Skies Full of Life: Microbes May Thrive in the High Atmosphere

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Image: NASA Earth Observatory




Each year, hundreds of millions of metric tons of
dust, water, and humanmade pollutants make
their way into the atmosphere, often traveling between continents on jet streams. Now a new study confirms that some microbes make the trip
with them, seeding the skies with billions of
bacteria and other organisms—and potentially
affecting the weather. What’s more, some of
these high-flying organisms may actually be able
to feed while traveling through the clouds, forming an active ecosystem high above the
surface of the Earth. The discovery came about when a team of
scientists based at the Georgia Institute of
Technology in Atlanta hitched a ride on
nine NASA airplane flights aimed at
studying hurricanes. Previous studies carried out
at the tops of mountains hinted that researchers were likely to find microorganisms at high
altitudes, but no one had ever attempted to
catalog the microscopic life floating above the
oceans—let alone during raging tropical storms.
After all, it isn’t easy to take air samples while
your plane is flying through a hurricane. Despite the technical challenges, the researchers
managed to collect thousands upon thousands of
airborne microorganisms floating in the
troposphere about 10 kilometers over the
Caribbean, as well as the continental United States
and the coast of California. Studying their genes back on Earth, the scientists counted an average
of 5100 bacterial cells per cubic meter of air, they
report online today in the Proceedings of the National Academy of Sciences. Although the
researchers also captured various types of fungal
cells, the bacteria were over two orders of
magnitude more abundant in their samples. Well
over 60% of all the microbes collected were still
alive. The researchers cataloged a total of 314 different
families of bacteria in their samples. Because the
type of genetic analysis they used didn’t allow
them to identify precise species, it’s not clear if
any of the bugs they found are pathogens. Still,
the scientists offer the somewhat reassuring news that bacteria associated with human and
animal feces only showed up in the air samples
taken after Hurricanes Karl and Earl. In fact, these
storms seemed to kick up a wide variety of
microbes, especially from populated areas, that
don’t normally make it to the troposphere. This uptick in aerial microbial diversity after
hurricanes supports the idea that the storms
“serve as an atmospheric escalator,” plucking
dirt, dust, seawater, and, now, microbes off
Earth’s surface and carrying them high into the
sky, says Dale Griffin, an environmental and public health microbiologist with the U.S.
Geological Survey in St. Petersburg, Florida, who
was not involved in the study. Although many of the organisms borne aloft are
likely occasional visitors to the upper
troposphere, 17 types of bacteria turned up in
every sample. Researchers like environmental
microbiologist and co-author Kostas
Konstantinidis suspect that these microbes may have evolved to survive for weeks in the sky,
perhaps as a way to travel from place to place
and spread their genes across the globe. “Not
everybody makes it up there,” he says. “It’s only
a few that have something unique about their
cells” that allows them survive the trip. The scientists point out that two of the 17 most
common families of bacteria in the upper
troposphere feed on oxalic acid, one of the most
abundant chemical compounds in the sky. This
observation raises the question of whether the
traveling bacteria might be eating, growing, and perhaps even reproducing 10 kilometers above
the surface of Earth. “That’s a big question in the
field right now,” Griffin says. “Can you view [the
atmosphere] as an ecosystem?” David Smith, a microbiologist at NASA’s Kennedy
Space Center in Florida, warns against jumping to
such dramatic conclusions. He also observed a
wide variety of microbes in the air above
Oregon’s Mount Bachelor in a separate study, but
he believes they must hibernate for the duration of their long, cold trips between far-flung
terrestrial ecosystems. “While it’s really exciting
to think about microorganisms in the atmosphere
that are potentially making a living, there’s no
evidence of that so far.” Even if microbes spend their atmospheric travels
in dormancy, that doesn’t mean they don’t have
a job to do up there. Many microbial cells are the
perfect size and texture to cause water vapor to
condense or even form ice around them,
meaning that they may be able to seed clouds. If these microorganisms are causing clouds to form,
they could be having a substantial impact on the
weather. By continuing to study the sky’s
microbiome, Konstantinidis and his team hope to
soon be able to incorporate its effects into
atmospheric models.
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