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When corals turn white, it is a sign that a reef is suffering from stress. This coral bleaching occurs when temperatures rise just two degrees above the normal peak temperatures. Healthy corals get their colour from microscopic algae living within their tissues, but at heightened temperatures, the relationship is no longer beneficial and the algal plant cells are expelled. This exposes the white chalky skeleton of the reef showing the corals' transparent tissues. The corals can survive this for short periods but if hotter conditions persist, they will die.