Read more: 鈥�Strange skies: Seven wonders of the atmosphere鈥�
Few creatures soar as high as the invisible denizens of Earth鈥檚 skies: bacteria and fungi. Many may be deliberate tourists, travelling the world by air; others appear to be grazing on chemicals in the clouds. It is even possible that life on Earth began up there in the stratosphere.
鈥淢any bacteria may be deliberate tourists, travelling the world by air; others appear to graze on chemicals in the clouds鈥�
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We have long known about airborne bacteria, but it was always assumed they were inert and accidental wanderers. Then in 1997, cloud droplets collected at 3000 metres in the Austrian Alps showed signs of a more active existence. Birgit Sattler of the University of Innsbruck in Austria and her colleagues saw bacteria growing and reproducing in the water. They also found organic acids and alcohols that might be a food supply. Sattler estimates that globally, life in the clouds may take up between 1 and 10 million tonnes of carbon a year ().
Some of these microbes might be altering our weather. Most rain clouds form when microscopic ice crystals start to grow on specks of material. These specks can be inorganic, but research in the 1970s showed that bacteria can do the job better. For example, the plant pathogen Pseudomonas syringae carries a protein adapted to trigger ice growth (). Since then, other bacteria and fungi have been found with similar abilities. 鈥淢icrobes can incite freezing at temperatures that abiotic nuclei cannot,鈥� says at Louisiana State University in Baton Rouge. He has found microbes in rain and snow at sites all around the world.
Many people doubted there were enough bugs up in the sky to affect cloud formation. Then in 2007 a team led by of the University of California, San Diego, flew a plane over Wyoming to take cloud samples. They found that about a third of ice nuclei contained biological matter, including bacteria, fungal spores and plant material ().
Christner suggests that this detritus hitches a ride on the water cycle. Blown off leaves and into the air, tiny organisms are tough enough to survive sunburn and dehydration for a while, but not indefinitely 鈥� so they make clouds to rain them back down. He suspects that these bacteria actually evolved to seed clouds. And since clouds reflect sunlight, aerial microbes may well play a role in climate change. It might even be the case that some of these airborne bacteria spread human diseases (Science, vol 308, p 73).
So far we have only had a fleeting glimpse of air life; no one knows how many species are blowing around. There are also no figures on how high life can go. 鈥淲e don鈥檛 know the upper boundary,鈥� says Christner. So his team has sent balloons into the stratosphere to see if anything lives there. They are still sifting through the results, but early signs are that some life survives at altitudes of more than 60 kilometres. This high up, conditions are strikingly similar to those on the surface of Mars 鈥� cold, dry, almost airless and fiercely irradiated 鈥� so studying life in the outer limits of Earth鈥檚 atmosphere could give us an insight into extraterrestrial existence.
Finally, in a radical proposal, Adrian Tuck of the National Oceanic and Atmospheric Administration in Boulder, Colorado, suggests that life may have arisen in the air. Primitive cells might have formed in fine water droplets in the stratosphere, he says. These droplets could have picked up nutrients from meteorites, with more complex chemistry sparked by ultraviolet rays from the sun.