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Why do tropical coral reefs need warm seawater?

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coral reef The Great Barrier Reef, off the coast of Queensland, Australia, is the world's largest coral reef.

Tropical coral reefs thrive in warm seawater because of their essential symbiotic relationship with zooxanthellae, a type of photosynthetic algae. These algae live within the coral polyps and provide them with food while significantly accelerating the rate at which the coral secretes its calcium carbonate skeleton.12

Coral reefs are best developed in waters with mean annual surface temperatures between approximately 23–25 °C (73–77 °F). While corals require these warm conditions to maintain their metabolic and growth processes, they are highly sensitive to temperature fluctuations. Prolonged exposure to water that is too warm can stress the corals, causing them to expel their symbiotic zooxanthellae—a process known as coral bleaching. If the zooxanthellae do not return, the coral colony may die, leading to the collapse of the reef ecosystem. Conversely, significant reefs generally do not occur where temperatures fall below 18 °C (about 64 °F).314