Here come the microbes! (Credit: China Foto Press/Barcroft Medi)

Cover your mouth when you cough!

We've all learned the hard way that microbial organisms, from bacteria to viruses, can be transported by air. But the extent to which organisms exist in the Earth's atmosphere is only now becoming clear.

There is good evidence that bacteria (or bacterial spores) can help nucleate water condensation, seeding clouds and encouraging precipitation. It has been speculated that this could even form part of a bacterial life-cycle, lofting organisms into the air, transporting them, and bringing them back to earth for fresh pastures.

There's also growing evidence for just how widespread airborne microbial 'ecosystems' might be. Scientists in Austria have found bacteria in cloud droplets at 10,000 feet as well as clear signs that these microbes are not just passengers - they're actually growing and reproducing in-situ in the super-cooled water environment. This suggests that clouds are quite literally another habitat for life on Earth, and with an average covering of 60% of the planetary surface represent a pretty major ecosystem.

Now a new study finds that dust plumes in the troposphere are carrying over 2,000 distinct species from Asia to North America - right across the Pacific Ocean. Some of these organisms are fungal, but at least 50% are bacterial, and make the trans-planetary journey in only 7-10 days when storms loft them as high at ten miles into the atmosphere.

This might not seem so surprising, we know that single-celled organisms occupy almost every niche on the planet. However, it does seem that the Asian microbes represent a distinct population that's usually only a trace on the continental USA - but when the wind blows their numbers on the western hemisphere definitely increase significantly. This means that there is real mixing of species going on, a microbial pollution that may have consequences for all manner of things, including local ecosystem function and even disease.

Now *that's* a dust storm. Before and after images of Mars, dust blankets the planet on the right (Credit: NASA/STScI Hubble imagery)

It's fascinating stuff. This kind of transportation must have been going on across all three to four billion years of life on Earth, leading us to wonder exactly what role it may have played in maintaining the global biosphere. It's also food for thought in considering the potential ecosystems of Mars, a place where planet-wide dust storms regularly loft particles high into the atmosphere.