Many types of coral rely on small algae known as zooxanthellae to survive. These algae help the coral reef in many ways. Their relationship is symbiotic, in that they each help one another in different ways. Different types of coral use these algae for different purposes. For the most part, the algae live within the coral's cells. The coral provides the algae with a protected environment and the compounds necessary for photosynthesis. These include carbon dioxide, produced by coral respiration, and inorganic nutrients such as nitrates, and phosphates, which are metabolic waste products of the coral.
Likewise, the algae helps the different types of coral produce oxygen which helps coral to remove waste. In addition to that, they provide the coral reef with the products of photosynthesis that are necessary for survival. Each of these photosynthesis products keep the coral reef alive and in healthy condition. These compounds, including glucose, glycerol, and amino acids, are utilized by the coral as building blocks in the manufacture of proteins, fats, and carbohydrates. Another compound that is important in coral survival is the synthesis of calcium carbonate (CaCO3). The exchanging of these different products and compounds is how many coral reef creatures survive, but it also contributes to the necessary means of coral reef building.
These algae are critical in the survival and growth of the coral reef. As much as 90% of the organic material they manufacture through photosynthesis is transferred to the host coral tissue. If these algal cells are expelled by the polyps, which can occur if the colony undergoes prolonged physiological stress, the host may die shortly afterwards. When the algae are expelled from the coral reef the coral cannot survive and will not be able to handle its typical lifestyle. As much as coral reef seems to be an independent living organism, they are actually quite dependent.
