Zooxanthellae cells provide corals with their color. Click on the image to learn more.

Most reef-building corals contain photosynthetic cells called zooxanthellae that live in their tissues. The corals and zooxanthellae have a mutualistic relationship. This means there is a positive relationship between two things and both benefit from the relationship. Corals provide the zooxanthellae with a protected environment, and the coral polyp cells produce carbon dioxide and water that the zooxanthellae need for photosynthesis. The zooxanthellae use energy from the sun to turn the carbon dioxide and water into oxygen and help the coral to remove wastes. Most importantly, zooxanthellae supply the coral with the building blocks of sugars and proteins, which are the products of photosynthesis. The coral uses these products to make proteins, fats, and carbohydrates, and produce calcium carbonate. This leads to coral growth and reproduction.

Coral polyps, which are animals, and zooxanthellae, the specialized cells that live within them, have a mutualistic relationship. Learn more about polyps up close.

The relationship between the zooxanthellae and coral polyp results in a tight recycling of nutrients in nutrient-poor tropical waters. In fact, as much as 90 percent of the organic material photosynthetically produced by the zooxanthellae is transferred to the host coral tissue. This is the driving force behind the growth and productivity of coral reefs.

In addition to providing corals with essential nutrients, zooxanthellae are responsible for the unique and beautiful colors of many stony corals. Sometimes when corals become physically stressed, the polyps expel their zooxanthellae and the colony takes on a stark white appearance. This is coral bleaching. If the corals go for too long without the nutrients that zooxanthellae provide, coral bleaching can result in the coral's eventual starvation and death.

Genetic research has found that zooxanthellae are very diverse and have different characteristics. Some coral species have only one type of zooxanthellae throughout their life, however, other corals switch between the types of zooxanthellae they host. This may be beneficial because some zooxanthellae are more resistant to high temperatures and coral bleaching. Sometimes, when a coral expels its zooxanthellae during a bleaching event, it can take up a different type of zooxanthellae, potentially making it more resistant to bleaching in the future.

Tiny cells called zooxanthellae live within most types of coral polyps. Learn more about zooxanthellae purpose.

Because of their intimate relationship with zooxanthellae, and these cells' special ability to photosynthesize, reef-building corals respond to the environment like plants. Reef-building corals require clear water so that sunlight can reach their zooxanthellae for photosynthesis. For this reason they are generally found only in waters with small amounts of suspended material, or water of low turbidity and low productivity. This leads to an interesting paradox—coral reefs require clear, nutrient-poor water, but they are among the most productive and diverse marine environments on earth.