I found this while searching this issue -
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First, for the original poster, you are correct. Bleaching occurs when a coral looses zooanthellae. Zooanthellae are single-celled algae that have co-evolved with coral. They are two separate organisms, but they have gotten so used to living together that most coral cannot live without their zooanthellae. The zooanthellae (like all algae) are photosynthetic. They absorb photons of light and use the energy to turn nutrients into sugars. The zooanthellae make more sugar than they need to survive, so they allow the coral to use some of it. In return, the coral provides physical protection and access to light for the zooanthellae (it keeps them out of the water column where they would risk being eaten by filter feeders, buried in the substrate, sunk into the depths, etc.)
As was said above, the process of recovery or "coloring up" is simply the few remaining zooanthellae growing within the tissue of the coral. If the zooanthellae population can recover before the coral uses up it's stored sugars, then it will live and recover. If not, it will die.
There are quite a few mistakes in the rest of this thread. The worst are about the actual pigments and their source and physiological location. All of the colorful pigments are produced by and reside within the zooanthellae and NOT the coral itself. The only "colorful" compounds that the corals make themselves are fluorescent proteins. These produce the glow you see under actinic light. If anyone has any references that state otherwise I would like to know about them. Here is a long review paper about pigments in simple biological systems. It goes way beyond the scope of this discussion, but there is a clearly defined section about corals and coral bleaching.
http://www.blackwell-synergy.com/doi/pdf/1...6.x?cookieSet=1
I hope this is a publicly available paper. I'm typing this from a university computer. If not, you'll have to go look it up at a science library.
Dawson, T L. "Light-harvesting and light-protecting
pigments in simple life forms" Color. Technol., 123, 129–142”
”I don't think it really matters what causes the bleaching, only how severe. All of the things you mentioned (including light shock) kill off the zooanthellae. Assuming that the tank conditions are corrected, it just becomes a race to see if the remaining zooanthellae can grow fast enough to provide enough sugar to keep the coral from starving to death.”
https://www.nano-reef.com/forums/topic/133973-how-do-bleached-corals-come-back/
