What process allows corals with algae to produce much of their food using sunlight?

Photosynthesis allows reef-building corals and their symbiotic algae to produce much of their food using sunlight.

Many corals host microscopic dinoflagellate algae called zooxanthellae within their tissues. The algae use light, carbon dioxide, and water to make organic compounds, some of which they transfer to the coral. In return, the algae receive shelter and access to nutrients and carbon dioxide produced by the coral’s metabolism.

This partnership helps corals build calcium-carbonate skeletons in clear, shallow water. It also explains why reef-building corals generally need strong light and why reefs are concentrated in warm, sunlit tropical seas. Corals can capture plankton too, so photosynthesis is not their only food source.

During prolonged heat stress, corals may expel their algae, causing coral bleaching. Bleaching does not immediately mean the coral is dead, but it removes a major energy source and can lead to starvation if stressful conditions persist. The algae are often called zooxanthellae in general discussion, although modern taxonomy places these symbionts in several lineages.