One of the few things that all life has in common is that it is made of carbon. Our hair, skin, muscles, and everything else that is or was once living has carbon in it. Nitrogen is used to build amino acids which are linked together to make proteins. DNA, the genetic material that determines what we look like, contains phosphorus. Corals are both animals and plants.
Corals are animals that allow tiny algae cells to grow in-between their two cell layers.
The animal is healthier with the algae in it because the plant can give it carbon. Remember that when humans breath, we take in oxygen and breath out carbon dioxide. Plants do the opposite - they use carbon dioxide and release oxygen. If you had a plant living inside of you, you would have first dibbs on all of that carbon. I also want to know whether the animal or the algae decides how much phosphorus the coral will take from the water.