Algae Are Friends not Food

Why did the limestone feel unappreciated? Because they thought they were being taken for granite! Fortunately, they are appreciated by coral reefs. Coral reefs are large underwater ecosystems that are home to thousands of different underwater species. In fact, they are commonly known as the rainforests of the sea due to their rich diversity.

In season 1 episode 6 of Blue Planet: Natural History of the Ocean- Coral Seas, the development of coral polyp’s (young coral) flesh is supported by limestone. Most of the growth of the skeleton occurs below the gut and moves upward. Below the gut of the polyp, the living tissue deposits a lattice of limestone. This limestone backbone creates the foundation for the coral reef that may extend for miles. This is known as biomineralization. Organic macromolecules present in between the coral skeleton and tissue layer are solidified and incorporated into the skeleton [1]. The limestone skeleton is built from precursors of calcium carbonate, which are formed, attached, and crystallized to the surface of the coral [2].

Although corals are animals, sunlight is vital to their survival. This is because they depend on single-celled algae that live inside of them for sustenance to survive and perform work. As a matter of fact, through photosynthesis, algae produce almost 90% of the food that corals consume [3]. The photosynthetic and endosymbiont dinoflagellates produce reduced carbon that the coral may use for their metabolism [4]. Specifically, this carbon fixation process allows inorganic carbon in the form of carbon dioxide to be converted into an organic or reduced form via a reduction-oxidation reaction. These interactions have been proven to be essential to the development of coral reef ecosystems [5]. The algae contribute to the maintenance of coral reefs by inducing the settlement of coral larvae and preventing bioerosion [6]. Light is known to enhance the calcification of some coral due to these symbiotic algae.

by Anu Thiru, Kevin Le, Princess Ekpo and Paridhi Agarwal

Blue Planet: Natural History of the Oceans, Season 1, Episode 6 (coral seas), starting at 5:54


  1. Raz-Bahat M, Erez J & B Rinkevich. 2006. In vivo light-microscopic documentation for primary calcification processes in the hermatypic coral Stylophora pistillata. Cell and Tissue Research 325: 361-368.
  2. Mass T, Giuffre AJ, Sun C, Stifler CA, Frazier MJ, Neder M, Tamura N, Stan CV, Marcus MA, & P Gilbert. Amorphous calcium carbonate particles form coral skeletons. Proceedings of the National Academy of Science of the United States of America 114: E7670-E7678.
  3. Berkelmans R & MJH van Oppen. 2006. The role of zooxanthellae in the thermal tolerance of corals: a ‘nugget of hope’ for coral reefs in an era of climate change. Proceedings of The Royal Society B 273: 2305-2312.
  4. Falkowski PG, Dubinsky Z, Muscatine L, & JW Porter. Light and the bioenergetics of a symbiotic coral. BioScience 34: 705-709
  5. LaJeunesse TC, Parkinson JE, Gabrielson PW, Jeong HJ, Reimer JD, Voolstra CR, & SR Santos. 2018. Systematic revision of symbiodiniaceae highlights the antiquity and diversity of coral endosymbionts. Current Biology 28: R873-R875.
  6.  Weiss A. 2017. Crustose coralline algae increased framework and diversity on ancient coral reefs. Plos Once 12: e0181637.

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