Life on Earth’s Ceiling
Recent studies have confirmed that microbes exist in the stratosphere, the atmospheric region between about 18 and 50 kilometers in altitude. (Baumgartner leaped into history from 39 kilometers up.) Biologists had once thought this zone was uninhabitable due to its low pressure, high radiation and absence of water and nutrients.
“Life surviving at high altitudes challenges our notion of the biosphere boundary,” says David Smith of the University of Washington in Seattle.
Smith is trying to take a more complete census of life in the stratosphere using mountain-top observatories and high-altitude balloons (although he admits being jealous of the lift capacity of Baumgartner’s balloon). With funding from the NASA Astrobiology Institute, he and his colleagues hope to say where these high-flyers come from, where they are going and what their evolutionary trajectory has been.
The research should provide insights into a class of hardy microorganisms that can survive at the fringes of what we Earthlings consider habitable, both here and elsewhere.
The life forms adapted to sailing the high winds of Earth may give scientists clues on what to look for in the soils of Mars.
A hint of life in the air
Reports of life in the upper atmosphere go back as far as the 1930s. Studies conducted on samples collected in high-altitude balloon and rocket flights claimed to find microbes as high as 77 kilometers, but it’s uncertain whether the bugs really came from high up, or were the result of surface contamination.
“I am skeptical of the pioneering flights,” Smith says. “Almost no controls for sterilization are reported in the papers.”
Getting reliable samples from the upper atmosphere is difficult and expensive. Researchers must process huge volumes of air to catch a single microbe, since the estimated concentrations are only a few microbes per cubic meter. Smith believes the first solid evidence of life in the stratosphere was a 2002 balloon study, which collected viable cells at altitudes between 20 and 41 kilometers above tropical India.
Born to spore
The observed microbes do not appear to be actively growing at high altitude.
“Our best picture is that microbes are simply enduring the upper atmosphere,” Smith says. “They are hitching a ride on prevailing winds until landing in a distant environment, sometime later.”
Many of these globetrotters are spore-forming bacteria. When placed under stress, these bacteria shut down their metabolism and shrink down in size. They form a hard shell-like shield around their exterior, while turning on internal defense mechanisms against UV damage to DNA.
“One could argue that the spore is an adaptation that evolved, in part, because of airborne transport,” Smith says.
Smith thinks these “search and revive” missions have only scratched the surface of what is up there. “Only about 1% of microorganisms can actually be cultured in the laboratory,” he says.
To see the beyond these one-percenters, Smith and his colleagues have begun using molecular-based techniques to identify biological material (both living and dead). Their atmospheric samples come from a “laboratory in the sky” at Mt. Bachelor Observatory in central Oregon, which rises 2.7 kilometers above sea level. Smith says this altitude gives them reliable access to the upper troposphere, which is just below the stratosphere.
Instead of trying to grow organisms in a petri dish, Smith and his colleagues extract DNA that they can match to a genomic library containing 60,000 microbial taxa. Preliminary results have shown microbes from every major domain of life can be found in the upper troposphere.
Jet stream feeds gene pool?
In order to sample higher altitudes, Smith is also working on a balloon mission with engineers from NASA Kennedy Space Center, called MIST (Microorganisms in the Stratosphere).
“We still have no idea where to draw the altitude boundary of the biosphere,” Smith says. Experiments like MIST will “address how long life can potentially remain in the stratosphere and what sorts of mutations it may inherit while aloft.”
Those mutations could potentially act like tiny “seeds” that fall down randomly over the face of Earth and potentially take root in a favorable environment.
“It is conceivable (though difficult to prove) that the evolution of complex life on our planet is owed, in part, due to airborne transport,” Smith says.