More news on bleaching

[Konadog]

http://www.sciencefriday.com/news/051107/news0511071.html

HEALTHY CORAL HIT HARDEST



A coral colony with white syndrome from the Great Barrier Reef. Credit: AIMS Long Term Monitoring Program
The healthiest coral reefs may be the most at risk for a disease sweeping the Indo-Pacific ocean. Outbreaks of “white syndrome,” a recently-discovered disease that attacks coral, are more likely to strike at times when sea surface temperature is above average and at places where the reef is flourishing with high coral cover, according to a new study in the journal Public Library of Science (PLoS) Biology.

“We unfortunately don’t know what the pathogen is,” says John Bruno, a marine ecologist from the University of North Carolina at Chapel Hill and co-author of the study. He says that some scientists suspect white syndrome is a type of bacteria because it behaves similarly to bacteria that wiped out coral in the Caribbean in the 1980s (dubbed “white band”).

First discovered in 1998 on the Great Barrier Reef off Australia, white syndrome slides over reefs, leaving a white ribbon of dead coral in its wake. “The band will move right across the colony chewing up the coral tissue,” Bruno says. This disease prompts coral to spit out their inhabitantsâ€"zooxanthellae (pronounced zo-zan-thell-ee), “single-celled plant-like animals that live in the coral tissue symbiotically,” Bruno explains. Without the zooxanthellae, all that is left is the white calcium carbonate skeleton.

An attack of white syndrome doesn’t necessarily kill the whole colony, but it does kill the infected tissue. Bruno says not much is known about how the coral recover from white syndrome, “mainly because this just started happening and we haven’t geared up to study it in depth yet.”

White syndrome is exacerbated by heat, not to be confused with coral bleachingâ€"which is not a disease but also a threat to corals induced by warming temperatures. White syndrome's frequency increased 20-fold during 2002, a particularly warm year. Bruno says: “The increase in temperature that we’ve been seeing in the last decade or so is combining with new diseases that have been appearing. And it essentially has been exacerbating them, making them more severe. There is a synergism between these two stressors.”

Particularly distressing to reef managers, Bruno says, is that the healthy coral reefs seem to be most susceptible to this disease. A reef’s health is measured by coral coverâ€"the amount of coral covering the bottom. “That’s kind of the surprising result and the irony, that the reefs where we’re doing the best job mitigating these other stressors like nutrient pollution, sediment run off and overfishingâ€"that’s where we’re seeing the big outbreaks.”

Bruno says one explanation for the vulnerability of healthy reefs is that the tightly packed reefs are also the places where corals are fighting most competitively for space. “Space to attach on to the bottom is their primary limiting resource,” Bruno says. “If you go out on a reef at night, it’s just a blood bath. They literally digest each other’s tissue right off of the skeleton.” The battle wounds may make the coral more susceptible to the disease.

-Flora Lichtman