Zooxanthellae are single-celled plants that live in the tissues of animals. These organisms are part of a group of dinoflagellates that are most often found as plankton. However, this particular group of species is best known for a mutualistic relationship they have with reef building corals.
Corals provide a protected environment and compounds needed for photosynthesis to zooxanthellae. In turn, zooxanthellae provide food as products of photosynthesis to coral. This gives corals a boost of nutrients, so they can secrete the calcium carbonate skeleton that serves as the foundation for coral reef. This important relationship represents a highly efficient exchange of nutrients in a nutrient-poor environment.
Corals are not the only hosts of zooxanthellae. Some other reef hosts include:
- Giant clams
- Gorgonian corals or sea fans
- Soft corals
- Sea whips
Zooxanthellae Diversity (1:50)
Andrew Baker discusses his research examining the variations among zooxanthellae.
Any of these animals can bleach and turn white. As with corals, bleaching occurs when environmental stress adversely affects the mutually beneficial relationship between the host organism and its zooxanthellae.
Until recently, scientists thought all corals shared the same species of zooxanthellae (Symbiodinium microadriaticum) but we now know there are at least nine groups (called clades), and there may be many species within these. Scientists have also discovered that some of these zooxanthellae are more able to resist bleaching. One of these, called ‘clade D,’ is particularly resistant to bleaching and enables the coral host to avoid the bleaching stress response.
Dr. Andrew Baker at the University of Miami is leading work to test out the possibility of transferring heat-resistant zooxanthellae to corals that host other species, in order to protect corals from bleaching. The progress of this work will be updated in this toolkit as more information becomes available.