Zooxanthellae are the microscopic algae that provide most reef-building corals with their main energy source.
The term traditionally refers to symbiotic dinoflagellates living inside coral tissues. Through photosynthesis, these algae use sunlight to make organic compounds, some of which the coral host receives. In return, the algae gain shelter, carbon dioxide, nutrients, and access to a sunlit habitat.
This partnership helps corals build calcium-carbonate skeletons and supports the high productivity of tropical coral reefs. The relationship is strongest in clear, shallow water where enough light reaches the reef. Corals can also capture plankton, but photosynthesis supplies much of the energy for many reef-building species.
Heat stress can disrupt the partnership. During coral bleaching, corals expel or lose much of their symbiotic algae, causing the tissue to appear white. Bleaching does not always mean immediate death, but prolonged stress can starve corals and cause extensive mortality.