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  EurekAlert 26 Apr 06
Coral Reef Resilience: Better Feeders Survive Bleaching

EurekAlert 26 Apr 06
Tiny polyps gorge themselves to survive coral bleaching
Nature

Columbus, Ohio: Certain species of coral have surprised researchers by showing an unexpectedly successful approach towards survival when seriously bleached. Their innovative strategy is gluttony.

The discovery, derived from experiments on coral reefs in Hawaii , provides new insights into how these tiny animals face a multitude of environmental threats. The report by Ohio State University researchers is published in the current issue of the British journal Nature.

During the past decade, reports have multiplied of major bleaching events that have damaged, if not destroyed, large portions of the world's fragile coral reefs. Scientists point to global warming as the cause and the victims are some of the tiniest creatures near the base of the undersea ecosystem.

Despite the apparent sturdiness of coral reefs, the creatures themselves are quite fragile. These tropical organisms survive in a narrow 4-to-6-degree C temperature range centered about 26 degrees C.

While the exact temperatures vary with individual species from location to location throughout the tropics, they all must live within that tight range. When the temperature climbs above that range, even by only two degrees, the result is a bleaching event.

Within a two-year window during the 1997-98 El Nino event, 16 percent of the world's coral reefs sustained serious bleaching due to increases in seawater temperature and the animals died.

"If the rain forests were dying off at this rate, we would all be panicking," explained Andrea Grottoli, an assistant professor of geological sciences at Ohio State and lead author of the study.

"The problem is that now, with the planet's climate warming, coral are living closer and closer to their thermal threshold, so it takes less of a warming event than it did before to cause a catastrophe."

Coral are symbiontic organisms that host one-celled algae within their bodies for mutual benefit. The coral polyp, a relative of jellyfish and anemones, provides a safe home within its cells for the algae while the algae convert sunlight into energy for the polyp Grottoli said that when the temperature of the waters around a reef exceeds that upper limit and stays there for more than two weeks or so, it triggers a bleaching event.

Once that happens, the symbiotic algae and the brown or green photosynthetic pigments inside are lost. The result is a 'bleached' white coral.

"In most cases, corals get 100 percent of their daily metabolic energy needs from the algae. Once they are gone, the coral polyp is left with only two alternatives: Draw energy from stored fats within its body, or eat organic matter and plankton in the surrounding water," she said.

But what has puzzled Grottoli and other researchers is why in some bleaching events, some corals quickly died off while others close by were able to recover.

To answer that, she returned to Hawaii Institute of Marine Biology where she has been studying corals for the past 13 years. There, she and her collaborators focused on two types of common coral that thrived on the local reefs, Montipora capitata, or 'rice' coral, and Porites compressa, 'finger' coral. They collected samples of both types and placed them in sets of tanks supplied with natural seawater. Water from the reef was filtered to remove any plankton and flowed through the tanks in the same way it did through their natural environment. In one set of tanks, the water was heated, mimicking the rising temperatures leading to a bleaching event. After a month, fragments of the coral were gathered from all of the tanks and put through a series of tests measuring energy reserves, photosynthetic rates and growth rates of the coral.

The results showed that both Porites and Montipora used up their internal energy reserves. However, after a month of recovery on the reef (where plankton is naturally available) Porites continued to use up its reserves while Montipora had somehow managed to completely replenish them.

To explain that, Grottoli and colleagues closely examined the bleached and healthy corals of the two species on the reef.

" We let them feed for one hour," Grottoli said. "Then we harvested them all, dissected each polyp and counted how many zooplankton each had eaten, how big they were and what species. That told us how much the coral had eaten."

Surprisingly, the researchers discovered that while the bleached Porites fed at its normal rate, bleached Montipora had increased its rate of feeding more than five-fold, allowing it not only to survive and repair but also replenish its internal energy reserves.

"We think that this means that coral like Montipora can switch how it gets its food so that it can sustain itself in a bleached state much longer than can corals like Porites," she said. "While bleached Porites is limited by how much energy reserves it has, bleached Montipora is not."

That's good news for Montipora and corals like it as the frequency, duration and intensity of warming events increases globally. But Grottoli warns that Montipora's resilience doesn't diminish the threat that bleaching events hold for the world's coral reefs. While it might survive while other species may not, on a global scale it is unlikely to re-colonize areas where less-resilient species died.

"Recent projections suggest that with the current rate of warming, as much as 60 percent of the world's coral reefs could be lost within the next 10 to 30 years," she said. "We have a delicately balanced ecosystem that is already highly stressed. It is very much interconnected and so far, we have royally messed it up."

Along with Grottoli, her graduate student Lisa J. Rodrigues, (now a postdoctoral fellow at Villanova University), and undergraduate student James E. Palardy, (now a PhD student at Brown University), also worked on the project. Support for this research came in part from the Andrew Mellon Foundation and the National Science Foundation.

EurekAlert 26 Apr 06
Coral Reef Resilience: Better Feeders Survive Bleaching

Global warming and other threats are killing coral reefs through a phenomenon known as bleaching. But why do some corals survive?

A new study, published in Nature, is the first to document a trait that helps some coral species live through, and recover from, bleaching. The survivors' secret: Ramped up feeding rates.

PROVIDENCE, R.I. Coral bleaching, a stress response that turns rainbow-hued reefs into bone-white graveyards, is damaging reefs worldwide. But some corals survive.

A Nature report identifies a new trait critical to coral resiliency--the ability to kick feeding rates into overdrive.

In an experiment with three species of Hawaiian corals researchers found that, when bleached, the branching coral Montipora capitata sharply increased its intake of tiny plankton, making it much more likely to bounce back. The findings suggest that any coral, regardless of shape or location, may recover from bleaching if it can ramp up feeding.

James Palardy, a Brown University graduate student and co-author of the Nature paper, said the results indicate that these corals may become the dominant species in reefs and could play a role in protecting these critical marine ecosystems.

"These 'super-feeders' have an ecological advantage, one that may protect reefs from extinction," Palardy said. "If our results hold up with other species, we may well see that these resilient corals are the future for our reefs."

Coral reefs reduce beach erosion, support tourism and serve as breeding grounds and habitat for fish. A 2006 report by the United Nations Environment Programme put the value of coral reefs at $100,000 to $600,000 per square kilometer per year.

But the UNEP report states that 30 percent of the world's coral reefs are severely damaged or dead and that 60 percent of remaining reefs will vanish by 2030. Several factors are to blame, from pollution to overfishing.

Scientists say the biggest new threat is global warming. Because corals are highly sensitive to temperature, even small amounts of warming can trigger bleaching. When water temperatures rise, coral expel single-celled algae called zooxanthellae, which live inside coral tissue and give corals their color and, more importantly, supply the bulk of their food energy. If bleaching persists, corals die, leaving behind ghostly limestone skeletons.

Some corals can survive bleaching. The reasons for this, however, aren't well understood.

Palardy and his colleagues had a hunch: In the absence of algae-derived nutrition, corals may tap energy reserves or increase feeding, a process where corals use their tiny tentacles to 'grab' passing plankton and stuff them into their stomachs.

To study the role metabolism and feeding might play in coral resiliency, Palardy, a Brown Ph.D. student in the Department of Ecology and Evolutionary Biology, collaborated with Andréa Grottoli, an assistant professor of geological sciences at Ohio State University, and Lisa Rodrigues, a post-doctoral research fellow in biology at Villanova University.

The team took chunks of three types of healthy Hawaiian corals--M. capitata, another branching coral called Porites compressa and the mounding coral Porites lobata--from colonies off the coast of Oahu and put them in eight outdoor tanks.

Water in four of the tanks was kept at 27° C, the typical reef water temperature. In the other four, water temperature was elevated to 30° C, warm enough to trigger bleaching. After 30 days in the tanks, the team measured chlorophyll concentrations, photosynthetic rates, and lipid levels in some corals. The remaining corals were returned to the reef to recover.

After two weeks on the reef, researchers covered some of the healthy and bleached corals with fine mesh boxes, which kept plankton out of reach. The boxes went over the corals eight hours a day, then removed for an hour each night for five days.

The goal: Empty their stomachs, so that the plankton the corals consumed could be accurately measured. Researchers dissected the corals and painstakingly counted plankton in their stomachs.

Researchers also created a system of measurements that gauged the corals' energy input from feeding.

Four weeks later, the scientists weighed the remaining corals and again measured chlorophyll concentrations, photosynthetic rates and lipid levels.

The results: P. compressa and P. lobata depleted their energy reserves during bleaching. And when these bleached and unbleached corals were compared, feeding didn't increase.

In contrast, the feeding rates of bleached M. capitata increased five-fold, allowing them to replenish their energy reserves--making it more likely that they'll survive and spawn after a bleaching event.

"The results were a surprise," Palardy said. "Previous studies showed that thick tissue layers or mounded shapes made corals resilient. But we found a new resiliency factor--feeding. In evolutionary terms, corals that eat more may win."

The National Science Foundation, the Mellon Foundation, and a William Penn Fellowship funded the work.
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