From Environment 360:

Consider the African rain dance. People in tribal costumes stamping the ground to make rain — it’s nonsense, you might say. Except that we now know it could actually work. If you have enough dancers, there may be no better way to make rain, because bugs in the soil and surface vegetation make exceptionally good cloud- and ice-condensation nuclei — and rain dances stir them up.

Microbes, it turns out, are the hidden players in the atmosphere, making clouds, causing rain, spreading diseases between continents, and maybe even changing climates as well. Eos, published by the American Geophysical Union, last month reported that bio-aerosols are “leading the high life.” In the Eos article, David Smith of the University of Washington and colleagues argue that microbes are “the most successful types of life on Earth” and are the unacknowledged players in many planetary processes, particularly in the atmosphere. It’s time we caught up with them.

Back in 1979, Russell Schnell of the University of Colorado was in western Kenya wondering why the tea plantations there held the world record for hailstorms. They occurred 132 days a year. He discovered that tiny particles of dead and decaying leaves in the soil bore a close resemblance to the tiny particles around which hailstones formed. They were, it turned out, far better adapted to the task even than man-made cloud seeding chemicals like silver iodide.

Schnell, who is now deputy director of the Global Monitoring Division of the National Oceanic and Atmospheric Administration, concluded that “the feet of hundreds of tea pickers going about their daily jobs” were to blame for the hail. By kicking the bits of leaf into the air, he said, the tea pickers must be providing the abundant ice-nucleators that created the hailstorms. He published in Tellus in 1982, revealing that the critical actors in this Kenyan drama were the bacteria, Pseudomonas syringae, that attached

‘Bioprecipitation’ is a hot topic, more so as we learn how much biological matter is in the atmosphere.

themselves to the leaves as they rotted — the tea pickers sent the leaf bits airborne as they walked the fields picking the tea leaves from the bushes.

Biologists have long known that many species of bacteria trigger frost damage on vegetation, with Pseudomonas syringae the most efficient. The bacteria have evolved a gene that promotes spontaneous ice nucleation at around minus 2 degrees Celsius, much warmer than would happen otherwise. Their ice-making skills allow them to break down the cell walls of the plants they feed on. But it seems they also use the same skill in clouds.

Mineral and salt particles are present in large numbers in clouds and can act as condensation nuclei. But many bacteria, as well as fungal spores and tiny algae, are the cloud condensation nuclei of choice because they can work at higher temperatures. Since the formation of ice is normally the first step in the creation of raindrops in clouds, they are probably critical in the creation of rain. “Numerous studies,” say Smith and his colleagues in Eos, “have shown that many… condensation nuclei responsible for climate and precipitation patterns are in fact airborne micro-organisms, living or dead.”

And that, Smith says, means any human activity that puts more bugs in the air is potentially a rain-making activity, whether it is tramping tea plantations or cooking up a big rain dance. “Exactly how higher concentrations of airborne micro-organisms will interact with other variables that drive weather and precipitation is a major unknown in the climate change equation,” he says.

Schnell’s original observation was largely ignored by the wider science community. But recent papers have made similar observations in other places. For instance, Brent Christner, a microbiologist at Louisiana State University, reported in Science in 2008 that he had found “ubiquitous and abundant” microbes in fresh snowfall sampled from Antarctica to Montana – between 70 and 100 percent of ice nucleators found in the snow were biological.

This, Christner points out, was especially remarkable since he was sampling snow in areas where there was no local vegetation. The microbes had traveled a long way to do their job. “It’s a wake-up call,” he says. “Biological particles do seem to play a very important role in generating snowfall and rain.”

Then in May this year, at a meeting of the American Society of Microbiology, Alexander Michaud of Montana State University in Bozeman reported finding high concentrations of bacteria in hailstones falling on his campus.

“Bioprecipitation” is a hot topic. And the more so as we learn how much biological matter there is in the atmosphere — more than 10,000 individual bacteria per cubic meter of air over the land, according to a 2009 study by Susannah Burrows of the Max Planck Institute for Chemistry in Mainz, Germany. These bacteria spend an average of about a week in the atmosphere; but while some stay close to the ground, others soar into the stratosphere, says Smith. Weather balloons have even found them in the mesosphere, up to 77 kilometers aloft, according to a forgotten study by Soviet scientist A. A. Imshenetsky, published in Applied and Environmental Biology as long ago as 1978 and uncovered by Smith.

More.

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