May 9, 2007

Global warming linked to White Syndrome in corals

Global warming is not only stripping corals of their food source, it is opening the door to rapid, widespread infection.

A group of researchers (led by a Dr. John Bruno) published a paper in PLoS Biology this week looking into a possible correlation between the spread of white syndrome among schleractinian corals and warmer temperatures at the Great Barrier Reef (GBR). The study found a positive correlation between warmer temperatures and outbreak.

More on schleractinian corals and how global warming is affecting them below the fold.

Schleractinian corals - such as the brain coral pictured - are known as "reef-builders"; literally, reefs exist because of this order of animals. Most of the schleractinians found on shallow reefs like the GBR are zooanthellates, meaning their tissues are home to tiny, dinoflagellate algae symbiotes that photosynthesize sugars for the corals (zooanthellae). The pigments of the symbiotes is also responsible for color

Which brings us to an important distinction: White syndrome is not bleaching. Coral bleaching is the result of the expulsion of these zooanthellae from the coral's tissues due to stress. This stress can be caused by any number of factors, but recent evidence ties the proliferation of bleaching to global warming.

Warming ocean temperatures are disrupting seasonal growth. The prolonged warm temps keep the zooanthellae from proliferating and subsequently the corals from producing adequate body mass in the winter, stressing the organism's physiology and leaving them susceptible to infection. The temps also seem to cause rapid growth in the pathogen.

The pathogenic cause(s) of white syndrome are virtually unknown, though Bruno and his colleagues suspect that it may have similarities to the "white diseases" found in corals in the Caribbean, which are caused by a group of pathogens.

The white syndrome was previously thought to be caused by a bacterial infection, but researchers back in March announced that they had found evidence that the corals were somehow committing suicide; their cells were prematurely going into programmed cell death (PCD). At the time, one of the researchers from the ARC Centre of Excellence for Coral Reef Studies had an idea about what was causing this:

"I think the smoking gun is climate change. We have had a series of hot summers recently in which corals in the shallows become bleached and literally 'worn out'. This leads to mechanisms that allow the coral to retract tissues that are no longer functioning due to this stress," says Professor Hoegh-Guldberg. "This is somewhat similar to Eucalyptus trees dropping branches when they run short of water."

This study would seem to support his assertion. Samples taken of infected corals at the GBR have been collected and are currently being studied.

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