Seaweed growth on coral reefs is normally controlled by plant-eating fish, but in many parts of the world, overfishing has dramatically reduced populations of these consumers -- allowing the seaweed to dominate. Understanding these harmful chemicals and the seaweeds that produce them, however, could lead to development of new management techniques aimed at protecting fish that consume the most harmful seaweed. Protecting these herbivores could help reduce the pressure on coral, potentially allowing recovery of some endangered reefs.

"We were able to isolate some of the key molecules responsible for the harmful interactions between seaweed and coral," said Douglas Rasher, a graduate student in the School of Biology at Georgia Tech. "These molecules are active at very low concentrations, suggesting that they need only to be expressed on the surfaces of the seaweed in minute concentrations to have damaging effects when they are in contact with the coral."

A May 2010 PNAS study published by Rasher and School of Biology professor Mark Hay showed for the first time that chemicals on the surfaces of seaweed could harm coral. To assess the scope of the coral-seaweed interaction, the researchers followed up their initial study by investigating interactions between eight different species of seaweed and three species of coral growing in the waters off the Fiji Islands. In 79 percent of the interactions studied, the seaweed chemicals harmed the coral.

"Though some corals were more resistant than others, what we have shown is that these seaweeds are generally bad for corals," said Hay, who has been studying coral reefs for more than 30 years. "At some level, these seaweed molecules can definitely kill the corals. But at other levels, what they are probably doing is cutting off the options for reefs to recover by making these reefs unreceptive to newly-arriving coral larvae. It is difficult for juvenile corals to colonize and grow through a chemically-toxic layer of seaweed."

Research on the coral-harming chemicals were reported October 17th in the online Early Edition of the journal Proceedings of the National Academy of Sciences (PNAS). The research has been supported by the National Science Foundation, the National Institutes of Health and the Teasley Endowment at the Georgia Institute of Technology.

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