zooxanthellae

Zooxanthellae are photosynthetic dinoflagellates that live within the tissues of many marine invertebrates, most notably reef-building corals. In corals they reside in gastrodermal cells and form a mutualistic association that supports a large portion of the host’s energy budget. Although the term has been used broadly, most corals harbor dinoflagellates from the family Symbiodiniaceae, including lineages assigned to the genera Symbiodinium, Cladocopium, Durusdinium, and Breviolum, among others.

In the symbiosis, zooxanthellae perform photosynthesis, fixing carbon dioxide into organic compounds that feed the coral host. In exchange, the coral provides inorganic nutrients such as nitrogen and phosphorus, a protected photic environment, and access to waste products that fuel the algae. The relationship can contribute substantially to the energy available to the coral, and under favorable conditions can support calcification and reef growth, making zooxanthellae central to tropical reef productivity.

Zooxanthellae are adapted to warm, clear, shallow marine habitats with ample light. They are sensitive to environmental stress, particularly elevated temperatures, high light, and pollution, which can disrupt photosynthesis and lead to coral bleaching—expulsion of the algae and a loss of color and energy source. Transmission of symbionts varies: many corals acquire zooxanthellae horizontally from the environment after larval release, while some species pass the symbionts vertically from parent to offspring.

Research on zooxanthellae informs understanding of coral resilience, reef biogeochemistry, and restoration strategies, and continues to illuminate the diversity and functioning of coral-algal mutualisms.