Coral bleaching

Reef-building corals live in partnership with tiny photosynthetic algae called zooxanthellae, which live inside the cells of the coral's tissue. The algae get shelter and raw materials from the coral, and in return they supply most of the coral's food through photosynthesis, as well as much of its color. Coral bleaching happens when stress breaks down this partnership and the coral expels the algae. Its tissue then becomes nearly transparent, and the white limestone skeleton shows through.

The main cause is a marine heatwave: sea temperatures staying around one degree Celsius above the usual summer maximum for several weeks can trigger bleaching, and the hotter and longer the event, the worse the damage. Strong sunlight, very low tides, unusually cold water, fresh water from heavy rain and polluted runoff can also cause it. A bleached coral is not dead, but it is starving and more prone to disease. If conditions return to normal soon enough, the algae can recolonize and the coral can recover, though it may grow and reproduce less for a time; if the stress lasts, the coral dies.

Mass bleaching events that affect reefs across whole regions have become more frequent as the oceans warm with climate change.