Scientists say Australia''s Great Barrier Reef suffered its most severe coral bleaching event in March.Corals are small marine animals that live together and form large hard structures called reefs.Bleaching usually happens when ocean water gets too warm,causing corals to release the healthy algae(海藻)living in their tissues.The process turns coral white and makes it sick,sometimes killing it.Reefs are critically important to the health of the ocean and all the life it holds.
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