Warm-weather friend

Good news is rare in research on global warming, but here's a hopeful discovery. Certain species of coral may be able to cope with warming seawater with a little help from their microscopic friends.

In return for a safe place to live, in the tissue of hard coral, single-celled algae of the genus Symbiodinium supply their hosts with photosynthesized sugars, and help calcify the coral's hard skeleton. When rising sea temperatures kill the algae or cause them to become toxic to their hosts, hard corals suffer bleaching and may die. But some corals harbor several strains of Symbiodinium, which differ in their response to light and temperature, and in some of their metabolic products. Investigators suspected that the algal strains might also alter the thermal tolerance of their hosts.

To test that idea, Madeleine J.H. van Oppen, a marine geneticist, and Ray Berkelmans, a coral ecologist, both at the Australian Institute of Marine Science in Townsville, transplanted colonies of Acropora millepora, a common Indo-Pacific hard coral, from their home waters on Australia's Great Barrier Reef to warmer sites on the reef. They also tested the colonies' thermal tolerance in the laboratory and genetically identified the strains of algae living inside. After a year, the investigators discovered, the transplanted corals increased their heat tolerance, a direct result of shifting the strain of Symbiodinium that dominates their tissues. Apparently, the corals initially take up an assortment of strains; if a strain with low heat tolerance is lost during high-temperature stress, a more heat-tolerant strain takes over.

Will hard corals survive the next century's hike in sea temperatures, predicted at between one and three Celsius degrees? Shuffling their Symbiodinium strains will probably not be enough to save the corals, say van Oppen and Berkelmans. But it may buy enough time to save them by reducing emissions of greenhouse gas. (Proceedings of the Royal Society B)

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