Coral Reef NOAA
May 28, 2016  

Symbiotic Algae

light microscope image of round yellow-greenish-brown Symbiodinium kawagutii cells, a type of zooxanthellae
close-up image of a coral polyp where the brownish-yellow round zooxanthellae cells are visible within the clear coral tissue
Top: Zooxanthellae are a type of algae found inside coral polyps of reef-building coral species. Many of the zooxanthellae are from the genus Symbiodinium, such as this image of Symbiodinium kawagutii under a light microscope. Photo Credit: Scott R. Santos/Department of Biological Sciences, Auburn University Bottom: Zooxanthellae within a polyp of Porites astreoides. Photo Credit: National Coral Reef Institute

Most reef-building corals have a mutually beneficial symbiotic relationship with a microscopic unicellular algae called zooxanthellae that lives within the cells of the coral's gastrodermis. The coral provides the algae with a protected environment and the compounds necessary for photosynthesis, which are metabolic waste products of the coral. In return, the algae produce oxygen and help the coral to remove wastes. Most importantly, they supply the coral with organic products of photosynthesis which are predominantly carbohydrates. These compounds are utilized by the coral as building blocks in the manufacture of fats, as well as the synthesis of their calcium carbonate (CaCO3) skeletons. Zooxanthellae often are critical elements in the continuing health of reef-building corals. As much as 90 percent of the organic material they manufacture photosynthetically is transferred to the host coral tissue. [a]

Another byproduct of the symbiotic relationship with zooxanthellae is color. Several million zooxanthellae live and produce pigments in just one square inch of coral. These pigments are visible through the clear body of the polyp and are what gives many reef-building coral their beautiful color. [b] The zooxanthellae range in color from golden-yellow to brown and give their hosts the same shade. Color can vary or change depending upon the intensity of light to which the coral and its symbionts are exposed. If the light intensity is lower than normal, the zooxanthellae cells may increase in number and may also produce more chlorophyll to maintain photosynthesis. If the coral is exposed to higher intensity light, photosynthesis may increase, which can produce dangerously high levels of oxygen in the coral's tissue. To counteract this, the coral may expel some zooxanthellae or the zooxanthellae may reduce the amount of chlorophyll within them. [c]

Mass expulsion of zooxanthellae leaves coral appearing white because coral tissue itself is mostly transparent and their calcium carbonate skeletons are white. When this happens on a large scale, the phenomenon is termed coral bleaching. [c] Corals begin to starve once they bleach. While some corals are able to feed themselves sufficiently, most corals struggle to survive without their zooxanthellae. If conditions return to normal relatively quickly, corals can regain their zooxanthellae and survive. This stress; however, is likely to cause decreased coral growth and reproduction, and increased susceptibility to disease. Bleached corals often die if the stress persists. Coral reefs suffering severe mortality following bleaching can take many years or decades to recover. [d]