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.