At first glance, the colonies that make up coral seem like a harmonious household. Tiny invertebrates provide steadfast shelter for microscopic algae which, in return, share the food they photosynthesize. But in tomorrow's Nature, ecologists report that some corals are unforgiving landlords: When conditions worsen, they evict algae that can't cope to make room for those that can. This flexibility could help protect some coral if ocean temperatures rise.
Rob Rowan, a coral reef ecologist from the University of Guam, was analyzing the microscopic algae, called zooxanthellae, living in two common Caribbean corals, Montastraea annularis and Montastraea faveloata, to determine what alga associated with each coral. But when he and his colleagues sat down with the data, they found that these massive corals sometimes hosted not just one, but up to three kinds of zooxanthellae.
The distribution varied according to the environment. When the researchers surveyed the three types of algae found in 46 lumps of coral, they found that two are more common in coral in shallow water, with its bright light and warm water. The third thrives in coral in deeper, darker water, although shaded coral anywhere was just fine. Removing some of this shade, the researchers discovered, to their surprise, that the dark-adapted zooxanthellae were ejected over 6 months.
This change also happens in natural conditions. The team measured changes in the prevalence of algae species in 1995, when unusually warm weather stressed the corals and caused them to expel the algae, leaving just the white skeleton behind. Only those corals where the deep-water zooxanthellae predominated turned white. The rest adapted by welcoming zooxanthellae that did better in shallow, warmer water.
Rowan says that this versatility could help coral cope with the temperature rises of global warming, which some scientists feared would bleach corals worldwide. "The resiliency that can be built [into a community] by having different [algal] taxa and different hosts is very powerful," comments Robert Buddemeier, an environmental scientist with the Kansas Geological Survey, Lawrence.