Coral bleaching refers to the acute release or loss of the symbiotic algae zooxanthellae from the coral tissue. (See “zooxanthellae” to learn about the symbiotic relation). The visible symptom of coral bleaching is the white skeleton underneath the corals tissue that lost its zooxanthellae colonies.
During nutrient deficiencies waste products created by the coral can no longer support the zooxanthellae colonies, which in return can not provide nutrients back to the coral.
Continued nutrient deficiencies will have a reduction of zooxanthellae in consequence. The interruption of this symbiotic relationship can be detected by the coral releasing zooxanthellae back into the water, by digesting dead algae as a direct food source, or by regulating colonies in exposing the algae to less and less light through limited openings by the polyp.
Probably the main cause of coral bleaching is excess nutrients especially nitrogen (nitrate).
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In addition to the nitrogen corals provide the algae with through their metabolism, excess nitrogen in the water is also available for utilization by the zooxanthellae as well as by the coral directly. This can increase the rate of the reproduction of the algae. “Overpopulation” can trigger the coral to either release algae into the water or it may limit light exposure causing algae colonies to die off.
As described in “zooxanthellae” a certain constant release and repopulation caused by adjustments due to environmental changes is natural. This is also often referred to as bleaching. The terminology also includes naturally replaced algae as they don’t live indefinitely.
Coral bleaching varies from a natural occurrence to a severe detrimental state. It is the latter that is cause for concern.
The progressive loss of zooxanthellae ultimately leads to a nutrient deficiency. It is this deficiency that allows the zooxanthellae to repopulate the coral tissue. Nevertheless it is important to provide the coral with nutrients, failing to do so will be result in the death of the coral.
Feeding corals may become necessary. However the need for feeding may indicate that the coral is dying off. If the algae are unable to take up food due a lack of energy to catch and digest the prey, this may also be a sign of the symbiotic relationship being out of balance. The most severe consequence again, would be a dying coral.
In short, bleaching describes a dysfunctional symbiotic relationship between corals and zooxanthellae and the consequential loss of symbiotic algae.
Contributing factors other then described above are an increased water temperature, metals and other pollutants (nitrate) low water current and starvation caused by UV radiation.
Low nitrate levels are essential for both, the algae, and the coral, in order for the symbiotic relationship to thrive.