Shallow water corals are small polyps that grow in crystal clear waters in the tropic at depths of less than 50 meters. They float around as microscopic plankton, find a hard surface and attach, then start to produce calcium carbonate exoskeletons that form gigantic, beautiful sculpted reef which are visible from space. How can they ever survive, much less prosper in such nutrient poor water? Symbiosis is the secret!
Symbiosis is two or more organisms living together in a mutually beneficial relationship. The two things work together so both can survive and thrive were either one alone could not, or could not as easily. Few places is this more evident or essential than on a coral reef. The following is a description and detailed account of the basis for Symbiosis on the coral reefs taken as a direct quote from a website currently being developed for educational purposes at this link by Logan Walters .
"The most celebrated form of symbiosis in the coral reefs can be seen in that of the coral polyp. The success of corals as reef builders is due largely to the mutualistic association with zooxanthellae. Zooxanthellae are unicellular yellow-brown (dinoflagelate) algae, which live symbiotically in the gastric-dermis of reef-building corals." (Murdoch, 1996)
"Through photosynthesis, zooxanthellae convert carbon dioxide and water into oxygen and carbohydrates. The coral polyp uses carbohydrates as a nutrient. In turn, the polyp provides food to the algae with its waste products. The algae store the waste as ammonia and break it down into nitrogen and phosphorus, which the algae use for energy." (Goreau et al., 1979)
Through this exchange, coral saves energy that would otherwise be used to eliminate the carbon dioxide. Zooxanthellae also promote polyp calcification by removing carbon dioxide during photosynthesis. Under optimum conditions, this enhanced calcification builds the reef faster than it can be eroded by physical or biological factors. Because of the need for light, corals containing zooxanthellae only live in ocean waters less than 100 meters deep. They also only live in waters above 20 degrees Celsius and are intolerant of low salinity and high turbidity. (Goreau et al., 1979)"
A simplified explanation is that the algae provide energy and nutrients that the polyps cannot obtain from the tropical waters while the polyp provides form and sturcture for the algae to live on as well as removes waste products. By working in unity, both thrive where alone neither could survive! Ironically enough, by removing the carbon dioxide, they reduce this gas in the atmosphere and helpl mankind as well, now it we could just help them back!