Microbes may be responsible for snow—and rain for that matter. They are certainly involved in much of the man-made snow that ski resorts use to cover for Mother Nature's winter lapses. Microorganisms, particularly bacteria, produce proteins in their cell walls that bind water—even if they are dead. In fact, they bind water in such an orderly fashion that water droplets freezing around a microbe almost mirror the natural lattice formation of ice. As a result, bacteria can help snow form at warmer temperatures than would otherwise be the case, which explains why some ski resorts add dead microbes to the mix in their snowmaking machines. And now scientists have discovered such biological precipitation catalysts in natural snows—in such far-flung locations as Montana and Antarctica.

Microbiologist Brent Christner at Louisiana State University in Baton Rouge and his colleagues collected snow samples from 19 different sites, including Bozeman, Mont., the French Alps, Ross Island in Antarctica and a glacier in the Yukon's Wheaton River Valley. They found microbes in all of the samples, and the highest concentrations in the least remote areas.

In an attempt to gauge whether these microbes were the catalysts of their snowflake vessels, the team exposed them to heat as well as an enzyme found in tears that punctures bacterial cell walls—both of which reduce the ice-forming ability of many microbes. They then placed the particles in purified water and discovered that they were no longer able to freeze water as effectively at warmer temperatures. "We would not expect that heat treatment to have any effect on dust particles," Christner notes. "It's good evidence that it's proteinaceous in origin, that it's biological."

Of course, this does not prove that the microbes were actually in the clouds, seeding the snowfall: "Just because we find an ice nucleator in rain or snow doesn't mean it originated in a cloud; it could be scavenged during precipitation," Christner says. But other researchers, including microbial ecologist Gary Andersen of Lawrence Berkeley National Laboratory in California, have found as many as 2,000 varieties of microbes in the air above Texas cities.

"It is clear that they are widely distributed in the atmosphere," Christner says. "If they are in the atmosphere, there is no reason they couldn't get into clouds."

Although it remains unclear which microbes may be most responsible for snowfall or rainstorms, one leading candidate is the plant pathogen Pseudomonas syringae, which infects wheat, corn and other crops. It is a major pest—and the target of genetic modification—because it causes immediate crop damage if the temperatures drop below freezing.

But it also shows up in clouds with this water-organizing protein on its cell walls—as do fungi, pollen and a host of other biological bits that boast the same property. "There are a large variety of organisms in nature that have this activity that we just haven't discovered yet," Christner says. For example, algae in the ocean can control local weather by releasing a volatile compound that helps promote cloud cover.

That means that Pseudomonas syringae and the other living microbes in the clouds might just be perpetuating themselves—and spreading—when it snows or rains.

7 Comments

1. 1. Suene123 06:39 AM 3/6/08

   Euuugh! does this mean eating snowflakes can make you sick? Uh oh...

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2. 2. Mary79 02:33 AM 3/13/08

   I really would like to know something more about this issue...someone can help me?

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3. 3. constantinidou 03:48 PM 4/14/08

   In a recent article by Christner et al., the authors present some new findings on the detection of biological ice nucleators in snow, extending the previous research efforts of Lindemann, Constantinidou, Barchet and Upper (1982, Appl. Environ. Microbiol. 44:1059), nearly 26 years ago. Being a co-author in the latter paper, it is fulfilling to realize that the potential significance of the early work of Lindemann et al. is finally acknowledged by the scientific community, as the publication of the article of Christner et al. in the well-respected journal of Science suggests. More importantly, it is hoped that this publication revitalizes research in bioprecipitation at times where discussions over the global climatic changes are ever more pertinent.
   To this end, the following brief historical background on the topic of biological ice nucleators including archival references that have been overlooked by Christner et al. is provided along with elucidating comments on certain important aspects. Concrete evidence that ice nuclei of biological origin enter the troposphere, as well as demonstration of their appearance in cloud and sub-cloud samples and in rain, was presented 42 years ago in the pioneer work of Gabor Valis group in Laramie Wyoming (Shnell and Vali 1976, J. Atmos. Sci. 33:1554). It was actually a few years earlier that, during the quest for atmospheric ice nuclei, the group reported appearance of leaf-derived freezing nuclei (Shnell and Vali 1972, Nature 236:163), and identified the bacterium Pseudomonas syringae as the active ice nucleus (Maki et al. 1974, Appl. Microbiol.28:456; for a review also see Upper and Vali, 1995 in Biological Ice Nucleation and its Applications, Lee et al Eds ch. 2). Subsequently (1982), members of Chris Uppers team in Madison Wisconsin, provided evidence that plants constitute a major source of airborne ice nucleation active (INA) bacteria, namely of the most common and most active nucleator Pseudomonas syringae (Lindemann et al. 1982, Appl. Environ. Microbiol. 44:1059). Indeed, viable INA bacteria were abandoning their host plants under drought weather, with flux calculations indicating in addition their transportation into the surface boundary layer. These bacterial ice nucleators could then be mixed throughout the depth of the troposphere and return to earth during snow or rain events, thus employing a transportation cycle well evolved to suit their unique nucleation ability. As a matter of fact, the group reported strong net downward fluxes of INA bacteria during rain storms (Constantinidou, Hirano, Baker and Upper 1990, Phytopathology 80:934). The authors concluded that: Viable INA bacteria in the troposphere are potentially in a suitable position to initiate precipitation process, although their involvement in these processes remains to be demonstrated.
   Pointing to the future, the actual involvement of INA bacteria in precipitation process, a quest pending since the 1982 Lindemann et al. article, still awaits to be demonstrated. Such a demonstration, if ever possible, will be the real breakthrough news.

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4. 4. Chemist99a 07:24 AM 6/29/08

   Hi there I set up this account just to comment having noticed you registered here. I do remember as I am sure you do too Steves early work on this. How are you doing these days. I now have a grandson and a ? on the way. Retirment is well relaxing in spite of my health problems. My new location is much warmer than Madison!

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5. 5. Connor 02:23 PM 11/10/08

   I think microchodes are fascinating organisms.

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6. 6. Chodeman69 02:07 PM 11/11/08

   I believe strongly that microchodes can often be found in suspicious places like the chode, for example. In the longrun resulting in an inability to come up with such reasoning as to why these fascinating organisms are found where they are.

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7. 7. kmondon 05:26 PM 3/7/12

   For example, algae in the ocean can control local weather by releasing a volatile compound that helps promote cloud cover.
   
   Would it be more accurate to indicate that algae can influence local weather?

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