Why do some coral reefs bleach when the water gets too warm?

Reef-building corals live in a partnership with microscopic algae called zooxanthellae, which supply much of their energy through photosynthesis. When sea temperatures rise above the corals’ normal range for even a few weeks, the relationship becomes unstable and the coral expels the algae. Without them, the coral turns white, a process known as bleaching, first documented in major modern events in the 1980s and now linked strongly to marine heat waves. If cooler conditions return quickly, corals can sometimes recover, but extended heat stress often leads to death and reef decline. The Great Barrier Reef, for example, has experienced several major bleaching events since 1998.