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The Effects of Photobleaching on Photosynthetic Pigments

There are a variety of factors that can cause photobleaching, or the loss of color, in photosynthetic pigments. This is especially true for coral reef habitats that are subject to changes in several environmental factors, such as temperature and light. To understand how and why photobleaching occurs, it is important to understand how photosynthesis plays a role in the life cycle of a coral.

Endosymbiosis
- Corals get their vibrant colors from the algae that resides within the polyps.
  
  Although plants are the only organisms that actively use photosynthesis for nutrition, other organisms may benefit from the processes. Basically, a plant converts light to energy in the form of oxygen and simple sugars. Corals are animals and do not independently carry out photosynthetic processes. Rather, corals have an endosymbiotic relationship with zooxanthellae---a single-celled algae that lives inside the coral's hard exoskeleton. This is a mutually beneficial partnership for both the coral and the algae. The coral's skeleton protects the algae while the zooxanthellae provide nutrients to the coral through photosynthesis. The algae in the tissues gives the calcitic skeleton of the coral the color that one sees with the naked eye.
Photobleaching
- The natural color of a coral without algae is white.
  
  Photobleaching on a small scale is a natural phenomenon. Corals are able to control the amount of nutrition they receive from their zooxanthellae partnership by expelling the organisms when the rate of photosynthesis is too high. Because the algae are the medium that gives a coral color, when the density is diminished, color pigmentation is lost. The skeletons of corals are composed of calcium carbonate which tends to be white in color. Thus, when the algae is lost, the corals return to their original color.
Causes of Photobleaching
- If the rate of global warming continues to increase, coral populations could be devastated.
  
  Photobleaching can be attributed to several factors. One is environmental stresses for the expulsion of zooxanthellae; the more extreme the stress, the more severe the photobleaching. Climate change or changing weather patterns like El Niño can cause a range of environmental changes that will cause photobleaching in corals. These changes could include a significant increase or decrease in water or air temperature, changes in salinity, or a buildup of carbon dioxide and methane gases. Similarly, changes in water depth can cause photobleaching by exposing the corals in increased ultraviolet radiation or by decreasing overall light exposure. According to "Encyclopedia of Earth," an increase in global temperatures by 2 degrees Celsius would decimate coral reef habitats which would be unable to adapt. (see Reference 3)
Recovery
- Despite its beauty, the coral reef habitats of the Indian Ocean have been severely affected by climate change.
  
  If the environmental disruption is minimal and conditions return to normal in a relatively short period of time, the zooxanthellae will rapidly divide, proliferate and color will return to the coral. However, repeated or continuous environmental stress will exacerbate the bleaching, causing long-term affects such as reduced reproduction rates or decreased growth. The more algae that is expelled, the more difficult it is for the coral to recuperate, and the organism will expire. Widespread loss of coral reef habitats have been evidenced in many locations, the most devastation being noted in the Indian Ocean, where 90 percent of corals have been lost to photobleaching, according to a report released by the European Commission on the Science for Environment Policy. (see Reference 4)

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