Why do bleached coral turn white?

So after doing some research this is what I have come up with.
(one informative site)
http://ag.arizona.edu/azaqua/algaeclass/symbios.htm
Zooanthellae do have a color. This color is usually brown or yellow, but sometimes can be red, green, or blue green, but not very often and in small quantities. Therefore the only color you will actually see from the zooanthellae, outside the tank, is brown or brownish yellow. Therefore when there are more zooanthellae inside the cells of the coral you will get a brown coloration. This may be from an elevated nutrient level or because the coral is not getting enough light and is excepting extra zooanthellae to compensate.

So why do corals bleach? There are many reasons why and I will explain a few.
It all begins with the coral expelling their zooanthellae. A few of the reasons they do this are...

• Warmer temperatures

-I’m not completely sure why they will expel during warmer temperatures. I think it has something to do with increased metabolism but not sure. I just know that studies have shown warmer temperature=less zooanthellae.

• Too much light or radiation

- Zooanthellae, being a type of plant, feed off of light. Although the nutrients the zooanthellae produce during photosynthesis are good for the coral too much can be harmful. If the coral believes that the zooanthellae are too active it will expel them. Zooanthellae create oxygen during photosynthesis which is used for cell respiration, which is good. Too much oxygen will oxidize lipids, acids, etc, which is bad.

• No need to protect zooanthellae. The actual pigment that you see (green, purple, red, etc.) is a pigment that the coral creates as a protection to the zooanthellae, much like the keratin in our own skin. Zooanthellae are sensitive to UV-A and UV-B and need protection. The more reflective the pigment the better the protection. Corals may also localize the pigment produced. Some SPS have colored tips that face the light. The shadowed sides of the SPS branches do not need as much protection and therefore do not have as much pigment. Zooanthellae also help with calcifying, or growing the coral, which will have an effect on the pigments localized in certain areas but that’s for another discussion.

This is why we add more light to “color up” our corals. Metal Halide lighting has higher levels of UV rays and therefore creates the need for more pigment protection of the zooanthellae. In conclusion, the coral will lose its zooanthellae, by its own account or by external factors, and then will quit producing protection pigments due to the missing zooanthellae, thus bleaching the coral. The coral will still be alive during this period of stress and will hopefully regain its symbiotic partner. Different species will be able to survive different amounts of time but all share the same outcome if the zooanthellae do not return, death.

Hope this helps answer your question.

 

If one has a lot of different corals in your tank, sometimes they will come back a different color if they are re-infected with a dinoflagellate (zooxanthellae) from a different clade. However, when a coral bleaches it rarely 100% bleaches so it will usually recover the same color.

If you have an SPS that is bleached fully, it's not going to get fed via photosynthesis. Put it in lower light and target feed it very tiny particulate foods (Coral Frenzy, Golden Pearls, etc). Even better is to just put it in your fuge (assuming you aren't running your lights 24/7).