 | 000 what is coral bleaching
Coral Bleaching Coral bleaching is the whitening of coral colonies due to the loss of symbiotic zooxanthellae from the tissues of polyps. This loss exposes the white calcium carbonate skeletons of the coral colony. Corals naturally lose less than 0.1% of their zooxanthellae during processes of regulation and replacement Brown and Ogden, 1993. However, adverse changes in a coral's environment can cause an increase in the number of zooxanthellae lost. There are a number of stresses or environmental changes that may cause bleaching including disease, excess shade, increased levels of ultraviolet radiation, sedimentation, pollution, salinity changes, and increased temperatures. Corals tolerate a narrow temperature range between 25 degrees Celsius and 29 degrees Celsius depending on location. Corals bleach in response to prolonged temperature change and not due to rapidly fluctuating temperatures. Lab experiments show that corals bleach when water reaches a constant 32 degrees Celsius Brown and Ogden, 1993. Experiments have shown that an increase in UV light causes coral bleaching. UV light experienced by corals can increase in calm waters. The amount of mycosporine-like amino acids in a coral's tissues helps to determine how much UV it can withstand without bleaching Gleason, 1993. The exact mechanism by which corals bleach or the trigger that induces bleaching is unknown. However, there are a number of hypotheses that attempt to answer this question. Laboratory experiments have shown that the zooxanthellae are released into the gut of the polyp and then are expelled from the polyp through the mouth Brown and Ogden, 1993. However, this has not been observed in nature. Another hypothesis is that stressed corals give algae fewer nutrients and thus the algae leave the polyp. Algae may produce oxide toxicity under stress, and these toxins may affect the polyps.www.uvi.edu/coral.reefer/bleach.htm
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 | 01 More about Coral Bleaching
Understanding Coral Bleaching. Coral bleaching refers to the loss of color of corals due to stress-induced expulsion of symbiotic unicellular algae . The corals that form the structure of the great reef ecosystems of tropical seas depend on a symbiotic relationship with photosynthesizing unicellular algae called zooxanthellae that live within their tissues. Zooxanthellae give coral its particular coloration, depending on the clade living within the coral. Under stress, corals may expel their zooxantheallae , which leads to a lighter or completely white appearance, hence the term "bleached". Coral bleaching is a vivid sign of corals responding to stress, which can be induced by any of: increased water temperatures often attributed to global warming starvation caused by a decline in zooplankton levels as a result of overfishing . solar irradiance photosynthetically active radiation and ultraviolet band light, changes in water chemistry silt runoff pathogen infections Some of these factors are anthropogenic , while others occur naturally. Once bleaching begins, corals tend to continue to bleach even if the stressor is removed. If the coral colony survies, it often requires weeks to months for the remaining symbiont population to reach a normal density. Following bleaching, corals may be recolonised by the same species of zooxanthellae, or by a different species. Different types of zooxanthellae respond differently to environmental conditions and may be more resistant to coral bleaching than other species. Some corals are known to host multiple clades of zooxanthellae within an individual coral. . Ability to withstand stress and bleaching and ability to recover from a bleaching event varies greatly across coral species. Large massive corals, such as Porites lobata is able to withstand extreme temperature shocks, while fragile branching corals, such as Acropora spp. are far more susceptible to dying following a bleaching event. Recent research has also shown that corals consistently exposed to low levels of stress may in fact be more resistant to bleaching. Factors that protect against mass coral bleaching are bleaching resistance, coral tolerance, reef recovery. Due to the patchy nature of bleaching, local climatic conditions such as shade or a stream of cooler water can reduce the risk of bleaching. Also, the health and genetics of both the coral and its zoozanthellae can influence the risk of bleaching. Other reef creatures have symbiotic zooxanthellae , which they may also expel under stressful conditions. Bleaching stress is also exhibited by soft corals ,giant Tridacna clams and some sponges . The Great Barrier Reef along the northeast coast of Australia suffered two mass coral bleaching events in the summers of 1998 and 2002. While most reef areas recovered with relatively low levels of coral death, some locations suffered severe damage, with up to 90% of corals killed. Other coral reef provinces have been permanently damaged by warm sea temperatures, most severely in the Indian Ocean . Up to 90% of coral cover has been lost in the Maldives ,Sri Lanka ,Kenya and Tanzania and in the Seychelles . Pathogen infection In 1996, Kushumaro, et al. reported that the agent for the coral bleaching in the Red Sea was an infectious bacteria attacking the symbiotic algae. The agent has been later identified as Vibrio shiloi . The pathogen is infectious only during warm periods; therefore, global warming would increase the occurance of conditions that promote the spread of infection. During the summer of 2003, coral reefs in the Red Sea appeared to gain resistance to the pathogen, and further infection was not observed . The main hypothesis for the emerged resistance is probiotic : inside the symbiotic communities of bacteria living with the corals. One species capable of lysing V. shiloi has gained prominence. This hypothetical bacteria has not yet been identified. American Psychological Association APA: Coral_bleaching. n.d.. Wikipedia, the free encyclopedia . Retrieved June 17, 2007, from Reference.com website: http://www.reference.com/browse/wiki/Coral_bleaching Chicago Manual Style CMS: Coral_bleaching. Reference.com. Wikipedia, the free encyclopedia .http://www.reference.com/browse/wiki/Coral_bleaching accessed: June 17, 2007. Modern Language Association MLA: "Coral_bleaching." Wikipedia, the free encyclopedia . 17 Jun. 2007. .
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