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 Now, a new study shows a surprisingly large share of those so-called nucleators turn out to be bacteria that can affect plants.
"Bacteria are by far the most active ice nuclei in nature," said Brent C. Christner, an assistant professor of biological sciences at Louisiana State University.
Christner and colleagues sampled snow from Antarctica, France, Montana and the Yukon and they report their findings in Friday's edition of the journal Science.
In some samples as much as 85 percent of the nuclei were bacteria, Christner said in a telephone interview. The bacteria were most common in France, followed by Montana and the Yukon, and was even present to a lesser degree in Antarctica.
The most common bacteria found was Pseudomonas syringae, which can cause disease in several types of plants including tomatoes and beans.
The study found it in 20 samples of snow from around the world and subsequent research has also found it in summer rainfall in Louisiana.
The focus on Pseudomonas in the past has been to try and eliminate it, Christner said, but now that it turns out to be a major factor in encouraging snow and rain, he wonders if that is a good idea. Would elimination of this bacteria result in less rain or snow, or would it be replaced by other nuclei such as soot and dust?
"The question is, are they a good guy or a bad guy," he said, "and I don't have the answer to that."
What is clear is that Pseudomonas is effective at getting moisture in a cloud to condense, he pointed out. Killed bacteria are even used as an additive in snow making at ski resorts.
Which raises the question, Christner said, of whether planting crops known to be infected by Pseudomonas in areas experiencing drought might help increase precipitation there by adding more nuclei to the atmosphere.
It has been known that microbes and insects and algae blow around in the atmosphere, Christner added, "but the atmosphere has not been recognized as a place where things are active. That has been changing in the last decade. In a cloud you've got water, organic carbon," everything necessary to support a microorganism.
Virginia K. Walker, a biologist at Queen's University in Kingston, Ontario, Canada, said other researchers have found bacteria serving as snow nuclei, but had not identified it as Pseudomonas.
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 "It's one of those great bacteria ... you can find them anywhere," said Walker, who was not part of the research team. "They are really interesting."
Charles Knight, a cloud physics expert at the National Center for Atmospheric Research in Boulder, Colo., wasn't surprised by the finding, however.
At relatively warm temperatures of just a few degrees below freezing, bacteria are "remarkably effective" at attracting ice formation, said Knight, who also was not part of the research group.
The study was supported by a Louisiana State University research grant.
In a second paper published online by Science, researchers report that the amount of dust blown into the tropical Pacific over the last half-million years has varied widely between warm and cold periods.
Dust also has important impacts on weather and climate ranging from serving as nuclei for rain to blocking some incoming radiation from the sun, and it also delivers minerals like iron that increase growth of plankton in ocean areas.
Cores of seafloor sediment were taken from locations across the tropical Pacific covering a period of 500,000 years.
Researchers led by Gisela Winckler of the Lamont-Doherty Earth Observatory of Columbia University found that dust deposited in the ocean peaked during cold periods and was less during warm periods. Using isotopes, the scientists traced the dust on the western side to Asia and that on the eastern side to South America.
They say the reasons for the change are complex but in general it tends to be windier in cold periods meaning more dust gets blown around.
They found that cold peaks occurred about every 100,000 years, with the last one at 20,000 years ago.
The research was supported by the National Science Foundation and the Earth Institute at Columbia University.
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On the Net:
Science: http://www.sciencemag.org/
[Associated
Press; By RANDOLPH E. SCHMID]
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