Aug. 28, 2008 -- In the oceans, it's not just corals that feel the heat from climate change.
New research finds that marine sponges suffer at the same elevated water temperatures that cause corals to bleach, suggesting that coral reef ecosystems may experience broad effects in a warming ocean.
Coral bleaching is a well-known phenomenon predicted to occur in warming oceans. It happens when the single-celled algae living within the corals disappear, leaving only the white coral skeleton behind.
Because of the symbiotic relationship between the algae and the coral -- the algae provides up to 90 percent of the coral's food -- the corals die when the algae leave.
But Nicole Webster of the Australian Institute of Marine Science in Townsville noted that corals are not the only marine creatures that rely on a symbiotic relationship with microorganisms to survive.
Microorganisms can make up as much as 40 to 60 percent of the weight of some sea sponges. Meanwhile, clams, nematode worms and even starfish can have bacteria or other microbes living on or in them, although not enough research has been done in many cases to know whether the relationship is truly a "you-scratch-my-back-I'll-scratch-yours" symbiotic association.
Webster tested samples of a common sponge from the Great Barrier Reef, Rhopaloeides odorabile, a brown, velvety sponge that can range from fist-sized to the size of a large beach ball -- and whose name derives from the observation that out of the water it smells like bile. "It's a very stinky sponge," Webster said.
She placed pieces of the sponge in water of different temperatures and observed the effect on the sponge and the microorganisms over about a month.
"The health of the sponge was unaffected at 27, 29 and 31 degrees Celsius [81, 84, and 88 degrees Fahrenheit]," Webster told Discovery News. "But at 33 degrees [91 degrees F], within only three days we saw signs of stress."
"The tissue got necrotic and the skeleton could be seen coming out of the tissue," she added.
Moreover, one of the symbiotic microbes Webster monitored totally disappeared from the sponge within the first 34 hours at 33 degrees. When Webster analyzed the microbial population overall, she found no change until 33 degrees, at which point the normal microbes were lost, while populations of pathogenic bacteria associated with coral bleaching and coral disease increased.
"We aren't really sure whether the sponge stress was causing the symbiont [the microbes] to disappear or whether the sponge was losing the symbiont and that was causing the stress," she added. "At this point we don't really have a handle on which came first."
Thirty-three degrees Celsius is a temperature that is occasionally reached today in isolated pockets of the Great Barrier Reef. "With the temperature increases that are predicted, we will see it more frequently," Webster noted.
Intriguingly, 33 degrees is the same temperature threshold at which coral bleaching is observed.
"I guess the implications of that are, as we see temperature elevations, we're going to start seeing some breakdowns in microbial relationships with marine organisms," she said. "Microbes are a relatively unstudied group, yet they have a really important role for the function of many marine systems."
Russell Hill, a marine microbiologist at the University of Maryland Biotechnology Institute in Baltimore, agrees. His recent results suggest that microorganisms associated with sponges may help to convert nitrogen into a form that sponges and other organisms can use.
It's still highly speculative, Hill said, but the combined findings suggest that if the microbes experience stress at elevated temperatures, "that could affect the entire nitrogen cycle of the reef ecosystem."
"The big take-home message is how important it is to do more research on the microorganisms in the whole reef ecosystem," he added. "Probably the key to good functioning of these reef ecosystems lies in the microbes."
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