Coral bleaching is the process in which heat-stressed corals expel or lose their symbiotic algae.
Most reef-building corals live with microscopic algae called zooxanthellae, which provide food through photosynthesis and help give corals much of their color. When unusually warm water or another stress disrupts this relationship, the algae may be expelled, leaving the coral tissue visibly pale or white.
Bleaching does not mean that the coral has instantly died. A bleached coral can recover if conditions improve, but prolonged or severe stress can cause starvation, disease, and death. Recovery also depends on the coral’s species, the duration of stress, and local conditions.
Bleaching differs from coral spawning, which is a reproductive event. It also differs from ocean acidification, which changes seawater chemistry and can make skeleton-building more difficult.