Sample a hurricane's air from a plane high in the stratosphere and, in addition to the expected water and grit, you'll find an abundance of microbes. Swept up from land and sea by the tropical cyclone's power, the skyborne bacteria persist in the atmosphere for days—and some may even thrive there.
A new survey of the tropospheric skies reveals that even as much as 10 kilometers up, microbes are a constant presence. Indeed, 17 individual bacterial species have been found in every air sample taken so far, suggesting that they may live way up there. The results of this 2010 survey conducted by scientists aboard NASA flights were published online in Proceedings of the National Academy of Sciences on January 28.
Exactly what microbes are doing so high in the sky remains unknown, though the 17 ubiquitous residents can apparently survive the extreme conditions and even gain nourishment from compounds that are abundant in the atmosphere. The tiny cells are also known to help the coming together of water vapor in the atmosphere that forms the ice crystals responsible for cirrus clouds or, lower in the atmosphere, the drops of water that then become clouds, rain or snow. The bacteria outnumber fungal spores, pollen and other living particles in the sky, based on the collection of all particles by filtering discrete samples of six cubic meters of air on a total of nine flights.
By sequencing the DNA found in these samples (rather than attempting to grow any bacteria in culture ,an often tricky process) the researchers determined that each cubic meter of rarified air contained more than 5,000 bacterial cells—a much higher concentration than anyone had expected. The sequencing also revealed that the 17 species, mostly from the families of Methylobacteriaceae and Oxalobacteraceae, were present from California across the U.S. and out into the Gulf of Mexico, suggesting they had mechanisms to protect the fragile cells from ultraviolet light and extremely cold and dry conditions.
As for how the microbes get to the sky, the routes may be the same as for the millions of metric tons of dust and salt that reach the atmosphere: the wind, sea spray and other physical phenomena. But judging by these samples, hurricanes and possibly other storms, help convey enormous amounts of a lot of different bacteria around the world.
Samples from Hurricane Karl and Earl (both in September 2010) showed that the big storms both rained out bacteria from the atmosphere and also swept up some species not usually found so high, including E. coli and Streptococcus, especially after passing over cities and other populated areas. In the case of Hurricane Earl, there were even soil microbes from that tropical cyclone's origins as a dusty wind in the Sahara Desert.
The results merit a rethink of the role microbes play in shaping the atmospheric effects we humans know as weather. After all, microbes already determine everything from the amount of oxygen in the atmosphere to the breakdown of minerals on land. Microscopic life may also play a key role in the chemistry of the atmosphere as well as whether it rains or pours. The deep blue sky may even qualify as an ecosystem. "I wouldn't be surprised if there is active life and growth in clouds," mused microbiologist Kostas Konstantinidis of the Georgia Institute of Technology, one of the research team, in a press release. "But we cannot say for sure now."