Certain corals resist bleaching.

• Position: Environmental Stress

Stressed corals lose the symbiotic algae that help them survive in a process known as bleaching, but researchers at the University of Georgia, Athens, have discovered that one subtype of the symbiotic algae which lives mostly in shallow-water corals of the Caribbean provides resistance to environmental stress.

All reef-building coral species host unicellular algae that live within cells on tentacle-bearing polyps. Yellow-brown algae called zooxanthellae support their hosts' survival by carrying out photosynthesis and providing nutrients that stimulate construction of reef coral colonies. In addition to their ability to provide carbon dioxide fixation at rates equivalent to those of forests, the algae lend color to coral tissues, which otherwise are pale.

Corals bleach by losing their beneficial algae when stressed. While corals usually can recover from short-term stressful episodes, prolonged events such as during an El Nino period can cause permanent bleaching and death of vast expanses of coral reef. The 1997-98 El Niho remains the strongest such event on record. That year, sea surface temperatures were the highest ever recorded, and coral reefs worldwide suffered extensive bleaching and mortality.

The researchers found, however, that clade A Symbiodinium has complementary mechanisms for surviving in its coral hosts during periods of warmer-than-normal water temperatures and intense late-summer sun. They also discovered that deeper-water corals host other Symbiodinium types that are more sensitive to elevated temperatures and light and bleach more extensively during periods of ocean warming. "It seems that clade A Symbiodinium has an extraordinarily high capacity to control excess light energy by cycling electrons through atypical cyclic pathways,"...