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Coral bleaching
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== Process == [[File:Coral Bleaching.jpg|thumb|{{center|}} Coral and microscopic algae have a symbiotic relationship. When water temperatures get too high, the algae leave the coral tissue and the coral begins to starve.<ref>{{Cite web |title=What is coral bleaching? |url=https://oceanservice.noaa.gov/facts/coral_bleach.html |url-status=live |archive-url=https://web.archive.org/web/20201220021310/https://oceanservice.noaa.gov/facts/coral_bleach.html |archive-date=20 December 2020 |access-date=10 January 2020 |website=NOAA National Ocean Service}}</ref>]] [[File:Climate change threats to coral reefs.png|thumb|[[Climate change]] will affect coral reef ecosystems, through [[sea level rise]], changes to the frequency and intensity of tropical storms, and altered ocean circulation patterns. When combined, all of these impacts dramatically alter ecosystem function, as well as the goods and services coral reef ecosystems provide.<ref>{{Cite web |last=US Department of Commerce |first=National Oceanic and Atmospheric Administration |title=How does climate change affect coral reefs? |url=https://oceanservice.noaa.gov/facts/coralreef-climate.html |access-date=2024-02-19 |website=oceanservice.noaa.gov |language=EN-US}}</ref> ]] [[File:Zooxanthellae.jpg|thumb|[[Zooxanthellae]], the microscopic algae that lives inside coral, gives it colour and provides it with food through photosynthesis]] The corals that form the great [[coral reef|reef]] [[ecosystem]]s of tropical seas depend upon a symbiotic relationship with algae-like single-celled [[flagellate]] [[protozoa]] called [[zooxanthellae]] that live within their tissues and give the coral its coloration. The zooxanthellae provide the coral with nutrients through [[photosynthesis]], a crucial factor in the clear and nutrient-poor tropical waters. In exchange, the coral provides the zooxanthellae with the [[carbon dioxide]] and [[ammonium]] needed for photosynthesis. Negative environmental conditions, such as abnormally warm or cool temperatures, high light, and even some microbial diseases, can lead to the breakdown of the coral/zooxanthellae symbiosis.<ref>{{Cite book |editor-last1=Dubinzk |editor-first1=Z. |editor-last2=Stambler |editor-first2=N. |last=Lesser |first=M.P. |title=Coral Reefs: An Ecosystem in Transition |pages=405β419 |date=2010 |publisher=Springer |location=Dordrecht |isbn=978-94-007-0114-4 |doi=10.1007/978-94-007-0114-4_23 |chapter=Coral Bleaching: Causes and Mechanisms}}</ref> To ensure short-term survival, the coral-polyp then consumes or expels the zooxanthellae. This leads to a lighter or completely white appearance, hence the term "bleached".<ref name=":1">{{Cite journal |year=1999 |title=Climate change, coral bleaching and the future of the world's coral reefs |journal=Marine and Freshwater Research |volume=50 |issue=8 |pages=839β66 |doi=10.1071/MF99078 |last1=Hoegh-Guldberg |first1=Ove |author-link=Ove Hoegh-Guldberg (biologist) |doi-access=free}}</ref> Under mild stress conditions, some corals may appear bright blue, pink, purple, or yellow instead of white, due to the continued or increased presence of the coral cells' intrinsic pigment molecules, a phenomenon known as "colourful bleaching".<ref name=":23" /> As the zooxanthellae provide up to 90 percent of the coral's energy needs through products of photosynthesis, after expelling, the coral may begin to starve.<ref name=":19" /> Coral can survive short-term disturbances, but if the conditions that lead to the expulsion of the zooxanthellae persist, the coral's chances of survival diminish. In order to recover from bleaching, the zooxanthellae have to re-enter the tissues of the coral polyps and restart photosynthesis to sustain the coral as a whole and the ecosystem that depends on it.<ref>{{Cite journal |vauthors=Nir O, Gruber DF, Shemesh E, Glasser E, Tchernov D |title=Seasonal mesophotic coral bleaching of Stylophora pistillata in the Northern Red Sea |journal=PLOS ONE |volume=9 |issue=1 |pages=e84968 |date=15 January 2014 |pmid=24454772 |pmc=3893136 |doi=10.1371/journal.pone.0084968 |bibcode=2014PLoSO...984968N |doi-access=free }}</ref> If the coral polyps die of starvation after bleaching, they will decay. The hard coral species will then leave behind their [[calcium carbonate]] [[exoskeleton|skeletons]], which will be taken over by [[algae]], effectively blocking coral regrowth. Eventually, the coral skeletons will erode, causing the reef structure to collapse.{{Citation needed|date=May 2021}}
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