Zooxanthellae, while challenging to pronounce (zoh-uh-zan-thel-ee), are not difficult organisms to understand. Zooxanthellae are single-celled, photosynthetic algae that live symbiotically within corals, anemones, jellyfish, and other aquatic organisms. They are a critical component of coral health and therefore are important for not only corals in the ocean but also any coral kept and grown in saltwater aquariums. Learn more about zooxanthellae below and why coral is happy to call these little guys their BFF.


What is Zooxanthellae?

Like I mentioned above zooxanthellae are single-celled, photosynthetic algae that live symbiotically within the tissues of hard or stony corals. Since they are single-celled, they have a simple structure by nature thus a single zooxanthella is composed of a single cell. This means that they reproduce by single cell division and the only difference in individuals is a direct result of mutations within the genome of the dividing cells. Otherwise all other individuals are exact clones of one another. Being photosynthetic algae they, like plants, take light energy and carbon dioxide from their external environment and fix them into glucose or sugars for energy or food.  Symbiotic refers to the relationship between the zooxanthellae and the coral. Specifically coral and zooxanthellae have a mutualistic relationship which refers to a reciprocal or positive/positive relationship between both organisms. That means that both the coral and the zooxanthellae benefit from each other.

Zooxanthellae are more specifically a yellow-brown type of dinoflagellate. Dinoflagellates as a group commonly form symbiotic relationships with other marine organisms, most often animals. There is great diversity in these organisms in terms of size, range, depth, and luminosity.

Zooxanthellae and coral biology

A critical component of coral biology is zooxanthellae; in fact the two really can’t exist without the other.

To review: Zooxanthellae are dinoflagellates that are present in the cytoplasm of many marine invertebrates like coral. They are photosynthetic algae that live in the tissues of coral organisms and many times give coral their vibrant hues. Zooxanthellae and coral have a mutualistic symbiotic relationship, thus they benefit each other by a cooperative existence. Coral gives the zooxanthellae a safe environment and compounds they need for photosynthetic events while zooxanthellae produces oxygen and helps the coral remove waste. Zooxanthellae also provides coral with essential products of photosynthesis like amino acids, glucose, and glycerol which is critical for the coral in order to make essential molecules such as fats, proteins, and carbohydrates. Coral uses the products zooxanthellae provides to produce calcium carbonate which they use to build their hard exterior. The relationship between zooxanthellae and coral reefs is so important that it drives the growth and productivity of coral reefs or the vast groupings of hard coral outgrowth in the ocean or aquariums alike.  This relationship is responsible for a tightly knit web of recycling nutrients in the often nutrient poor tropical ocean waters coral is most commonly found in. It is so prominent and important in the ocean, that 90% of organic matter that is photosynthetically produced in seawater is produced by zooxanthellae and is transferred to a host coral tissue. Impressive!


Besides the critical productivity and recycling of nutrients, zooxanthellae are also the component of coral that give stony or hard coral species their beautiful, vibrant colors.  Zooxanthellae is not a permanent part of coral. In stressful or less than optimal conditions they can leave or expel from the coral tissue. These conditions could include any range of water quality or a taxi the zooxanthellae is exposed too.  This could include but is not limited temperature, pH, dissolved oxygen, salinity, light, and water movement.  When an exodus of zooxanthellae occurs from a coral, the coral becomes a white or bleached color. This of course gave rise to the nickname “coral bleaching” which sadly is very prevalent in ocean news today. Coral decline is often associated with coral bleaching or coral appearing bleached or less bright. Coral bleaching is a term used so frequently today that it is easy to read about it and not understand what is actually happening to coral reefs that suffer such a loss. Since zooxanthellae is what gives coral their characteristic vibrant hue, when the zooxanthellae is expelled it causes the coral to become white or appear bleached. While coral bleaching brings the coral closer to mortality, it does not kill the coral instantaneously.  In other words, the coral survives the bleaching events but it leaves the coral very vulnerable and at high risk.


Coral bleaching events are occurring in massive areas all around the globe due to rising ocean temperatures and pH. It is without a doubt that changing ocean temperatures and pollution go hand in hand with this horrible decline of coral.  In the most recent news, the National oceanic and atmospheric administration (NOAA) has released a new prediction on the fate of coral reef systems around the globe. They suspect that within the next two years that 6% of coral reefs will be destroyed due to bleaching events. While 6% may seem insignificant at first glances that could lead to more than a third of coral disappearing forever. It is important before buying coral for an aquarium to understand the critical condition coral health is in the ocean, and to understand the responsibility we have in protecting, conserving, and saving coral.


Sadly, coral cannot survive very long without their zooxanthellae.  Zooxanthellae provide coral organisms with so many vital components that without them, the coral will eventually die. This relationship is one of the most intimate and conversely dependent mutualistic relationship seen among organisms in the ocean.  This closeness causes an animal (reef building coral) to respond to stimuli in their environment like a plant would.  Zooxanthellae need light for photosynthesis thus healthy coral organisms are found in clear water that sunlight can easily penetrate.  These waters tend to be areas of low turbidity (calm water) and also low productivity. These facts are what create the coral reef paradox that is part of what makes coral reefs such important ecosystems: they require clear, calm, nutrient-poor water but are one of the most productive and diverse ecosystems on the planet.


Why Zooxanthellae is important in your home aquarium system

If you have a coral in your home aquarium system you already probably know the importance of zooxanthellae and how to monitor it in your tank. Thus it is critical to monitor your system’s light, pH, temperature, and so on to make conditions optimal for the relationship of your coral organism and the zooxanthellae within their tissue. This varies from species to species so make sure to do thorough research on your specific coral and check with the grower that you got your coral from. However, the majority of color loss or zooxanthellae loss comes from light fluctuation.

Now, if your coral becomes stressed in any way and they expel their zooxanthellae and the zooxanthellae dies, coral growing sites and specialty stores carry zooxanthellae cultures that can aid your coral and possibly even save the specimen. In addition to restoring your coral, zooxanthellae cultures can enhance coral color and lengthen polyp growth.

Overall, after examine coral biology and the basics of Zooxanthellae, it is no wonder we can call the two BFFs; they need each other not only to survive but to flourish as well. To learn more about how to mail order or buy corals check out my article in references!


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