英语听力：自然百科 海洋天堂大堡礁:堡礁内外—23（在线收听）

This bleaching occurs because corals can only live in a narrow temperature range, healthy corals get their color from microscopic algae living in their tissues, these manufacture food for the corals by photosynthesis, but when the temperature rises just 2 degrees above the normal summer maximum, the algal cells are expelled because they no longer benefit the coral. The bleaching effect is the white chalky skeleton showing through the coral's transparent tissues, but they are not dead, not yet, they can survive in this bleach state for several weeks, if the temperature drops, the corals aquire new algae from plankton floating by, but if the warm water persists, the coral dies. Coral bleaching hadn't been seen on the great barrier reef before the 1980s, due to global warming, bleaching is now more common, and cyclones are likely to be more frequent too, and there is something even more insidious, temperature is rising because more and more carbon dioxide from human activity enters the atmosphere, this dissolves in sea water turning acidic which can stop coral growth, if they can't build their chalky skeletons, reefs will start to crumble.