James Weiss of Journey to the Microcosmos had the opportunity to examine samples of coral underneath a powerful microscope. When he cut into the coral, he noticed that it released dinoflagellates onto the surface. These particular dinoflagellates, known as zooxanthellae, maintain a symbiotic relationship with the coral, providing it with the nutrients to form the hard shells and brilliant colors often associated with coral.
The zooxanthellae live their algae lives within the coral, absorbing light and photosynthesizing it into nutrients. But those nutrients aren’t really for them. About 90% of what the zooxanthellae makes gets sent to the coral instead. …In exchange, the zooxanthellae get the protection of their coral host. .…Also, the overall effect is beautiful, with zooxanthellae providing much of the color we associate with corals.