Without symbiotic algae, the coral can die from starvation, or become so weakened by a lack of food, that it succumbs to harmful bacteria
(Mao-Jones 2010), and/or seaweeds which can poison and kill coral on contact.
...
As coral reefs operate very near to their upper limit of heat tolerance
(Glynn & D'Croz 1990), bleaching en masse happens when the surface waters get too warm above their normal summer temperature, and are sustained at this warmer level for too long.
...
The huge mass bleaching in the Caribbean in 2005, a non El-Nino year, and again this year is a prime example of this
(Eakin 2010) .
...
Over the last 30-40 years 80% of coral in the Caribbean have been destroyed
(Gardner 2003) and 50% in Indonesia and the Pacific
(Bruno & Selig 2007). Bleaching associated with the 1982 -1983 El-Nino killed over 95% of coral in the Galapagos Islands
(Glynn 1990),
...
Studies from around the world show no signs of bleaching dating back many thousands of years, until recent decades
(Abram 2003),
(Aronson 2003). In the Caribbean there are no signs of previous mass bleaching dating back 220,000 years
(Pandolfini & Jackson 2006)
...
However this a only small proportion, major reef-building coral species seem incapable of forming long-lasting partnerships with these heat tolerant algae
(Coffroth 2010), and the coral polyp themselves have a very poor genetic ability to adapt to warming
(Csaszar 2010).
...
Goods and services derived from coral reefs are very roughly estimated to be between $172 to $375 billion dollars per year
(Martinez 2007).
...
Ocean Acidification in particular is a large looming threat
(Veron 2009).