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Until then, no one had seen life in a rain cloud. "We were not going in purposely looking for bacteria," she says. But she has since seen similar results in two more cloud-sampling campaigns. The Sierra flights in early 2011 found that most ice crystals contained dust from deserts on other continents, along with what seemed to be bacteria. When Prather and DeMott sampled clouds over the Caribbean island of Saint Croix in July 2011, they again saw signatures of both dust and cells in the ice. The dust itself seemed to come from North Africa.
DeMott amassed a library of 100,000 ice-forming particles collected from these flights. He has only begun to identify them, but in electron microscope images he finds that some of them resemble bacteria. Colleagues of Franc (who recently passed away) will test them soon, looking for ice nucleation genes.
Some researchers still skeptical
Despite these results, some researchers remain skeptical that microbes could be important for cloud formation. Corinna Hoose, a cloud physicist at the Karlsruhe Institute of Technology in Germany, thinks that the number of ice-forming cells that Christner found in snow is too low to make much difference. She estimates that airborne microbes contribute less than one per cent of all cloud ice. "Dust and soot particles are much more abundant," she says. "They are much more important than biological particles."
That has been the dominant view for decades, and dominant views do not die easily. But Prather, Christner, and Sands are chipping away at it. Lab studies have yet to turn up a common mineral that triggers freezing as effectively as bacteria like syringae do. "These organisms," Christner says, "are able to catalyze ice formation at a temperature warmer than any other naturally occurring particle."
And Prather may actually have underestimated the abundance of ice-forming biological particles in her samples. Even though she saw lots of dust in the icy patches of clouds, 60 per cent of those dust grains also contained carbon, nitrogen, and phosphorus, suggesting that they are not just minerals. They might have cells lurking inside them — or at least the guts of dead cells splattered on their surfaces. It's an issue that Prather chats about with her colleagues plenty these days. "I don't think it's the dust itself" that's forming ice, she now says, "I think it's the bio."