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Coral bleaching
Coral bleaching is the loss of intracellular endosymbionts (''Symbiodinium'', also known as zooxanthellae) through either expulsion or loss of algal pigmentation. The corals that form the structure of the great reef ecosystems of tropical seas depend upon a symbiotic relationship with algae-like unicellular flagellate protozoa that are photosynthetic and live within their tissues. Zooxanthellae give coral its coloration, with the specific color depending on the particular clade. Some scientists consider bleaching a poorly-understood type of "stress" related to high irradiance; environmental factors like sediments, harmful chemicals and freshwater; and high or low water temperatures.〔 This "stress" causes corals to expel their zooxanthellae, which leads to a lighter or completely white appearance, hence the term "bleached". Bleaching has been attributed to a defense mechanism in corals; this is called the "adaptive bleaching hypothesis," from a 1993 paper by Robert Buddemeier and Daphne Fautin.〔''The Coral Reef Era: From Discovery to Decline: A history of scientific investigation from 1600 to the Anthropocene Epoch'', James Bowen, Springer, 6 Jan 2015, ISBN 978-3-319-07478-8, DOI 10.1007/978-3-319-07479-5〕 Bleached corals continue to live, but growth is limited until the protozoa return.
== Causes ==
Bleaching occurs when the conditions necessary to sustain the coral's zooxanthellae cannot be maintained. Any environmental trigger that affects the coral's ability to supply the zooxanthellae with nutrients for photosynthesis (carbon dioxide, ammonium) will lead to expulsion.〔 This process is a "downward spiral", whereby the coral's failure to prevent the division of zooxanthellae leads to ever-greater amounts of the photosynthesis-derived carbon to be diverted into the algae rather than the coral. This makes the energy balance required for the coral to continue sustaining its algae more fragile, and hence the coral loses the ability to maintain its parasitic control on its zooxanthellae.〔
Physiologically the lipid composition of the symbiont thylakoid membrane affects their structural integrity when there is a change in temperature, which combined with increased nitric acid results in damage to photosystem II. As a result of accumulated oxidative stress and the damage to the thylakoid of chloroplasts there is an increase in degradation of the symbiosis and the symbionts will eventually abandon their host. Not only does the change in temperature in the water increase the chances of bleaching, but there are other factors that play a role. Other factors include an increase in solar radiation (UV and visible light), regional weather conditions, and for intertidal corals, exposure to cold winds.〔Baker, Andrew. Glynn, Peter. Riegl, Bernhard. 2008 "Climate change and coral reef bleaching: An ecological assessment of long-term impacts, recovery trends and future outlook". ''Estuarine, Coastal and Shelf Science''. 80: 435–471〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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