Seaweeds are the 'tricky chemical assassins' of the reef


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One species of seaweed has been found capable of increasing its production of anti-coral compounds to compete with reef-building corals — but at a price.

Researchers examining the chemical warfare taking place on Fijian coral reefs have found that Galaxaura filamentosa — a species of seaweed known for its toxicity to corals — increases its production of noxious anti-coral compounds when placed into contact with reef-building corals. But as it competes chemically with the corals, the seaweed grows more slowly and becomes more attractive to herbivorous fish, which boost their consumption of the skirmishing seaweed by 80 percent.

This appears to be the first demonstration that seaweeds can boost their chemical defences in response to competition with corals. However, determining whether such responses are common or rare awaits additional studies with a broader range of seaweeds and corals.

"Competition between corals and seaweeds can cause dramatic changes in seaweed physiology, both in terms of their growth and their defines," said Douglas Rasher, who was a graduate student at the Georgia Institute of Technology when the research was conducted. "These changes have potentially cascading effects throughout the rest of the reef community."

The findings may also challenge the popular notion that plants cannot change rapidly and strategically in response to their environments.

"We tend to think of plants as being fixed in their behaviour," said study co-author Mark Hay, a professor in the Georgia Tech School of Biology. "In fact, plants such as these seaweeds assess their environment continuously, altering biochemically what they are doing as they compete with the coral. These algae somehow sense what is happening and respond accordingly. They may appear passive, but they are really the tricky chemical assassins of coral reefs."

An experimental coral rack was deployed on a shallow reef within the marine reserve at Totua, Viti Levu, Fiji and used for research into chemical warfare between seaweeds and corals. Fragmented pieces of a common coral, Porites cylindrica, were glued into cement cones and placed on the rack where they were allowed to grow for two years.

At the start of the experiment, half the coral samples were dipped into bleach to kill the living organisms, leaving only the calcium carbonate skeletons which then served as the control group.

The seaweeds were then allowed to interact with the corals and coral skeletons for eight days.

Studies showed that Galaxaura became more potent — nearly twice as damaging — when it was in contact with living Porites cylindrica coral, compared to that which had only been in contact with coral skeletons.

However, researchers also found that herbivorous fish consumed 80 percent more of the seaweed portions that had been in contact with a living coral. In addition, when they took extracts from treatment seaweed and control seaweed and applied them to a palatable seaweed species not previously used in the experiment, they also found that fish preferred the seaweed coated with extracts from the portions that had been competing with corals, indicating that competition had compromised the seaweed’s chemical defences against herbivores.

The researchers now want to study chemical defence in other seaweeds to determine if what they’ve seen is common among tropical seaweeds that engage in chemical warfare. For now, they don’t know if the chemical defences evolved to compete with coral or perhaps for another reason, such as fighting off harmful microbes.

The fact that corals may cause seaweeds to increase their anti-coral defences could help explain why coral reefs rarely bounce back once they begin a decline and become dominated by seaweeds. The research also demonstrates the importance of studying broad interactions among numerous species within complex communities like coral reefs.

The study was published in the journal Proceedings of the Royal Society B. 

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