Bacteria, snow mix in forecast
Study says bond can harm plants
WASHINGTON - Those beautiful snowflakes drifting out of the sky may have a surprise inside - bacteria.
Most snow and rain form 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 socalled nucleators are 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 reported their findings in today'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 were present to a lesser degree in Antarctica.
The most common bacterium 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 found it in summer rainfall in Louisiana.
The focus on Pseudomonas syringae 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 whether that is a good idea. Would elimination of this bacterium 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 syringae 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 syringae 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 them as Pseudomonas syringae.
"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."