Published online 28 February 2008 | Nature | doi:10.1038/news.2008.632

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Some microbes are frequent flyers in clouds.

Rain brings microbes back to Earth along with water. Punchstock

The same bacteria that cause frost damage on plants can help clouds to produce rain and snow. Studies on freshly fallen snow suggest that ‘bio-precipitation’ might be much more common than was suspected.

Before a cloud can produce rain or snow, rain drops or ice particles must form. This requires the presence of aerosols: tiny particles that serve as the nuclei for condensation. Most such particles are of mineral origin, but airborne microbes — bacteria, fungi or tiny algae — can do the job just as well. Unlike mineral aerosols, living organisms can catalyse ice formation even at temperatures close to 0 ºC.

The effect of the biological ‘ice nucleators’ on precipitation has been a mystery, not least because no one has yet been able to detect them in clouds.

Cloud counters

Now a team, led by Brent Christner, a microbiologist at Louisiana State University in Baton Rouge, has managed to catalogue these rain-making microbes by looking at fresh snow collected at various mid- and high-latitude locations in North America, Europe and Antarctica.

They filtered the snow samples to remove particles, put those particles into containers of pure water, and slowly lowered the temperature, watching closely to see when the water froze. The higher the freezing temperature of any given sample, the greater the number of nuclei and the more likely they are to be biological in nature. To tease apart these two effects, the team treated the water samples with heat or chemicals to kill any bacteria inside, and again checked the freezing temperatures of the samples.

In this way they found between 4 and 120 ice nucleators per litre of melted snow. Some 69–100% of these particles were probably biological. The results are published in Science today1.

The researchers were surprised to find ‘rain-making’ bacteria in all samples; the snow from Antarctica had fewer than that from France and Montana, but it still had some. The results add evidence to the idea that microbes can safely travel long distances in clouds, and suggest that substantial biology-driven precipitation occurs everywhere on Earth.

"It is a wake-up call reminding us that some of the most active catalysts in clouds are being widely ignored, says Christner. "Biological particles do seem to play a very important part in generating snowfall and rain, especially at relatively warm cloud temperatures.”

Microbe water cycle

Most rain-making bacteria make their living as pathogens, using their ability to promote freezing at relatively warm temperatures to break the cell walls of the plants that they feed on. Some scientists note that this freezing ability also means that the bacteria get out of clouds and back to Earth more quickly, which is to the microbes' advantage.

“It is quite plausible that the organisms might be using their ice-nucleating ability to get out of the atmosphere,” says Tim Lenton, an Earth-system scientist at the University of East Anglia, UK.

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This idea — that bacteria are at an advantage if they can travel distances in clouds and then return to Earth — which Lenton developed ten years ago with the late Bill Hamilton2, features in the Gaia hypothesis. The Gaia theory holds that that living and non-living parts of the Earth are a complex interacting system, in which living things have a regulatory effect that promotes life overall.

But humans also have a big effect on these regulatory processes. Changes in land-use, forestry and agriculture, such as expanding monoculture, changes the composition of microbes in the atmosphere. As biological components seem to have a large role in how rain forms, such changes may affect rainfall and climate in many places on Earth.

“It is about time for atmospheric and climate scientists to start thinking about the implications,” says Christner.