Airborne Bugs May Trigger Rain To Get Home
Rain is just airborne bacteria's way of getting home. That's one startling conclusion from an analysis showing that rain-making bacteria are widely distributed in the atmosphere.
Particles of soot and other tiny pieces of inorganic debris are important "seeds" of precipitation.
The idea that bacterial cells could have the same function is not new, but what has not been appreciated until now is the extent to which biological particles are involved in 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."
The particles that Christner is referring to come from bacteria with proteins on the surface of the cell that facilitate ice formation.
Christner and colleagues collected snow soon after it had fallen from 19 locations, in France, the US and Antarctica, and found evidence of ice-forming proteins and DNA-containing cells at all sites.
The results mean that rain-making bacteria are present in the atmosphere, and probably in clouds too.
Christner's colleague Pierre Amato has sampled directly from clouds, and found that they contain viable bacteria, although whether they are rain-makers is not yet known.
"There's no reason [rain-making bacteria] couldn't be in clouds too," says Christner.
This suggests the intriguing possibility that the surface protein has 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, bacteria finding themselves up there might not want to stay.
"If a bacterium gets aerosolised into the atmosphere and has this protein on its surface," says Christner, "it could facilitate its own precipitation."
Pseudomonas syringae grows on the leaves of plants and is well known as a plant pathogen.
(image left: example of aforementioned bacteria on tomato leaf, from avrdc.org)
But Christner says it should be possible to plant crops that use other rain-making bacteria to seed their own clouds, and make their own rain.
"In places that suffer drought you could plant crops that harbour bacteria to increase precipitation," he says.
Journal reference: Science (DOI: 10.1126/science.1149757)