Snow Flurries, Bacteria Likely
byCHRISTOPHER JOYCE
Theo Allofs/zefa/Corbis
New research finds that a large portion of the atmosphere's ice particles form around bits of bacteria.
Shawn Doyle, Brent Christner, LSU
Top, an ice crystal lattice that was formed in the laboratory from bacterial culture. Bottom,Pseudomonas syringaecells, stained green, trapped within individual ice crystals.
Courtesy of Brent Christner
Brent Christner, lead author of the study, demonstrates the researchers' mode of transportation in the field. / March 3, 2008
Next time you're in a snowstorm, look up — and get a face-full of bacteria. It turns out there are bacteria up in the clouds, and some of them actuallycreateice crystals.
The inspiration for this research started in Montana. Brent Christner had a colleague there who was puzzled by some nasty plant bacteria that kept infecting his wheat crops. No matter what he did, he couldn't get rid of them. He suspected they might be airborne. So, Christner says, his friend cooked up a strange experiment involving a petri dish and an airplane.
He flew up in the airplane, "opened a window and held a petri plate outside," Christner explains. And in fact, bacteria grew.
The bacteria — which normally live on plants — were falling out of the sky.
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That was over 20 years ago. Christner went on to become a microbiologist himself, currently at Louisiana State University, and also an expert in ice and snow. He knew that at certain temperatures, particles floating in the clouds cause water vapor to crystallize around them. And he remembered the flying bacteria.
"It really intrigued me ... the idea that if these bacteria were blown into the atmosphere and actually got into a cloud that they could induce precipitation," Christner says.
So Christner packed up his skis and went looking. He collected snow and ice and melted it down. Sure enough, he found the particles — or "nucleators," as they're known among ice experts — and was surprised to realize that the most active ones were bits of bacteria.
"In every sample that we've looked at we've found them," he says. He found these ice-bugs all over the place — France, Montana, the Yukon. And in some pretty unlikely places.
"We analyzed fresh snowfall from places like Antarctica where there aren't any plants around," Christner says. "They were still present."
Christner says these bacteria have a special protein that gives them their ice-making powers.
"It's a protein that mimics the lattice of an ice crystal," he explains. "So it enhances ice crystal formation."
In fact, there are several kinds of bacteria floating around in the sky that could be making snow or even rain, according to Steven Lindow, a plant scientist at University of California, Berkeley. He says scientists have suspected these bacteria may be using the atmosphere like an aerial freeway.
"They could leave plants, reach the upper atmosphere, (and) become ingrained in atmospheric particles that would fall later as rain or snow," Lindow says. "This would bring them back to earth perhaps even as raindrops onto a new plant where they could find a new home and start the cycle again."
He says the research, which appears in the latest issue of the journalScience, helps confirm that idea. Brent Christner adds that the findings pose an interesting possibility. For example, people might consider growing plants that harbor these bacteria in drought-prone places — in essence, he says, using a green thumb to make it rain.
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Most snow and rain forms in chilly conditions high in the sky and atmospheric scientists have long known that, under most conditions, the moisture needs something to cling to in order to condense.
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 was 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.
"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.
© MMVIII The Associated Press. Educational Purposes Only / Questions:
Condense:
Nucleator:
Ice Nuclei (Nucleus)
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Where is the bacteria living? How is it getting in the air?
What does he mean when he says he doesn’t know if they are a “good guy or a bad guy”?
Precipitation:
Atmosphere:
How could bacteria be used to make snow at ski resorts?
Reflective Comment or Question:
Why do you think Charles Knight wasn’t surprised?
Write a 6-7 sentence reflection on this article. What do you now know about snow and bacteria? What do you think about this new information? Will you look at snow differently now or about the same? Why?