Zooxantellae The Partner in a Coral’s Mutualistic Relationship

Corals have a symbiotic relationship with the unicellular algae zooxanthellae. They are found within the coral polyp in cells that line the gut. They are yellow-brownish marine algae of the family Dinophyceae, which also include many free-living dinoflagellates. They spend their entire lives in the coral polyp going through photosynthesis and reproduction. This coexistence is very important for both organisms (called symbionts) and profits both of them. Cytosymbiosys takes place between the coral and the zooxanthellae. Cytosymbiosis occurs when one organism lives in the cells of another organism. Zooxanthellae seem to be present in almost all corals. This is especially for Leptoseris fragilis, a very interesting coral. This coral lives at depths where sunlight is absent and photosynthesis cannot possibly take place. Corals, not including deep-sea coral, are not usually found far from the surface past 100 meters where light cannot penetrate. The reason for this is that symbiotic zooxanthellae require light for photosynthesis, the biological process that converts light energy and carbon dioxide (CO₂) into food in the form of sugars. The corals are also limited to the warm water (above 20° Celsius), and avoid low salinity and high turbidity.

Zooxanthellae need light to photosynthesis. They clean the coral body of many different chemicals. One of these chemicals includes carbon dioxide. Carbon dioxide is a byproduct in both metabolism and the production of the skeleton. It is important to remove carbon dioxide from corals because the process accelerates the production of the skeleton. The zooxanthellae also remove nitro-compunds, another important byproduct of metabolism. For corals, nitro-compounds are noxious, but are necessary for zooxanthellaes to live.

See: Some information about Zooxanthellae

An interesting fact about zooxanthellae is that they are actually ejected from coral polyps during times of darkness or freshwater flooding. These are conditions that happen to be unfavorable for zooxanthellae photosynthesis. It is unknown what mechanisms control the zooxanthellae population in each coral polyp, but it does not appear that the corals absorb them. It is more likely that the coral polyp simply ejects the older, less active zooxanthellae to make room for new ones.

Even more interestingly is a discovery by Robert K. Trench and colleagues at the University of California, Santa Barbara (UCSB) where specific zooxanthellae have been shown to adapt to specific coral species.

There are corals that do not have zooxanthellae and therefore do not take part in a symbiosis. But only few coral species not living in symbiosis with Zooxanthellae produce reefs.

Corals without Zooxanthellae

Some such coral live in crevices beneath the enormous coral structures.

Examples include:

Solitary cup corals - These corals can exist in waters with low salinity, low temperatures, and greater depths as deep as 6000 meters in the deep sea.
Astrangia - A specific type of solitary cup coral which can be found encrusted to shells and rocks as far north as Cape Cod.
Lophelia - Located in Norwegian fjords where waters are extremely cold.

As one would expect, the rate of growth for non-symbiotic corals is very low, so much so that they are not likely to form massive reefs.