RAIN is just airborne bacteria’s way of getting down to the ground. That’s the startling conclusion of an analysis of snow samples from around the world, which reveal that rain-making bacteria are ubiquitous in the atmosphere.
Particles of soot and other tiny pieces of inorganic debris are important “seeds” of precipitation. That’s why particles of silver iodide and dry ice are sometimes used to encourage rainfall. The idea that bacterial cells could also trigger rain is not new, but until now no one had appreciated the sheer extent to which biological particles apparently contribute to rainfall.
“They are everywhere in the atmosphere,” says Brent Christner of Louisiana State University, Baton Rouge. “It’s hard to believe they couldn’t have some impact on precipitation.”
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“They are everywhere in the atmosphere. It’s hard to believe they couldn’t have some impact on precipitation”
The particles that Christner is referring to come from bacteria with “ice-nucleating” proteins on their cell surface. For example, the bacterium Pseudomonas syringae has a protein which binds water molecules together in a pattern that mimics an ice crystal’s lattice, allowing ice to form at warmer temperatures. When the bacterial ice crystals fall from the clouds, they produce snow, or if they melt, rain.
Christner and his colleagues collected snow soon after it had fallen from 19 locations in France, the US and Antarctica, and found evidence of ice-nucleating proteins at all the sites (Science, ).
This means that rain-making bacteria are present in the atmosphere. “There’s no reason they couldn’t be in clouds too,” says Christner.
If ice-nucleating proteins are found in clouds, this would raise the intriguing possibility that the proteins evolved as part of the bacterial life cycle. Many bacteria get swept up into the atmosphere, and although there are nutrients and water present in clouds, they might not wish to stay long.
“If a bacterium gets aerosolised into the atmosphere and has this protein on its surface, it could facilitate its own precipitation,” says Christner.
“The really interesting thing is that the majority of ice nucleators in their samples are biological,” says Tim Lenton of the University of East Anglia, Norwich, UK. Since bacteria make their living at ground level, triggering rain may be a way to get themselves out of the clouds, he adds. “Airborne dispersal is likely to be effective, but it only works if you can get out of the air again.”
It should be possible turn this ability to our advantage, says Christner. “In places that suffer drought you could plant crops that harbour bacteria to increase precipitation.” P. syringae grows on the leaves of plants and is well-known as a pathogen that causes frost damage. So farmers could only use rain-making bacteria in places where it doesn’t get cold enough for ground frost to form.
