well firstly its important to understand corals themselves.
Are they animal, mineral, or plant?
This has been argued by scientists for years, but essentially they are all 3.
The coral is made up of a CaCO3 skeleton (mineral), within the skeleton live coral polyps (animal) and within the polyp live dinoflagellates, or zooxanthellae (plant).
The coral and dinoflagellates form a mutualistic relationship, meaning that both the coral and algae benefit from each other.
Corals can get anywhere up 98% of their nutrition from the dinoflagellates themselves. Dinoflagellates also promote and allow the corals to grow their mineral skeleton.
Now what can healthy corals be affected by:
Sedimentation (smothers corals and disallows photosynthesis of the dinoflagellates), Pollution, Salinity levels (high or low), Storms, being loved to death, high and low sea temps, crown of thorns sea star (big problem here on the GBR), algae blooms, destructive fishing methods and cyanide fishing.
After 6-8 weeks of this continued stress (above) corals will begin to expell their dinoflagellates. Now when these dinoflagellates are released, it DOES NOT mean that they are DEAD! This is a common mistake made. When corals first bleach (lose dinoflagellates) they will only expel some, not all. If the stress discontinues the corals are able to rejuevenate. However, if these stresses continue for a long period of time then, yes, the coral will die.
As I mentioned before, dinoflagellates give the coral 98% of its energy and nutrition, without these dinoflagellates, corals die.
To put it simply, scientists are still trying to determine the exacts causes and triggers of coral bleaching. It seems that Global Warming is one of the factors, so a part from reducing your carbon footprint, we can't do much.
HOWEVER, any of you divers out there CAN help by doing coral research when diving... the coral watch charts are especially designed to be simple and extremely easy to use! hey if i can use it...anyone can!
Sorry about the essay...but its complicated and i'd rather u understand it as 'the key to conservation is education!'
Happy to answer any more questions!!!