Forecast: Snow with a high probability of bacteria

Bacteria, snow mix in forecast

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.

Those beautiful snowflakes drifting out of the sky may have a surprise inside — bacteria. 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 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.

"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.

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Brent Christner, LSU professor of biological sciences, in partnership with colleagues in Montana and France, recently found evidence that rain-making bacteria are widely distributed in the atmosphere. These biological particles could factor heavily into the precipitation cycle, affecting climate, agricultural productivity and even global warming. Christner and his colleagues published their results on Feb 29 in the journal Science.
Christner's team examined precipitation from global locations and demonstrated that the most active ice nuclei -- a substrate that enhances the formation of ice -- are biological in origin. This is important because the formation of ice in clouds is required for snow and most rainfall. Dust and soot particles can serve as ice nuclei, but biological ice nuclei are capable of catalyzing freezing at much warmer temperatures. If present in clouds, biological ice nuclei may affect the processes that trigger precipitation.

The concept of rain-making bacteria isn't far-fetched. Cloud seeding with silver iodide or dry ice has been done for more than 60 years. Many ski resorts use a commercially available freeze-dried preparation of ice-nucleating bacteria to make snow when the temperature is just a few degrees below freezing.

"My colleague David Sands from Montana State University proposed the concept of 'bioprecipitation' over 25 years ago and few scientists took it seriously, but evidence is beginning to accumulate that supports this idea," said Christner.

But, what makes this research more complicated is that most known ice-nucleating bacteria are plant pathogens. These pathogens, which are basically germs, can cause freezing injury in plants, resulting in devastating economic effects on agricultural crop yields.

"As is often the case with bacterial pathogens, other phases of their life cycle are frequently ignored because of the focused interest in their role in plant or animal health," said Christner. "Transport through the atmosphere is a very efficient dissemination strategy, so the ability of a pathogen to affect its precipitation from the atmosphere would be advantageous in finding new hosts."

It is possible that the atmosphere represents one facet of the infection cycle, whereby the bacteria infects a plant, multiplies, is aerosolized into the atmosphere and then delivered to a new plant through atmospheric precipitation.

"The role that biological particles play in atmospheric processes has been largely overlooked. However, we have found biological ice nuclei in precipitation samples from Antarctica to Louisiana -- they're ubiquitous. Our results provide an impetus for atmospheric scientists to start thinking about the role these particles play in precipitation," said Christner. "This work is truly multi-disciplinary, bridging the disciplines of ecology, microbiology, plant pathology and climatology. It represents a completely new avenue of research and clearly demonstrates that we are just beginning to understand the intricate interplay between the planet's climate and biosphere."