Coral reefs, whose decline was earlier linked with coastal pollution, global warming and sunscreen, are majorly affected by the chemicals released from seaweed.

Scientists have identified and mapped the chemical structure of molecules used by certain species of seaweed to inhibit the growth of reef-building coral.

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, he said.

Research on the coral-harming chemicals was reported Tuesday in the online Early Edition of the journal Proceedings of the National Academy of Sciences (PNAS).

According to the researchers, some of the corals are more resistant than others but seaweeds are bad for corals.

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, they added.

The scientists using a technique called bioassay-guided fractionation, divided up compounds in these seaweed extracts by the degree to which they could be dissolved in oils versus water, or by their size.

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