What is the symbiotic algae found in coral tissues called?

The symbiotic algae found in coral tissues are commonly called zooxanthellae. They are microscopic, photosynthetic dinoflagellates that live inside the cells of many reef-building corals.

The relationship is mutually beneficial. Using sunlight, the algae produce organic compounds such as sugars and other nutrients that help power coral metabolism, growth and calcium carbonate production. In return, the coral provides the algae with shelter, carbon dioxide and compounds containing nutrients such as nitrogen and phosphorus.

“Zooxanthellae” is a familiar collective term rather than the name of one single modern species. Many of these symbionts were historically grouped under the genus Symbiodinium, but genetic research has shown that coral-associated dinoflagellates include a diverse group now placed within the family Symbiodiniaceae.

A common mix-up is calling zooxanthellae seagrass or phytoplankton. They are algae, but they live within host tissues rather than simply drifting in open water. When corals are stressed and expel them, the coral can lose its color and undergo bleaching.