These algae are known as zooxanthellae and their varied colors are visible in healthy corals.

Zooxanthellae provide food and oxygen to the coral, allowing their host to direct more energy toward constructing its calcium carbonate skeleton.

Bleached corals lose their zooxanthellae and turn white. In extreme cases of bleaching, corals die.

SOURCES

Brown 1997, Coles 2001, Coles and Brown 2003, Glynn 1996, Hoegh-Guldberg 1999, Pecheux 1997, Salm and Coles 2001 Web links:   Australian Institute of Marine Science http://www.aims.gov.au/pages/research/ coral-bleaching/coral-bleaching.html Global Coral Disease Database, World Conservation Monitoring Centre, United Nations Environment Program http://www.unep-wcmc.org/marine/ coraldis/guide.htm Dave Krupp. Powerpoint presentation on Coral Nutrition & Symbiosis, from Biology 200 Coral Reefs, Windward Community College http://krupp.wcc.hawaii.edu/BIOL200/ powerpnt/corlnutr/ NOAA Coral Reef Watch http://coralreefwatch.noaa.gov/ Pecheux 1997: http://mars.reefkeepers.net/USHomePage/ USArticles/bleaching/bleaching1.html Salm and Coles 2001 http://www.conserveonline.org/2001/10/ w/CoralBleechingMPAsWorkshop;internal &action=buildframes.action