Earth’s stratosphere is a mysterious superhighway for microbes

The stratosphere is each stunning and hostile — a purgatory between Earth and house, between life and demise.

Hover 30 kilometers above the bottom, and you might be practically twice as excessive as any raincloud on Earth. The floor of the planet curves beneath you. A diaphanous movie of blue stretches over that horizon, representing the disconcert­ingly skinny layer of ambiance that envelops all life as we all know it. Above that, the sky resembles black interplanetary house.

You’d rapidly die right here. The air strain is one p.c what it’s at sea degree. As you gasped for oxygen, your blood would boil inside you, inflicting your pores and skin to welt like bubble wrap.

However the stratosphere holds loads of life — tiny single-celled microbes that by some means navigate excessive dehydration, temperatures as little as −60° Celsius and intense DNA-damaging ultraviolet radiation that will kill most life on Earth’s floor.

“When you took a microbe from these altitudes and you set them on the floor of Mars, they wouldn’t even know the distinction,” says microbiologist Brent Christner of the College of Florida in Gainesville.

When Christner and his workforce began searching for life excessive within the ambiance over a decade in the past, they meant to seek out the higher limits of Earth’s liveable zone. They hoped that this, in flip, may present whether or not life may persist on the chilly, radiation-pummeled floor of Mars, the place the ambiance is simply as skinny. However when Christner’s graduate scholar Noelle Bryan despatched sampling balloons to 38 kilometers above Earth’s floor, she was completely shocked by what they discovered: “We didn’t hit an altitude the place we couldn’t discover one thing [alive],” says Bryan, who’s now a senior analysis supervisor at Mass Common Brigham in Boston.

In 2025, Christner’s workforce revealed one thing much more stunning. The microbes the researchers discovered weren’t the form of extremophiles that you just’d count on on the fringe of house. They regarded nothing just like the unique organisms that inhabit excessive habitats on Earth, resembling boiling sizzling springs, deep subterranean rocks or swimming pools of concentrated acid.

As an alternative, they turned out to be a number of the identical humdrum bugs that develop on our crops, our gardens, even our pores and skin. These well-known critters might dwell a secret double life that few folks imagined: flying world wide at two to a few occasions the peak of a business jetliner.

The ambiance “is sort of a freeway system,” Christner says. It permits microbes “to maneuver globally in durations of weeks,” crossing oceans and settling in new habitats. This realization expands our view of the biosphere and evolution. It may reshape our understanding of how pathogens unfold round Earth. And it may rework how we search for life on different worlds — on Mars, within the clouds of Venus and even on exoplanets light-years away.

Trying to find microbes within the sky

Scientists have puzzled for a century what kind of life, if any, may exist excessive within the ambiance. For many years, their explorations yielded solely tantalizing hints.

In 1935, the U.S. Military Air Corps launched a helium balloon to a top of greater than 22 kilometers over the Black Hills in South Dakota, setting a world document. Two pilots in a pressurized gondola recorded air temperatures, magnetic fields, radiation ranges and dozens of different measurements. In addition they dropped a sterile sampling gadget designed to gather airborne microbes on its method down. After falling midway, it closed and parachuted again to Earth with its pattern sealed inside.

Scientists managed to develop 10 forms of micro organism and fungi from the pattern that had been ostensibly collected throughout its fall by the decrease stratosphere. However on this period earlier than scientists knew about DNA, they by no means discovered rather more than the shapes of the cells and the temperatures and vitamins that they wanted to develop. A lot later, in 1974, Soviet scientists launched a number of rockets outfitted with sticky microbe collectors 48 to 77 kilometers into the mesosphere — the atmospheric layer above the stratosphere, the place circumstances are much more excessive. The collectors parachuted again to the bottom and arrived with just a few residing cells embedded in them, which scientists once more managed to develop within the lab.

That Soviet work is extensively cited, Christner says, “however there’s simply no method to know what they sampled” and whether or not it got here from contamination at floor degree. He doesn’t imagine that they really discovered one thing alive at that altitude.

A brand new era of high-altitude research began within the early 2000s.

NASA was flying its ER-2 plane (a civilian model of the U-2 spy aircraft) 20 kilometers up within the stratosphere to gather cosmic mud that filters down from house and use it to review the historical past of the photo voltaic system. Microbiologist Dale Griffin, then with the U.S. Geological Survey in St. Petersburg, Fla., organized to have sticky microbe catchers taken on a number of of these flights from 2003 to 2008.

Griffin grew a number of sorts of micro organism from these samples. And in contrast to his predecessors, he subjected them to a easy DNA evaluation referred to as genetic barcoding that used a brief sequence from a single gene to get a normal thought of which household or genus they belonged to. These micro organism had been associated to a number of that had been identified from distant islands and volcanic soils, Griffin discovered. He speculated that eruptions might need lofted them into the stratosphere.

Taken collectively, the U.S. and Soviet observations didn’t reveal a lot. They trusted rising microbes within the lab, however scientists can’t develop 99 p.c of the microbes that exist in most environments as a result of they’ll’t re-create the right rising circumstances. As such, the findings didn’t reveal what number of residing cells had been current within the stratosphere. They typically didn’t establish the species. The scientists didn’t check whether or not these microbes may survive the acute chilly, desiccation or UV radiation discovered within the stratosphere. And so they by no means recognized the existence of those microbes on Earth’s floor — whether or not the creatures lived on crops, slurped uncooked sewage or grew within the armpits of NFL linebackers.

Christner was at Louisiana State College in Baton Rouge when he and Bryan began their seek for life within the stratosphere in 2008. They deliberate to start out low and pattern progressively greater. They’d attempt to develop the cells they collected, as different folks had beforehand finished.

However they might additionally depend the entire variety of residing cells, one thing nobody had ever finished earlier than at that altitude.

They anticipated that at some altitude, the variety of residing cells would drop to zero. This is able to outline the outer limits of life on Earth — and trace on the attainable limits of life on Mars.

Bryan spent three years constructing a collection of microbe collectors, utilizing Styrofoam and balsa wooden bought at Pastime Foyer and Residence Depot. She launched these contraptions into the air dangling from helium balloons. Throughout a dozen launches in Texas and Louisiana, she tried to pattern airborne microbes from as excessive as 25 kilometers. Every gadget included a management chamber that didn’t open in the course of the flight, permitting her to examine whether or not the samples had been contaminated with ground-level microbes. She step by step improved her design, bringing the degrees of contamination all the way down to virtually zero.

By 2013, Bryan was utilizing a extra superior system developed by LSU engineer T. Gregory Guzik. Its rugged circuits allowed it to succeed in even greater altitudes, the place the skinny air may cause electrical energy to arc from one wire to a different, damaging a tool. She and Guzik used this method in a pivotal collection of balloon launches over Fort Sumner, N.M. These can be the final samples that she collected for her Ph.D. analysis, and they’d rework the endeavor from a cool science undertaking into a serious discovery.

A scientific balloon bonanza

The city of Fort Sumner crouches amid an arid plain of grass and yucca in jap New Mexico. The Nineteenth-century gunslinger Billy the Child is buried in a cemetery on the south finish of city. Simply north of city is the airport, consisting of a corrugated metal hangar and cracked tarmac. The ability is sometimes used for launching scientific balloons, which profit from the realm’s secure climate and flat, undeveloped terrain.

On the morning of August 21, 2013, Bryan, Guzik and a number of other different folks carrying laborious hats emerged from the hangar greedy the tether of a spherical white helium balloon the scale of a small elephant. It bobbed and tugged impatiently as they walked it into an open discipline and let go. It surged upward, pulling its payload into the sky.

A downward-pointing digital camera captured video because it rose. The payload swung lazily because the desert shrank right into a hazy, wrinkled mosaic. A big jet — maybe a Boeing 737 — streaked by far beneath, tiny and silent. Later, the balloon was tossed by violent winds, marking its passage into the stratosphere.

When the balloon reached 18 kilometers, an electrical motor whirred, opening a compartment and exposing a sticky collector to snag airborne cells. Minutes later, because the balloon handed 23 kilometers, the compartment door closed. An electrical heating coil severed the balloon tether, sending the payload parachuting again to Earth.

In the meantime, Bryan drove west on Freeway 60 throughout empty rangeland whereas a fellow scholar tracked the balloon by way of GPS. She ultimately veered off the highway onto the dusty plain and after a number of miles discovered her germ catcher nestled between a yucca and a prickly pear cactus.

This was one in all six balloons that Bryan, Guzik and colleagues launched throughout a three-week stint in 2013; some reached as excessive as 29 kilometers. A seventh launch on a bigger NASA balloon reached 38 kilometers throughout that very same interval.

However looking back, the August 21 launch would loom particularly giant. That afternoon within the airport hangar, Bryan examined the pattern for ATP, an vitality molecule present in residing cells. “I actually thought all people was going to be useless,” she says. However check after check revealed the presence of ATP, prompting her to suppose, “Oh my God, they’re alive.”

Again at LSU, Bryan spent a number of months incubating her samples. The one from August 21 spawned a glistening orange bacterial colony, which she named L6-1. Samples from different launches spawned yellow, pink and black colonies. The colours most likely indicated well-known pigments that protected the cells by blocking some UV radiation.

Bryan assumed that these still-unidentified microbes would turn into powerful cookies — perhaps belonging to a bunch referred to as Deinococcus that may survive intense radiation, or to a different group resembling Bacillus that may compact itself into inanimate spores in a position to survive boiling. However as she remoted and barcoded their DNA repeatedly in 2013 and 2014, a really totally different image emerged.

Just one out of a dozen or so micro organism turned out to be the form of spore-former that Bryan anticipated. L6-1 and a number of other others belonged to a genus referred to as Curtobacterium that always lives on crops. Others belonged to teams that inhabit soil.

It “ought to have been nothing however spore-formers,” Bryan says. It “ought to have been nothing however extremophiles.” The truth that these critters gave the impression to be neither offered a thriller that she desperately needed to unravel. Totally unraveling it will take a decade of labor.

Plant pathogen superhighway

Bryan spent most of 2014 and 2015 doing work that was essential to her Ph.D., together with rigorous calculations and lab experiments to transform the cell counts from her balloons into estimates of what number of dwell cells are floating round within the stratosphere.

These calculations would finally get printed with Christner and Guzik in 2019 within the ISME Journal. The work urged that at 24 kilometers excessive, the focus of residing cells was 100,000 per cubic meter. At 36 kilometers, the air nonetheless held practically 8,000 cells per cubic meter.

In 2016, Bryan turned again to her thriller microbes for a brand new set of experiments. She examined their tolerance to UV-C radiation — an ultraviolet band that damages DNA and kills cells much more potently than the UV-A and UV-B bands that trigger sunburns. UV-C is absent on Earth’s floor as a result of it’s blocked by the ozone layer, nevertheless it’s ample within the mid-stratosphere.

Bryan in contrast her stratospheric germs to a bacterium referred to as Deinococcus radiodurans that may survive roughly 1,000 occasions the radiation dose deadly for a human. D. radiodurans was lengthy thought-about the hardest organism on Earth, after its unintended discovery in 1956. Scientists working with the U.S. army had been attempting to sterilize canned meat with gamma radiation. After they later opened the cans, they discovered the meat putrid, spoiled by these micro organism that had by some means survived.

D. radiodurans can be uniquely proof against UV-C radiation. However when Bryan in contrast L6-1 to the supermicrobe, she was shocked on the outcomes. Her dainty little germ, accustomed to the cool, shady canopies of crops, was simply as tolerant to UV-C as D. radiodurans. And a few of her different plant and soil micro organism weren’t far behind.

“Clearly I [fudged] up,” she remembers considering — although utilizing a spicier verb. However whilst she repeated the experiments over a interval of months, searching for potential errors, the outcomes stayed the identical.

This second in 2016 was when the workforce’s understanding “all form of got here collectively,” Christner says. Curtobacterium, the plant- and dirt-dwelling genus that L6-1 belonged to, was “fairly ample within the ambiance,” he says. “Nearly each time we put up a balloon and picked up samples, whether or not it’s within the excessive troposphere or within the stratosphere, we’re accumulating one thing that’s extremely just like this.”

Discovering that L6-1 was proof against radiation and drying hinted that it wasn’t up there accidentally. It urged that these plant-dwelling micro organism, carefully associated to species that had been well-known, had a secret expertise that nobody suspected: surviving, a minimum of briefly, on the fringe of house.

This was the state of data when Bryan completed her Ph.D. in 2017. Her discoveries had adopted a path nobody anticipated. However the story wasn’t full.

As Bryan started a postdoctoral fellowship at MIT, Christner moved from LSU to the College of Florida, the place he started to look at one other intriguing facet of L6-1. Based mostly on Bryan’s earlier DNA barcoding, L6-1 and a number of other of the opposite stratospheric micro organism had been carefully associated to a infamous crop pathogen referred to as Curtobacterium flaccumfaciens. It had plagued Midwestern farms from the Twenties to the Seventies, inflicting total bean fields to brown and wilt. It had step by step pale earlier than re-emerging round 2003.

Hoping to seek out out whether or not the stratospheric micro organism had been additionally pathogens, Christner reached out to 2 plant pathologists in 2020, Brian Kvitko on the College of Georgia in Athens and Robert Harveson on the College of Nebraska in Scottsbluff. Positive sufficient, they discovered that L6-1 brought about bean crops to yellow and wilt. So did two different Curtobacterium microbes Bryan had collected from as excessive as 29 kilometers.

That very same yr, Christner’s new graduate scholar, Adam Ellington, sequenced the complete genome of L6-1, revealing it as a brand new species, which they named Curtobacterium aetherium. It matched the DNA sequence of one other unidentified pathogen that Harveson remoted from a diseased millet crop in Nebraska years earlier than.

The invention that C. aetherium was a plant pathogen was lastly printed in 2025 in Microbiology Spectrum. “Plant pathogens within the stratosphere — that was the furthest factor that we anticipated to run into,” Christner says.

Harveson, a worldwide authority on Curtobacterium, had lengthy believed that these pathogens unfold on contaminated seeds. He was shocked to study that a minimum of some members of its genus may survive so excessive within the ambiance. “I don’t doubt that it may transfer that method,” he says. “I’m simply amazed that so lots of these items survived.”

“Brent [Christner] and his group are form of the pioneers for this,” Harveson now says. What they discovered may “change our thought processes about motion of ailments.”

Life finds a method

The benefits of touring lengthy distances by the ambiance could possibly be enormous. By surviving these journeys, a microbe may cross oceans and sprinkle down on new ecosystems, diversifying into numerous niches. A pathogen like C. aetherium may escape areas the place crops develop resistance and land on new hosts. Proof has been constructing for a while that plant- and soil-dwelling microbes are nicely fitted to making the leap.

Micro organism that dwell on leaves are identified to expertise frequent drying, which might harm their DNA in a way just like radiation. So a few of them can already restore their DNA moderately nicely. Leaf micro organism can even loft into the air with stunning ease. One landmark research printed in 1982 discovered that in a heat day in a bean discipline, microbes levitated into the air — practically 20 billion cells per hectare per hour — lifted by breezes, thermal updrafts or electrical fields that coalesce across the drying leaves. Micro organism might even encourage this by clumping onto tiny leaf hairs the place air currents can extra simply carry them.

As soon as airborne, microbes most likely have a number of methods of getting 10 or 20 kilometers into the air. Main thunderstorms, mud storms, hurricanes and volcanic eruptions can quickly carry them. Smoke plumes from giant wildfires can carry billions of dwell cells into the air, and in some instances can attain the stratosphere.

Ellington did work that means C. aetherium may have originated from a leaf-squatting, levitating ancestor, which then developed new diversifications for surviving excessive altitudes because it obtained lofted greater.

In a single experiment, Ellington checked out C. flaccumfaciens, the crop pathogen that’s carefully associated to C. aetherium however has not been discovered excessive within the ambiance. He discovered that it’s already reasonably tolerant to UV-C radiation. When he uncovered it to 12 rounds of UV-C, its survival at excessive doses elevated by one other 20- to 80-fold. This bolsters the concept some plant-dwelling micro organism have a pure aptitude for evolving excessive radiation safety in the event that they’re steadily lofted excessive into the ambiance.

When Ellington examined the genes that C. aetherium activates throughout UV-C publicity, he discovered three particularly notable DNA restore enzymes — suggesting that its specific model of UV-C resistance could also be well-suited for the excessive ambiance.

The “most intriguing” was an enzyme referred to as spore photoproduct lyase, Ellington says. It repairs an uncommon sort of UV harm that happens solely when a cell is so dehydrated that its DNA re-arranges from the conventional double helix right into a tighter, crystalline kind. A cell that’s drifting within the stratosphere is “basically in a desiccated state,” Ellington says. So having this enzyme may permit it to restore a kind of DNA harm that solely occurs in these circumstances.

Ellington discovered that C. aetherium additionally makes use of two totally different photolyase enzymes. Not like many different DNA restore proteins, these don’t require food-derived vitality to do their job. As an alternative, they restore DNA utilizing daylight. This can be a “enormous benefit for an organism that’s attempting to outlive” within the ambiance, the place meals is scarce however daylight is plentiful, Ellington says. He and colleagues printed the findings in March 2025 in Microorganisms.

Meals shortage might signify a serious problem for microbes within the ambiance. Some micro organism might produce other methods for dealing with it. Whereas sampling clouds drifting over distant Réunion Island within the Indian Ocean, aerobiologist Pierre Amato on the College Clermont Auvergne in France discovered a pressure of Methylobacterium that harnesses daylight to provide the vitality molecule ATP. This might permit it not solely to restore its DNA with out burning valuable energy, but in addition to do numerous different duties, like manufacturing proteins.

Microbiologist Chris Greening of Monash College in Clayton, Australia, believes that airborne micro organism can even faucet one other hidden vitality supply. He has discovered that many micro organism residing on bushes, in soils and even drifting just a few meters up within the air can oxidize hydrogen and carbon monoxide, that are hint gases within the ambiance. He’s just lately discovered that some microbes may even burn hydrogen at –40° Celsius, a temperature just like the stratosphere’s. Along with releasing vitality, hydrogen oxidation truly produces water as its sole combustion product. “So, it’s hydrating the cell,” Greening says. “That is perhaps simply sufficient for some cells to keep away from desiccation.”

The work finished thus far has revealed a coherent story about how and why C. aetherium and a few of its cousins may have developed to outlive excessive within the ambiance. However many different tales should disguise among the many 1000’s of different residing cells that Bryan retrieved. She managed to develop solely 12 or so species of micro organism from the stratosphere, however for each sort of microbe that did develop within the lab, 100 to 1,000 others might have been current however didn’t develop within the culturing circumstances that she used. A few of them may even be extra stunning.

“The subsequent query can be to grasp the entire group” up there, says Burak Erkorkmaz, a microbiologist on the College of Gothenburg in Sweden.

He envisions utilizing a way within the stratosphere that he employed a number of years in the past, sampling air at floor degree in Israel. He sequenced DNA from the entire cells that he collected, figuring out a whole lot of species. Doing this within the stratosphere would offer a fuller image of the species probably current there. It might trace at whether or not they, like C. aetherium, carry genes for surviving at excessive altitudes and the way that may match into their evolutionary area of interest.

Scientists are already testing that strategy. In 2018, NASA’s C-20A jet sampled microbes from as much as 12 kilometers over California, just under the stratosphere. Preliminary outcomes confirmed DNA sequences for microbes that inhabit soil, rotting leaves and the anatomic nooks and crannies of people or animals, seemingly derived from farmlands of the Central Valley, immediately upwind.

Many questions stay. Though Bryan estimated that C. aetherium can hunker down for just a few months or weeks at 20 to 36 kilometers, returning to Earth quickly sufficient to outlive the journey would nonetheless require some luck. For that reason, even C. aetherium most likely makes most of its profitable flights at decrease altitudes, say 5 to fifteen kilometers, the place circumstances are extra gentle — although nonetheless deadly for many microbes. Besides, merely discovering that these micro organism can survive within the stratosphere has enormous implications. It’s spurring new ideas about the place life may survive in different worlds. Together with ones which might be hellishly sizzling.

From Earth to exoplanets

The lead-melting floor of Venus is inhospitable to life as we all know it. However scientists have observed some tantalizing chemical signatures in a cloud layer 48 to 60 kilometers above the floor, the place temperatures and atmospheric pressures are just like these at Earth’s floor. Potential traces of a number of gases, together with oxygen, methane, ammonia and phosphine, have fueled hypothesis that Venusian microbes may inhabit these clouds, producing these gases by their metabolism.

If such cloud-dwelling microbes exist, they must survive periodic bouts of nastiness as they sank into decrease, hotter layers of the ambiance. In 2021, Bryan and Sara Seager, a planetary scientist at MIT, proposed within the journal Astrobiology that these Venusian microbes may survive the way in which C. aetherium appears to in Earth’s ambiance: by briefly drying out and hunkering down, then re-awakening as they’re swept again into cooler, wetter cloud layers.

Micro organism that Bryan remoted from the stratosphere in 2013 may additionally information the seek for life in different photo voltaic programs which might be light-years away.

For instance, the critters grown within the lab produced these vivid pigments which might be identified to guard microbes from UV radiation. Final November, astrobiologist Lígia Coelho of Cornell College urged that scientists may use this trait to establish distant planets with microbial life of their atmospheres.

Such life may also have to guard itself from UV radiation. By analyzing pigments from Bryan’s stratospheric micro organism, together with C. aetherium, Coelho recognized their particular patterns of absorbance and reflectance at sure wavelengths of sunshine.

“We’d be capable to seize [these patterns] with future telescopes” taking a look at Earth-sized exoplanets, Coelho says. The preliminary candidates can be small planets orbiting close by stars resembling Proxima Centauri, which is about 4 light-years from Earth.

“That is opening a complete new paradigm” for searching for life on exoplanets, she says — particularly on Earthlike planets with a variety of liquid water and clouds that may obscure the planetary floor.

However discovering C. aetherium within the stratosphere might be most related for fascinated about life on Mars. C. aetherium itself most likely couldn’t survive on Mars as a result of it requires oxygen, which is essentially absent there. However its sheer toughness means that different microscopic Earthlings most likely may.

Astrobiologist Andrew Schuerger on the College of Florida in Gainesville has spent years searching for candidates. In 2025, he and Rachel Harris, an astrobiologist with Harvard and NASA, reported that a microbe referred to as Methanosarcina barkeri comes surprisingly shut.

M. barkeri usually lives in sewage, landfills, rice paddies and lake bottoms with out oxygen. It obtains vitality by changing carbon dioxide and hydrogen into methane.

As a result of M. barkeri typically lives underground and underwater, one would hardly count on it to thrive within the excessive dryness and skinny ambiance on Mars. Nonetheless, Schuerger and Harris discovered that even at low, Marslike gasoline pressures — just like Earth’s stratosphere — it continues to provide methane.

For Mars fanatics, that’s massive information. Orbiters and rovers have repeatedly detected methane gasoline seeping into the planet’s ambiance, a possible byproduct of microbial life beneath the floor. Because of the low strain and dryness on the Martian floor, these methane-making critters might need to dwell kilometers underground, making them practically not possible for a Mars rover to seek out.

However discovering that M. barkeri is energetic at such low pressures “opens up your entire crust from the floor [downward]” as attainable habitat on Mars, Schuerger says. If a Mars rover drilled 20 centimeters into the bottom in the precise place, it may probably encounter tiny single-celled Martians — in the event that they exist.

The niches the place microbes may dwell just under the Martian floor may nonetheless be fairly small, says astrobiologist Lynn Rothschild at NASA Ames Analysis Heart in Mountain View, Calif. However Rothschild imagines Martian microbes utilizing the floor of the planet the way in which C. aetherium appears to make use of Earth’s ambiance: as a hostile atmosphere that may be tolerated for a interval time to maneuver from one place to a different.

She imagines windstorms on Mars carrying not solely mud, but in addition Martian microbes swept up with it. Microbes “can be floating round, simply searching for a spot to land [where] they’ll survive” and develop for some time, Rothschild says. If that’s the case, then a rover may discover life the way in which a fence finds blowing leaves.

Years might cross earlier than the teachings from Earth’s stratosphere discover their method into the seek for life on Mars or Venus. However for Bryan, the joy of it started years in the past, whilst she made her first tentative journeys to Pastime Foyer, gathering supplies that she would use to catch micro organism excessive up within the sky.

“For me it was house exploration,” Bryan says. “It was uncharted territory.”