Scientists Say: Zooxanthellae

These microorganisms, a group of algae, live in the tissue of coral

Algae called zooxanthellae live in the tissue of some sea animals, including many corals. The algae give the coral its color and also some food in exchange for shelter.

NOAA Okeanos Explorer Program, INDEX-SATAL 2010/NOAA Photo Library/Flickr (CC BY 2.0)

Zooxanthellae (noun, ZOH-uh-zan-THEL-ay)

This word describes the microorganisms that dwell in the tissue of some ocean animals, including many corals. Zooxanthellae are single-celled algae. Theyhave a symbiotic relationship with coral. That means the algae and coral each help the other out. The algae photosynthesize, turning light and carbon dioxide into food that they share with the coral. The algae help corals get enough energy to build reefs. The algae also provide oxygen and remove some of the coral’s wastes. In return, the coral shelters the algae and shares some nutrients with them.

But global warming and rising sea temperatures could spell trouble for these partnerships. When the algae are stressed by too-hot conditions, corals sometimes kick the algae out. This is called bleaching. The corals now look bone white because they lack the zooxanthellae that have given them their vivid hues. If a bleached coral doesn’t find new algae to live move in, the corals will eventually die.

In a sentence

Heat waves, like one in 2016 that bleached one-third of Australia’s Great Barrier Reef, can cause corals to expel their zooxanthellae.

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Kelsey Roberts/USGS/Flickr
When water temperatures are too hot, corals may kick out their symbiotic algae. This causes the coral to bleach, losing color like this bent sea rod coral. If corals don’t find new algae to partner with, they can die.

Carolyn Wilke is a former staff writer at Science News for Students. She has a Ph.D. in environmental engineering. Carolyn enjoys writing about chemistry, microbes and the environment. She also loves playing with her cat.

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