In my previous articles I explained that corals have two ways of obtaining nutrition. They can use their animal parts, or polyps, to expose their fleshy tentacles to the water current to catch plankton, but the second method is far more important. The symbiotic algae that live in the polyps’ tissues, zooxanthellae, convert sunlight and carbon dioxide into sugars and oxygen, using photosynthesis. As you know now, these zooxanthellae produce 90 to 98 percent of the corals’ organic nutrients; they are the main source of the polyps’ organic nutrient requirements.
Teamwork
These two methods of feeding complement each other in an astonishing way. Just imagine: Any type of grazing fish could eat a free-living single-celled alga, but by placing themselves in the tissue of the polyp they’ve at least minimized the chance of becoming a snack. The polyps will fire nasty, stinging cells at any fish that comes too close, and the algae receive waste products from the polyps, such as carbon dioxide and water, which they use to carry out photosynthesis.
Polyps also benefit from this mutualistic relationship. During photosynthesis, the algae create sugars, fats and oxygen, which the polyp uses for respiration. The tight recycling of products between the polyp cells and the zooxanthellae is the driving force behind the growth and productivity of coral reefs and is the single most important cause of their existence.
Creating the skeleton
Corals secrete their skeleton through the outer layer of the skin, located on the underside of all polyps. This layer contains specialized cells, which continuously secrete calcium and bicarbonate ions to the external environment, which will in turn form the calcium carbonate skeleton. The skeleton by itself is usually white.
Thanks to the photosynthetic activities carried out by the zooxanthellae, the skeleton of a reef-building coral is able to develop two to three times faster in light than in darkness. Many species are able to grow a little under half an inch a month this way. According to some estimates, tropical corals are able to deposit about 22 pounds of calcium carbonate per 11 square feet per year, and continue to build massive reefs if conditions remain stable.
Unfortunately, due to climate change the oceans are becoming warmer and more acidic, inhibiting coral growth. So tread lightly on our planet, and think of the remarkable mutualistic relationship before your eyes next time you dive on a coral reef.
