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. If the animal lets the algae have as much as it wants, the algae may start growing so quickly that it will need to use all of its carbon for itself. One idea that I am testing is that the animal restricts the amount of phosphorus that the plant gets to control the plants growth. The interesting thing here is that if you take the algae away, the animal can only take up about a tenth of the phosphorus that it can with the algae present.