Some corals like it hotter

The heat-tolerant algae that live symbiotically within some corals may enable their hosts to adapt to the warmer water temperatures projected to accompany long-term climate change, field studies suggest.

Many coral polyps that make up shallow reefs draw much of their nourishment from algae that live within the polyps. Those algae, all from the genus Symbiodinium, capture the energy of sunlight and pass along some stored energy to their hosts, says marine biologist Andrew C. Baker of the Wildlife Conservation Society in New York City.

Many species of the genus die off if the water temperature suddenly spikes by even a few degrees. That happened during an El Niño that began in 1997. Warmer-than-normal water temperatures bathed reefs off the Pacific coast of Panama, and many coral colonies died or temporarily lost their algal symbionts.

In 1995, 43 percent of the colonies hosted more heat-tolerant species of Symbiodinium, a grouping known as clade D. These corals didn’t show ill effects of heat when observed in 1997, says Baker. By 2001, 63 percent of the colonies at the Panamanian sites hosted clade D symbionts. Baker and his colleagues present their findings in the Aug. 12 Nature.

Results of the Panamanian field surveys, as well as of studies on reefs in the Persian Gulf, Red Sea, and the western Indian Ocean, suggest that heat-tolerant clade D symbionts become more abundant on reefs after heat-induced damage, which is called bleaching. It’s not yet clear whether that change stems from higher survival rates among hosts inhabited by clade D algae or from those symbionts’ colonization of recently bleached polyps, says Baker.

In either case, he notes, these algae may permit some corals to adapt to the increased water temperatures expected to result from global warming.