Similar to forests on land, the most important source of energy for tropical shallow water coral reefs is light. Photosynthetic algae, called zooxanthellae, live within the tissues of reef-building corals and provide them with oxygen and the products of photosynthesis, including glucose and amino acids. The corals, in turn, use these products as the energy source for building calcium carbonate skeletons and growing more living tissue. Other ecologically important reef organisms, such as macroalgae and turf algae, depend on light for growth and reproduction as well, making light the driving force behind the growth and overall productivity of coral reef ecosystems.