This document discusses the productivity of coral reef ecosystems. It notes that coral reefs are the most productive marine ecosystem due to the symbiotic relationship between corals and algae called zooxanthellae. The zooxanthellae live in the coral tissues and produce nutrients through photosynthesis, upon which the coral depends. Factors like light, temperature, and nutrient recycling influence the productivity of coral reefs. The high productivity supports a diversity of species and plays an important role in fisheries and coastal protection.