dinoflagellate algae, commonly called zooxanthellae, inside their tissues. The algae provide much of the coral’s energy through photosynthesis and contribute pigments that give healthy colonies their characteristic colors.

When seawater becomes unusually warm, the partnership can break down. Corals expel the algae or lose their photosynthetic pigments, leaving the pale calcium-carbonate skeleton visible through their transparent tissue. That is why the event is called bleaching.

Bleached corals are not automatically dead. If stressful conditions end, algae can return and the coral may recover. Prolonged bleaching, however, can cause starvation and death. Pollution, intense sunlight, and unusually low temperatures can also contribute, but sustained marine heat is a major modern trigger.