Certain Species Of Seaweed Can Harm Corals
Scientists studying coral reefs in and around the South Pacific have found that some seaweed can cause coral bleaching and suppress photosynthesis by emitting anti-coral chemicals.
As part of their study, researchers have identified and mapped the chemical structure of molecules used by certain types of marine seaweed that kill or inhibit 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 the algae could be a factor in the worldwide decline of coral reefs.
During the long-term study, scientists observed numerous coral reefs being affected by the toxic seaweed. But experts have not been sure if the algae contributed to coral decline or benefited from it.
“Now our research suggests that, once corals decline due to a combination of global and local stresses, some seaweeds use chemical warfare to suppress the recovery of remnant adult corals and new coral recruits,” study co-author Douglas Rasher, a doctoral candidate at the Georgia Institute of Technology in Atlanta, told Brian Handwerk of National Geographic News.
Rasher said it is unlikely that seaweed developed chemical weapons specifically to attack coral, since interactions between the two organisms have been historically infrequent on unspoiled reefs.
“We hypothesize that [seaweeds] may have evolved to produce these compounds on their surfaces in response to competition with microbes or attacks from [grazing animals], which they´ve been fighting for millions of years,” he explained to Handwerk.
“It´s only in ecologically recent times–perhaps over the last 30 or 40 years–that seaweeds have begun to have an impact on corals,” he said.
The seaweed takeover on reefs could be a result of two key forces: agricultural and sewage runoff. Both sources dump large amounts of nutrients on near-shore reefs, stimulating algae growth.
Seaweed growth on coral reefs is normally controlled by plant-eating fish, but in many parts of the world, overfishing has significantly reduced fish populations, allowing seaweed to flourish, and, dominate the coral reefs.
Understanding these chemicals and the seaweeds that produce them could lead to development of new management techniques aimed at protecting fish that consume the most harmful seaweed. Protecting these fish could help reduce the pressure on coral, potentially allowing endangered reefs to recover.
Study co-author, Mark Hay, a marine ecologist at the Georgia Institute of Technology in Atlanta, said that the reefs dotting Fiji´s coastline, where the study was undertaken, were in a battle royal against toxic seaweed. In areas where fishing is restricted, coral dominates. But in those areas where Fijians fish most, the seaweed flourishes, extending their tendrils over nearly 60 percent of the ocean floor, turning the waters around them sludgy green.
Hay said such “seaweed-covered parking lots” are not just unique to Fiji. Recent studies have hinted that this phenomenon may be widespread.
Rasher and Hay´s study, reported in the online Early Edition of the journal Proceedings of the National Academy of Sciences (PNAS), has been supported by the National Science Foundation, the National Institutes of Health and the Teasley Endowment at the Georgia Institute of Technology.
To investigate the covert struggle between the reef and seaweed, the authors strung 8 different species of Fijian seaweed across growing corals, including A. millepora colonies. They found that many of these algal species wielded a poisonous touch.
They found that, in less than 2 weeks, the test coral often began to discolor and even die where it rubbed against the seaweeds. Using fake seaweeds made of plastic the team found no such effect, adding weight to the toxic seaweed theory.
The team then picked apart the seaweeds to identify their killer make-up. What they discovered was a key chemical ingredient found in each of the specimens called terpenes, which some algae use to sicken fish that feed on them. Terpene extracts alone killed off corals, they found.
But it seemed that some algae were more liberal with their toxins than others, noted Hay. When one exceptionally nasty specimen called turtle weed rubs against A. millepora, for instance, wide bands of dying tissue encompass the coral´s affected areas.
Turtle weed is so nasty that most marine herbivores avoid it on sight — except for a species of rabbitfish that quivers with excitement whenever it spots the not-so-common algae. Hay noted that if Fijians develop a taste for this one species of rabbitfish, turtle weed may begin to grow out of control and dominate the corals around Fiji, and perhaps, the rest of the world.
Hay would like to work with Fijians to identify and protect the fish that are most responsible for staving off deadly growth of toxic seaweeds, giving sensitive corals a chance to rebound.
“It´s certainly a novel finding,” John Bruno, a marine ecologist at the University of North Carolina, Chapel Hill, told Daniel Strain from ScienceNOW. But not all seaweeds are poisonous, he added.
Many experts argue that algae — toxic or not — rarely kill off corals on a mass scale. Instead, these opportunistic organisms may simply be capitalizing on the slow death of the invertebrates due to pollution, climate change, or other factors.
Bruno said, however, that the seaweeds Rasher and Hay studied would likely be exceptionally toxic to young corals that have yet to grow to their massive sizes.
Jennifer Smith, a marine ecologist at the University of California, San Diego, told ScienceNOW that terpenes from seaweed are almost certainly not the only reason for the mysterious global decline of corals. Overfishing, pollution, and global warming are among the biggest overall contributors.
Smith and her colleagues found in a 2006 study that some California algae could also dominate coastal waters and choke off corals. 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 told ScienceNOW.
In Rasher and Hay´s study, they found the seaweed harmed coral on 79 percent of the interactions studied.
“Though some corals were more resistant than others, what we have shown is that these seaweeds are generally bad for corals,” Hay, who has been studying coral for more than 30 years, said in a recent press release. “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.”
The researchers hope to learn more about the toxic compounds in the seaweeds studied and how they evolved. They also want to identify all the species of fish that consume the toxic 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.”
“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.”
Image 1: Healthy corals in the Votua Marine Protected Area of the Fiji Islands are shown in this photo. Georgia Tech researchers are studying chemicals produced by seaweed species that can harm coral. Credit: E.H. Hay
Image 2: Scientist Douglas Rasher prepares small coral colonies for seaweed-coral competition experiments on a reef in Fiji. Credit: M.E. Hay
Image 3: By embedding seaweed chemicals in gel pads on window screen and placing these on corals in the field, researchers identified seaweed molecules that damaged corals. Credit: D.B. Rasher
Image 4: An Acropora coral showing dead branches near the green alga Chlorodesmis. This seaweed chemically damages corals upon contact. Credit: D.B. Rasher
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