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Coral reefs are built by colonies of small animals called coral polyps. Many reef-building corals live in partnership with microscopic algae inside their tissues. The algae use sunlight to produce energy and provide much of the food the coral needs, while also giving coral its colour. When seawater becomes unusually warm or corals experience other severe stress, this partnership can break down. The coral expels the algae and becomes pale or white, a process known as bleaching. Bleached coral is not necessarily dead, but it has lost a major energy source. If stressful conditions continue, the coral may starve, become diseased or die. Large-scale bleaching events have become more frequent as ocean temperatures rise. Reefs can recover when conditions improve, especially if other pressures such as pollution and overfishing are limited. Protecting coral reefs therefore requires both local conservation and action on the global causes of climate change.
