Divers in the Caribbean, expecting a brilliantly hued seafloor, are sometimes shocked to find corals with pale, washed-out spots--a phenomenon called coral bleaching. Biologists know that bleaching is a response to stress. A rise in water temperature, for example, can kill the symbiotic algae that give corals their colors. But researchers haven’t been able to explain the peculiar patterns of the bleaching. Corals in shallow water might be fine, while a few feet deeper some corals are bleached on top, others only on the sides. Now new research explains how an overall change in water conditions can affect only certain parts of corals. Moreover, bleaching may not be a fatal blight, and corals may be more adaptable than scientists had thought.
Rob Rowan, who works at the Smithsonian Tropical Research Institute in Balboa, Panama, has a background in molecular genetics-- atypical for a marine biologist. I realized that although people studied corals a lot, very few studied the symbiotic algae that live inside them, he says. People really didn’t have a clue how many different species there were, which types lived in which corals--things that struck me as pretty fundamental. Conventional wisdom in biology holds that any one species of coral houses just one algal species, sheltering it in exchange for the sugars produced by the algae’s photosynthesis. But Rowan has found otherwise.
He studied DNA extracted from different spots on coral and identified up to three different species of alga in the same coral. He also noticed a pattern in their distribution. Two species, which he called A and B, preferred spots on corals with a lot of light. Species C was adapted to low light. Suddenly the bleaching patterns made sense. In shallow water, species C hugs the shaded sides of colonies. In deeper and darker water, C can stretch out over the tops of corals. But C ventures out of the shade at its peril. On top of coral it lives at the limits of its light tolerance, and any slight additional stress can kill it, leaving behind a bleached stretch.
Bleached coral, however, is not necessarily dead. The ones Rowan observed recovered after a few months when other symbiotic algae began to move in. This sets a precedent for the idea that corals are more resilient to climate change than people had believed, he says. Maybe the algae- coral associations we see today are optimal for the planet now. They might bleach and go through hell, but then they can look for new combinations and start up again.