Scientists Say: Zooxanthellae | Science News for Students

Scientists Say: Zooxanthellae

These microorganisms, a group of algae, live in the tissue of coral
Sep 16, 2019 — 6:30 am EST
a yellow octocoral

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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an underwater photo of a bleached coral
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.    
Kelsey Roberts/USGS/Flickr

Power Words

(more about Power Words)

algae     Single-celled organisms, once considered plants (they aren’t). As aquatic organisms, they grow in water. Like green plants, they depend on sunlight to make their food.

carbon     The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

carbon dioxide     (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

cell     The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

coral     Marine animals that often produce a hard and stony exoskeleton and tend to live on reefs (the exoskeletons of dead ancestor corals).

global warming     The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.

hue     A color or shade of some color.

microorganism     A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

nutrient     A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.

reef     A ridge of rock, coral or sand. It rises up from the seafloor and may come to just above or just under the water’s surface.

symbiosis     (Adj. symbiotic) A relationship between two species that live in close contact. A species that lives this way, offering substantial help to the other species, is sometimes called a symbiont.

tissue     Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.