<P> Around coral reefs, lagoons fill in with material eroded from the reef and the island . They become havens for marine life, providing protection from waves and storms . </P> <P> Most importantly, reefs recycle nutrients, which happens much less in the open ocean . In coral reefs and lagoons, producers include phytoplankton, as well as seaweed and coralline algae, especially small types called turf algae, which pass nutrients to corals . The phytoplankton are eaten by fish and crustaceans, who also pass nutrients along the food web . Recycling ensures fewer nutrients are needed overall to support the community . </P> <P> Coral reefs support many symbiotic relationships . In particular, zooxanthellae provide energy to coral in the form of glucose, glycerol, and amino acids . Zooxanthellae can provide up to 90% of a coral's energy requirements . In return, as an example of mutualism, the corals shelter the zooxanthellae, averaging one million for every cubic centimeter of coral, and provide a constant supply of the carbon dioxide they need for photosynthesis . </P> <P> Corals also absorb nutrients, including inorganic nitrogen and phosphorus, directly from water . Many corals extend their tentacles at night to catch zooplankton that brush them when the water is agitated . Zooplankton provide the polyp with nitrogen, and the polyp shares some of the nitrogen with the zooxanthellae, which also require this element . The varying pigments in different species of zooxanthellae give them an overall brown or golden - brown appearance, and give brown corals their colors . Other pigments such as reds, blues, greens, etc. come from colored proteins made by the coral animals . Coral which loses a large fraction of its zooxanthellae becomes white (or sometimes pastel shades in corals that are richly pigmented with their own colorful proteins) and is said to be bleached, a condition which, unless corrected, can kill the coral . </P>

Where are coral reefs located in the world