By Blane Perun
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Coral zooxanthellae are the very livelihood of the coral as they have a symbiotic relationship. This means the survival of each hinges on the other’s existence. Coral zooxanthellae are intracellular endosymbionts that have a brownish gold coloration. Coral zooxanthellae may be found in protozoa, marine animals, and scleractinian corals.
The coral zooxanthellae are autotrophs and they provide the coral with important energy created through photosynthesis. A great percentage of the coral’s energy needs, up to 90% in some cases, may be provided by the coral zooxanthellae. The tradeoff is that the coral provides the coral zooxanthellae a place to reside as well as protection and nutrients.
The nutrients are generally in the form of phosphorus or nitrogen that is found in waste material. The coral also plays into the symbiotic relationship by providing the carbon dioxide that is necessary for photosynthesis to occur. The coral zooxanthellae have a controlled population based on the availability of nutrients as well as light.
Reef building corals, called hermatypic corals, depend on coral zooxanthellae. Because of this the photic zone determines their ultimate growth. Because of the relationship between corals and coral zooxanthellae, corals have been successful and developed throughout the world’s tropical waters. The symbiotic relationship between coral zooxanthellae and coral is threatened when environmental stress occurs. This is based on the fact that the coral zooxanthellae either die due to digestions or expulsion. When coral zooxanthellae no longer reside within the coral something known as coral bleaching occurs.
This means the coral begins to lose its color because the density of coral zooxanthellae declines or because the actual color of the zooxanthellae is lost. Another potential problem that could cause coral bleaching is when the concentration of the cnidarians’ cellular pigment, Green Fluorescent Proteins, is lost or significantly lowered. What results is coral that is white and skeletal in appearance. The coral will eventually die if the conditions that caused the coral zooxanthellae to die or leave are not reversed. If the conditions do change, however, then the coral zooxanthellae will return to the coral.
There are many environmental stresses faced by the coral zooxanthellae. For example, hurricanes and other tropical storms may create disturbances and stress for the coral zooxanthellae. During low tide, when the coral is exposed to the air then coral zooxanthellae will be stressed as a result of the solar radiation. Pollution is another stressor that can harm coral zooxanthellae.