Zooxanthellae: The Algae Inside Your Corals, Explained

A coral that was neon green in the store photo is brown three weeks after it went in your tank. Another one, same tank, has gone ghost white. Both problems are about the same thing: the single-celled algae living inside the coral, called zooxanthellae. Too many and the coral browns. Too few and it bleaches.

What zooxanthellae are

Zooxanthellae (zoh-uh-zan-THEL-ee) are single-celled dinoflagellate algae that live inside the tissue of most reef-building corals, as well as anemones, giant clams and many soft corals. They used to be lumped together as one genus, Symbiodinium. In 2018 LaJeunesse and colleagues split the family Symbiodiniaceae into seven genera, including Cladocopium and Durusdinium, which is why newer papers stopped saying “clade C” and “clade D.”

The cells are tiny, about 6 to 13 microns across, and there are a lot of them: hundreds of thousands to millions in every square centimeter of coral tissue, roughly the area of a fingernail. They sit in the coral’s inner tissue layer, where light reaches them through the clear outer layer.

Why corals need them

The deal is simple. The coral gives the algae a protected home plus the carbon dioxide and nutrients from its own metabolism. The algae photosynthesize and pass sugars and other organic compounds back. According to NOAA, as much as 90 percent of the organic material the zooxanthellae make goes to the coral. That is how a coral can grow in clear tropical water that holds very little food, and it is why light is the first number that matters for almost every photosynthetic coral you buy.

It also explains why stony corals grow skeleton faster in the light. Much of the energy for building calcium carbonate comes from those sugars.

Why corals go brown or pale

The algae have their own pigment, mostly chlorophyll, which gives a coral a green-brown base color. The bright pinks, purples, blues and greens come from fluorescent proteins the coral makes itself, partly as sunscreen for its algae. The Woods Hole Oceanographic Institution notes corals make more of these pigments in bright light. Put those two facts together and coral color is a balance:

• More algae carrying more chlorophyll turns the coral brown. In a 1998 study of Stylophora pistillata, N. Stambler found added ammonium roughly doubled algae density under low light, and that each algal cell carried more chlorophyll as light dropped.
• Less protective pigment dulls the bright colors. A 2018 study on Hydnophora grandis found corals turned their fluorescence down to 30 to 60 percent of maximum under low light.
• Too few algae and the coral goes pale, then white. That is bleaching.

In practice, a coral browning in your tank usually needs more light, lower nitrogen, or both. A coral going pale needs the opposite: less light, or a check on temperature and alkalinity. Our montipora care guide walks through both directions for montipora.

Bleaching vs STN vs RTN

Bleaching is the coral losing its zooxanthellae, or the algae losing their pigment. The tissue is still there, just clear, so you see the white skeleton through it. That is different from STN and RTN, where the tissue itself dies and peels off. A bleached coral can recover if the cause stops in time. Tissue that has peeled off does not come back. Our bleaching vs STN vs RTN guide shows how to tell which one you have.

The common causes in a home tank are heat, a jump in light (a new fixture or a coral moved too high), and an alkalinity swing. Nutrient balance matters too. Wiedenmann and colleagues reported in Nature Climate Change in 2013 that corals with plenty of nitrogen but too little phosphate were more likely to bleach under heat and strong light. It is one likely reason reefers who strip phosphate to zero sometimes see corals pale out.

Can you add zooxanthellae?

Yes, and corals can take them up. In a 2004 study in Science, Lewis and Coffroth bleached a Caribbean gorgonian, Briareum, by keeping it dark for 12 weeks until its algae dropped below 1 percent of normal. When they added algae to the water for six weeks, symbiont density rose 9 to 31 times. Most coral larvae pick up their first zooxanthellae from the water around them, so the route exists.

Bottled products such as PhycoPure Zooxanthellae are cultured zooxanthellae in a concentrated suspension. The maker suggests them during coral acclimation and to support the coral-algae relationship. What is not established is how much a dose changes recovery in a home reef where the coral still holds some of its own algae, which is the usual case. In most cases a bleached coral repopulates from the algae it still holds once the cause is fixed. So the order is: fix the heat, light or alkalinity problem first, then use a zooxanthellae product as support, not as the fix.

PhycoPure Zooxanthellae 4 oz PhycoPure Zooxanthellae 8 oz Phyto-Plasm Zooxanthellae 250 ml

See all phytoplankton and algae feeds.

How light and nutrients shift the balance

Our reef tank lighting guide has PAR targets by coral type, and new arrivals that stay closed are covered in new coral not opening after shipping.

The animals that farm algae

Corals are not the only hosts. Giant clams keep their zooxanthellae in a network of channels in the colorful mantle, which is why clams need strong light, as our clam guide explains. Anemones like the rock flower anemone carry them too, and so does nearly every coral we sell, from the hydnophora to the stylophora.

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The Underwater Gardener is a reef aquarium store in Key West, Florida. Livestock ships UPS Next Day Air on Tuesdays and Wednesdays.