The symbiotic algae that live inside the flesh of many reef-building corals are called zooxanthellae. Zooxanthellae are a type of microscopic, single-celled algae that thrive in the protective environment provided by the coral. These algae process the corals’ metabolic waste in a photosynthetic process; thus, the waste produced by the coral is in turn transformed into energy for the algae. The algae’s resultant photosynthesis produces oxygen as well as the carbohydrates necessary for the coral to create necessary fats and skeletal systems.
Approximately 90% of the photosynthesis byproducts created by the zooxanthellae algae are transmitted to its host coral. Thus, zooxanthellae and many reef-building coral maintain a mutually beneficial, or symbiotic, relationship upon which the majority of the corals’ growth occurs (National Ocean Service, 2011).
The symbiotic relationship between coral and zooxanthellae can be viewed as more desirable than the alternative of corals’ consumption of phytoplankton due to the higher rate of photosynthesis provided by the former (Scott & Jitts, 1977). The presence of zooxanthellae algae might affect the coral animals capable of living in the environment necessary for the algae’s existence. Zooxanthellae require clear water that maintains a fairly stable, warm temperature with fairly minimal variance (Baker, 2003).

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Changes in water depth, clarity, or temperature can drastically affect coral. Many corals become diseased or being to die as a result of increased light due to the zooxanthellae’s increased photosynthetic production creating excess oxygen in the coral. When corals’ become over oxygenated, they expel zooxanthellae to attempt to stabilize. This process on a broad scale often results in coral bleaching, a term that refers to the lightening/whitening of the coral on account of its tissue withering away due to insufficient fuel. Though corals can recover from this, conditions must improve fairly quickly and recovery may take decades (Baker, 2003; National Ocean Service, 2011).

  • Baker, A. (2003). Flexibility and Specificity in Coral-Algal Symbiosis: Diversity, Ecology, and Biogeography of Symbiodinium. Annual Review of Ecology, Evolution, and Systematics, 34, 661-689.
  • National Ocean Service. (2011, May 13). Symbiotic Algae. US Department of Commerce: National Oceanic and Atmospheric Association. Retrieved from
  • Scott, B., & Jitts, H. (1977). Photosynthesis of phytoplankton and zooxanthellae on a coral reef. Marine Biology, 41, 307-315.