Scientists investigating the chemical warfare that takes place on Fijian coral reefs have found that one species of seaweed increases its production of noxious anti-coral compounds when placed in contact with reef-building corals. The study was published in Proceedings of the Royal Society B and it shows that as this seaweed competes chemically with the corals, its growth slows. The seaweed becomes more attractive to herbivorous fish, which increase their eating of the combating seaweed by 80 percent.
The findings are the first to show that seaweeds can increase their chemical defenses in answer to competition from corals. Whether such responses are common or exceptional, however, will take further study with a broader range or seaweeds and corals.
“The important takeaway is that competition between corals and seaweeds can cause dramatic changes in seaweed physiology, both in terms of their growth and their defense,” 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.”
Now a postdoctoral research associate at the Darling Marine Center at the University of Maine, Rasher collaborated with Mark Hay, a professor at the Georgia Tech School of Biology. The team used coral reefs as outdoor laboratories to study the chemical gestures that happens during coral-seaweed war. They also estimated how herbivorous fish influence the connections and the long term health of reefs.
“We previously found that chemical warfare is fairly common among seaweeds and corals, and that several seaweed species are particularly harmful to corals,” Rasher said. “This research explored the degree to which seaweed allelopathy – chemical warfare – is dynamic, how it changes in response to competition, and also whether competition changes the efficacy of other seaweed defenses used against herbivores.”
The study results also defy the earlier held perception that plants cannot change quickly and strategically in response to their environments.
“We tend to think of plants as being fixed in their behavior,” said Hay. “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.”
Two seaweed species were selected: Galaxaura filamentos – which is known for its toxicity to corals – and Sargassum polycystum – which does not chemically damage corals. Pieces of a common coral, Porites cylindrica, were fragmented, then glued into cement cones and placed on a rack on a reef located in the shallow ocean off the coast of the Fiji islands. The coral fragments grew in the racks for two years.
To start the experiment phase, the researchers dipped half the corals in bleach to kill the living organisms and leave the calcium carbonate skeletons. These served as a control group for the experiments.
Samples from both species of seaweed were collected, and then divided in two with half of each sample being assigned as either a treatment group or a control group. Treatment seaweeds were placed with the living corals, and the control seaweed group was placed with the coral skeletons. Both groups were allowed to interact with their coral hosts for eight days, then a portion were removed and chemical compounds were extracted. These compounds were embedded into small gel strips that were then adhered to other living corals. This allowed the researchers to test the toxicity of the compounds.
“We saw that Galaxaura, the chemically rich seaweed and the species we knew was allelopathic, had up-regulated its chemistry to become more potent – nearly twice as damaging – when it was in contact with the living coral, compared to those individuals that had only been in contact with the coral skeletons,” Rasher said. Sargassum extracts, on the other hand, did not damage the corals.
Up to this point, the researchers kept the seaweed and corals separated from herbivorous fish. Seaweed samples, both those on living corals and those not, were placed on nylon ropes in a location available to fish. Fifteen pairs of these samples were placed at different locations.
“We saw that for the non-allelopathic seaweed, Sargassum, fishes didn’t differentiate – they consumed both the treatment and control seaweeds at equal rates,” Rasher said. “But given the option to choose between treatment and control Galaxaura, fishes consumed 80 percent more of the seaweed portions that had been in contact with a living coral.”
All the factors that might have played into making the chemically venomous seaweed more edible to fish are unknown, however those seaweed portions that had been competing with coral had less effective chemical defenses against fish. Extracts were taken from both the treatment groups and the control groups, and applied to a palatable seaweed species not previously used in the testing. Fish preferred the seaweed coated with extracts from the portions that had been competing with corals. This suggests that competition had compromised the seaweed’s chemical defenses against herbivores.
The researchers want to focus future efforts on studying chemical defenses in other seaweeds to conclude if what they’ve seen is typical among tropical seaweeds engaging in chemical warfare. It is still unknown if the chemical defenses evolved to compete with coral or to fight off harmful microbes.