Coral Bleaching

1. Why arecoral reefs important,and whatarepossibleexplanationsforthe phenomenon known as coral bleaching?

On the surface, coral bleaching looks exactly like what you’re envisioning right now: white, bleached-out coral reefs, which is quite a departure from the colorful structures we all know and love.

-Coral  reefs  areone  ofthe  most biologicallyproductiveeco- systemsonEarth.Most peoplehave  seen imagesofbrightly coloredfishesand otherreef-dwellingorganisms,yetmanydo not understand why these systems are personally important. Cyphastrea serailia and Pocillopora, damicornis were bleached in the light when water temperature was raised to 33°C for 3 h. We use the term ‘bleached’ in this context to mean that significant numbers of zooxanthellae were released from the coral host in comparison to controls at lower temperature. At 25°C few zooxanthellae could be sampled (60 cells ml–1 in the filtered supernatant), whereas after 3 h at 33°C the zooxanthellae density in the sampled water. The exact nature of this release is not fully understood. We use the term expelled zooxanthellae’ to describe the zooxanthellae that are released, to signify that there is probably a process of active expulsion by the host. Below 33°C there was no sign of bleaching, i.e. only a few zooxanthellae were released and these may have been senescent cells. Light was necessary for the bleaching to occur. However, we deliberately chose a rather low that still elicited bleaching because photoinhibition occurred at higher light levels.

-Symbiosis between polyps and zooxanthellae are common in corals. It is a mutualistic relationship (meaning both partners benefit from the interaction). The polyps are Cnidarians (same group as jellyfish). They lay down a calcified ‘skeleton’ and predate on small marine organisms using stings to catch prey. The zooxanthellae are basically a type of plankton that photosynthesis to produce energy from sunlight.




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