Corals are threatened by global warming

news that coral reefs are in hot water. Corals, which are sessile animals related to sea anemones, host within their tissues algae that provide them with both food and their tourist-attracting colours. But as temperatures rise, the photosynthetic mechanisms of these algae go haywire. Instead of molecular oxygen, the normal waste product of photosynthesis, they start generating highly reactive and therefore toxic oxygen-rich compounds, such as peroxides. If they do this to excess, their hosts expel them, bleaching the coral concerned white. Bleached corals can linger for a while, but starved of their symbionts’ supply of nutrients they are vulnerable to disease. Eventually, if the temperature does not drop far enough to permit the algae to be readmitted, they die.