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The clouds are alive



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Analyzing what, if anything, Christner's team collected on that 37,200-metre flight will take months. But they have promising results from earlier launches to lower levels of the atmosphere.

Back at their lab Christner and Bryan examine two dozen petri dishes containing bacteria collected from flights that peaked between 3,000 and 24,000 metres. They have yet to sequence and ID the microbes' DNA, but just looking at the petri dishes gives them some clues. Bright-colored colonies dot many of the dishes — reds, oranges, pinks, and yellows. "Those are natural sunscreens," Christner says. Colored carotenoid pigments (similar to those in many plants, including the carrots that the compounds are named after) can neutralize damaging ultraviolet light.

Christner holds up a petri dish containing colonies mottled with beige, black, and white: an indication that these bacteria produce dehydration-resistant spores that could help them survive at extreme altitudes. They are probably some sort of Actinomyces (a group of bacteria that live in soils and include species that make streptomycin and other antibiotics), but the species could be new to science. "We don't know," Christner says, looking at a splotch. "That might make an antibiotic that nobody's ever seen before."

Searching the skies for high-living microbes may also lead to insights concerning some species that we already do know about. In the 1950s researchers funded by the U.S. military tried to sterilize canned meat by blasting it with radiation. When they opened the cans, they were surprised to find the meat rotten: It had been fermented by a bacterium, now called Deinococcus radiodurans, that is exceptionally resistant to radiation. The species carries a muscular set of enzymes that stitch its DNA strands back together as quickly as radiation splinters them apart. Deinococcus can survive 5,000 times as much radiation as human cells, but no natural environment on Earth comes close to those levels of irradiation. "So what the hell has an organism like this evolved tolerance for?" Christner asks.

John Battista, who studies the microbe at LSU, just upstairs from Christner, thinks Deinococcus's DNA repair enzymes primarily help it survive dehydration in its native desert environment. But radiation tolerance could also allow the bacterium to join that 16-kilometre-high ecosystem. Windstorms in places like the Gobi Desert could easily pick up Deinococcus and propel it around the world. "If it managed to get into the upper atmosphere," Battista says, "it has all the tools it needs to survive." Deinococcus and other similarly hardy microbes may be lurking in the samples Christner's team is culturing.

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