While airborne dust is known to be an abundant component in the formation of rain and snow, a study [pdf] in February found that bacteria lofted into the atmosphere might also be a big part of the rainmaking mix.
For more than 20 years, scientists have tossed around the idea that bacteria play a role in precipitation. The speculation began because of the curious case of the bacterium Pseudomonas syringae, a plant pathogen that promotes frost damage on crops. A protein on the cell surface of P. syringae binds water in a way that mimics the structure of an ice crystal, and this helps start the transformation of cold water into ice. Moreover, studies suggest that bacteria travel from one patch of farmland to another by evaporating into the sky and then raining back down.
But are bacteria abundant enough to affect the weather the way dust does? Microbial ecologist Brent Christner of Louisiana State University collected fresh snow from across the globe and tested it for biological particles that could induce ice formation. He found them in all 19 samples, including snow taken from Montana in the dead of winter, when there is almost no deciduous vegetation, and even in samples from Antarctica.
The results suggest that bacteria can travel far from their plant hosts while maintaining the ability to make ice; they also hint at how airborne bacteria could play a role in the spread of plant disease. The next step is to collect bacteria directly out of clouds and identify the different species. “There are likely to be a whole fleet of organisms that can do this that we don’t know about,” Christner says.