Within the tissue of most reef-building corals live small organisms that are capable of photosynthesizing or changing light energy into food. Although corals are carnivorous and feed on zooplankton, they receive much of their energy and oxygen as byproducts of zooxanthellae photosynthesis. The zooxanthellae also promote the rate of calcium carbonate production by the coral colony, thus, promoting growth. The relationship between coral colonies and zooxanthellae is called symbiosis. This is a mutual relationship between species in which one or both benefit.
Symbiotic relationships are common among organisms living on coral reefs.
Coral reefs can be found in waters surrounding land masses that fit within a narrow range of physical parameters. Tropical waters are generally warm, clear, low in nutrients, and have a stable temperature and salinity. Ocean currents can alter the physical characteristics of an area reducing or expanding the area where reefs may develop. The Florida Keys is one example in which the warm waters of the Gulf Stream expand the range of corals north of the tropics. Like land plants, seagrasses produce oxygen.