19 October 2011

Coral-killer seaweeds and fishes that eat them

Seaweeds can contain chemicals that harm corals, and a study has for the first time identified these chemicals. These chemicals can prevent young corals from growing and can make adult corals ill. Thus making it difficult for reefs to grow and recover.
Rabbitfish (Family Siganidae) trapped in driftnet
One of the seaweed-eating fishes found on our shores,
a rabbitfish is caught in a driftnet found on Tuas, Sep 2011.
Seaweeds are usually controlled by seaweed-eating fish. Thus overfishing of such fish can affect the health of reefs.

Chemical Warfare On the Reef: Molecules Used by Certain Seaweed to Harm Corals
ScienceDaily 17 Oct 11;

Scientists for the first time have identified and mapped the chemical structure of molecules used by certain species of marine seaweed to kill or inhibit the growth of reef-building coral. Chemicals found on the surfaces of several species of seaweed have been shown to harm coral, suggesting that competition with these macroalgae could be a factor in the worldwide decline -- and lack of recovery -- of coral reefs.

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.

Research on the coral-harming chemicals will be 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.

"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."

In the 2010 study, the researchers determined that seaweed harmed coral only when their surfaces touched. That meant the harmful compounds were likely hydrophobic chemicals that dissolved in oil rather than water.

To identify the specific harmful compounds, the researchers produced extracts from the surfaces of the two most harmful seaweeds. Using a technique called bioassay-guided fractionation, they divided up compounds in these seaweed extracts by the degree to which they could be dissolved in oils versus water, or by their size.

They then placed gels containing the extracted compounds into contact with the most sensitive coral, Acropora. The coral samples had been placed into metal racks located on healthy coral reefs near Votua Village in the Fiji Islands. Similar gels not containing the extracts were used as controls -- and produced no effect on the coral.

By studying the effects of the extracts on the coral with a pulse-amplitude-modulated fluorometer to assess changes in coral photosynthesis, the researchers narrowed the list of suspect molecules. From the two most chemically-damaging seaweed species, the researchers isolated and identified four toxic molecules for detailed chemical analysis. Those compounds, identified as acetylated diterpenes and loliolide derivatives, are from a class of organic compounds known as terpenes.

The four molecules -- two from a green alga known as Chlorodesmis fastigiata and two from a red alga known as Galaxaura filamentosa -- were brought back to Georgia Tech for further study. They were analyzed by Julia Kubanek, a professor with joint appointments in the School of Biology and the School of Chemistry and Biochemistry, and by graduate student Paige Stout. Kubanek and Stout, who were also involved in a related project aimed at discovering potential drug compounds on the coral reefs, mapped the structures of the toxic molecules.

For the future, the researchers hope to learn more about the compounds and how they evolved in seaweed. Contact between seaweed and coral would have been limited on pristine reefs, so Hay and Rasher believe the molecules may have evolved as part of defense against microbes or herbivorous fish. They also want to identify the species of fish that consume the seaweeds.

"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."

Coral is impacted by both global and local stressors. The global problems, including changing ocean pH and rising temperatures, can't be controlled locally. But issues such as overfishing and pollution can be controlled, and easing those local stresses may help the coral survive the global challenges.

"It's becoming clear that the problem for coral is not just one factor," said Rasher. "The decline of coral reefs results from a complex interaction between many factors. Our study shows that regardless of what factors are driving coral decline, once algae become established, they can suppress the recovery of coral."

Journal Reference:

Douglas B. Rasher, E. Paige Stout, Sebastian Engel, Julia Kubanek, and Mark E. Hay. Macroalgal terpenes function as allelopathic agents against reef corals. PNAS, 2011 DOI: 10.1073/pnas.1108628108

Toxic Seaweed Poisons Coral Reefs on Contact
Daniel Strain ScienceNOW 17 Oct 11;

"Attack of the killer seaweed" may sound like a cheesy horror flick, but for many coral species, murderous multicellular algae have become real-life villains. A new study of reefs in the South Pacific suggests that some algae can poison coral on contact. This chemical warfare may be increasing the pressure on struggling reef communities worldwide, researchers say.

Along the reefs dotting Fiji, overfishing has pitted corals against algae in a battle royale. On swaths of coastline where fishing is restricted, corals such as the tall and branching Acropora millepora rule, says study co-author Mark Hay, a marine ecologist at the Georgia Institute of Technology in Atlanta.

But where Fijians spear lots of herbivores such as bird-beaked parrotfish, few fish remain to prune back the region's seaweeds, a blanket term for many types of big algae. These algae then creep in, extending their tendrils over close to 60% of the ocean bottom, Hay estimates, and turning waters a sludgy green. Such "seaweed-covered parking lots" aren't unique to Fiji, either, he says.

Recent studies have hinted that this ocean greenery may be carrying out a subtle chemical war on sensitive reefs. To investigate this covert struggle, Hay and colleagues strung eight different species of Fijian seaweed across growing corals, including A. millepora colonies. True to the researchers' suspicions, many of these algal species seemed to wield a poison touch. In less than 2 weeks, the test coral often began to discolor and even die where it rubbed against the seaweeds, the team reports today in the Proceedings of the National Academy of Sciences. Faux seaweeds made of plastic had no such effect.

Hay and colleagues then mashed up several of these seaweeds to identify their killer concoction. The key ingredient turned out to be chemicals called terpenes, which some algae use to sicken fish that feed on them. Terpene extracts alone killed off corals, the researchers found. But some algae seemed to be more liberal with their toxins than others, Hay notes. When one particularly nasty specimen called turtle weed (Chlorodesmis fastigiata) rubs against A. millepora, for instance, wide bands of dying tissue girdle the coral.

This seaweed is so nasty, in fact, that most marine herbivores avoid it on sight—except for one species of rabbitfish that quivers with excitement every time it spots this not-so-common algae. That interaction highlights the importance of prudent fishing practices, he adds. If Fijians developed a particular taste for that one rabbitfish, for instance, turtle weed might begin to grow out of control, launching its bid for world, or at least South Pacific, domination. Hay would like to work with Fijians to identify and protect the herbivores most responsible for trimming back deadly seaweeds, giving sensitive corals a fighting chance.

"It's certainly a novel finding," says John Bruno, a marine ecologist at the University of North Carolina, Chapel Hill. But not all seaweeds are poisonous, he adds. Many scientists argue that algae—toxins or no—rarely kill off adult corals en masse. Instead, these opportunistic organisms may simply be capitalizing on the slow death of the invertebrates due to pollution, climate change, or other factors. He adds, however, that the seaweeds Hay and colleagues studied would likely be exceptionally toxic to young, coin-sized corals that have yet to grow big and hale.

Terpenes from seaweed are almost certainly not the only reason for the mysterious global decline of corals, says Jennifer Smith, a marine ecologist at the University of California, San Diego. Most scientists rank overfishing, pollution, and warming oceans among the biggest overall contributors. But corals may suffer from other nasty tricks played by seaweed. In a 2006 study, Smith and colleagues sleuthed out that some California algae could take the epidemic route to domination. In the lab, these seaweeds leak huge quantities of dissolved carbon that then fuels the spread of potentially infectious microbes on coral surfaces. "You can imagine that [algae and corals] have evolved over the years different mechanisms for battling each other and fighting these turf wars," Smith says.

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