Zooxanthellae

Zooxanthellae are symbiotic dinoflagellates of the genus Symbiodinium (and related genera in the family Symbiodiniaceae) that live inside the tissues of reef-building corals, giant clams, and other hosts. Through photosynthesis they supply up to 90% of the coral’s energy as fixed carbon, which subsidizes the high calcification rates that build aragonite skeletons and reefs. The symbiosis depends on warm, clear, sunlit water. Thermal stress of about 1 to 2 degrees Celsius above the local summer maximum disrupts it: the coral expels the zooxanthellae, loses its color and its main energy source, and bleaches. Prolonged loss leads to starvation and coral mortality.