Seaweed launches chemical warfare on corals

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While many SPS coral keepers will be familiar with the chemical warfare that can occur between their prized corals, known as allelopathy, it might come as a surprise to learn that one of the bigger dangers might be seaweed!

In recent decades many tropical reefs have suffered from corals bleaching and a subsequent slow recovery of bleached coral species. Now, research reported in the online Early Edition of the journal Proceedings of the National Academy of Sciences (PNAS), shows that chemicals found on several species of macroalgae is capable of killing or inhibiting the growth of a range of corals, including one of the more strongly affected species of bleaching – Acropora.  

Conducting experiments in waters off the Fiji Islands, scientists have been able to identify and map the chemical structure of the molecules used by the macroalagae in question to kill or inhibit reef-building coral.

The chemicals, identified as acetylated diterpenes and loliolide derivatives, are from a class of organic compounds known as terpenes and are found on the surfaces of the seaweed. These potent allelochemicals were extracted from the red alga Galaxaura filamentosa, and the green alga Chlorodesmis fastigiata .

"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."

Scientist Professor Mark Hay who has been studying coral reefs for more than 30 years says: "Though some corals were more resistant than others, what we have shown is that these seaweeds are generally 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 colonise and grow through a chemically-toxic layer of seaweed."

Typically, seaweed growth on coral reefs is controlled by herbivorous fishes, but overfishing has dramatically reduced numbers of some fish and the seaweed has been able to dominate as a result.  

"We hope that this information will inform the Fijians to help them make decisions about fisheries management that could help protect the reefs," said Rasher. "We hope to give them scientifically-guided management tools for maintaining healthy reefs, or for restoring degraded reefs suffering from local human disturbance."

For more information see the paper: Macroalgal terpenes function as allelopathic agents against reef corals by Douglas B. Rasher, E. Paige Stout, Sebastian Engel, Julia Kubanek, and Mark E. Hay.

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