Coral reefs are very beautiful and interesting marine ecosystems found in the shallow seas formed by the corals. Corals are the structures formed by the calcium carbonate secreted by inhabiting colonies of tiny invertebrates belonging to a class of animals called Cnidarians.
Most of the corals support the growth of photosynthetic unicellular algae called Zooxanthella (also called as Symbiodinium) which in turn derives its nutrition from algae in an association called symbiosis. Corals are found in clear and shallow waters of not more than 200 feet depth where there is abundant sunlight for the survival of Zooxanthella.
A recent study by Associate Professor Kazuhiko Koike and Ms. Lisa Fujise from Hiroshima University showed that any increase in the temperature leads to the bleaching of corals. At a favorable temperature of 27°C, the corals expel a reasonable amount of Zooxanthella to maintain the density of the algae and stable carbon concentrations. However at temperatures of 30°C considered moderate thermal stress, Zooxanthella were damaged.
The damaged Zooxanthella were selectively expelled with or without digestion by the corals, which could be an adaptation to moderate thermal stress in order to get rid of the dead Zooxanthella. This mechanism of adaptability to moderate stress during extended duration of environmental stress (thermal stress) leads to the removal of most or all of the Zooxanthella that could not withstand the thermal stress hence leaving the corals bleached.
The temperature variations affecting one of the partners of the symbiotic association may wipe out the entire ecosystem. Global warming and other factors affect ecological balance resulting in threat to the existence of several organisms every day. Bleaching of coral reefs is just an example out of many.
For further reading: http://bit.ly/1wiCnPS