Corals reefs are truly masterpieces of nature, often described as oases in an oceanic desert. Commonly thought to be nearly devoid of plankton and nutrients, the translucent, azure waters bathing coral reefs are, in fact, teeming with microbial life. The concentrations of nutrients are low, but they are processed and recycled rapidly through the microbial food chains. The traditional notion that coral reefs sustain high rates of gross primary productivity is valid, but the contribution of various sources and variations in productivity among reef zones are still emerging into a coherent picture. Current debate rages over the precise relationship between corals and symbiotic dinoflagellates. Calcification and photosynthesis in zooxanthellate corals cannot be considered in isolation from the effects of changes in light, nutrient concentrations and coral heterotrophy and may be limited by such factors. Zooxanthellate corals respond differently, however, to nitrogen and phosphorus enrichment.