Chemical & Engineering News Digital Edition - Institute of Materia ...

SHUTTERSTOCK
TEN BUCKS says that the last time you
looked up at the sky on an overcast day,
musings about the behavior of bacteria in
the clouds did not flit across your mind.
Many of us think of the sky as an evolving
mixture of inanimate entities: water vapor,
nitrogen, oxygen, ozone, aerosols, polluting
particles, and increasingly, carbon dioxide.
But a more comprehensive list would
include pollen, algae, dandruff, and bacteria.
On average, these so-called biogenic
aerosols, which are swept up into the atmosphere
by wind, account for about 20%, by
mass, of the particulate matter in the sky.
In the air above tropical rainforests, that
figure can be as much as 75%.
“When you unravel the chemical composition
of the atmosphere, you quickly
find that there are a lot of biological particles
up there,” says Ulrich Pöschl, an
atmospheric scientist at the Max Planck Institute
for Chemistry in Mainz, Germany.
Although microbiologists have long
known that bacteria float about in the sky—
more than 4 miles from Earth’s surface—
the bugs were seen simply as “severely
stressed-out passive passengers” in the air,
accidentally windswept out of their more
surface-bound niches, says Cindy E. Morris,
a microbiologist at France’s National Institute
of Agronomic Research, in Montfavet.
After all, our blue sky can be a pretty
harsh environment: It’s cold. There are oxidants.
And there is intense ultraviolet light
that can wreck the heartiest of genomes.
SCIENCE & TECHNOLOGY
BACTERIA IN CLOUDS
Microbial METEOROLOGISTS investigate how
airborne microbes might influence weather
SARAH EVERTS, C&EN BERLIN
FLUFFY
BUSINESS
Microbes live
in clouds, but
what exactly
are they doing
up there?
Worst of all, the sky is a dry, desiccating
place for biological cells.
But bacteria are resilient: They are famous
for eking out an existence in extreme
habitats. Think ice, the plaque on your
teeth, deep ocean thermal vents, even airplane
fuel tanks. Why not the atmosphere?
It is only recently that scientists have begun
to think that bacteria, in particular, may
not be up there just for the ride. In 2006, the
first workshop on microbial meteorology
was held in France, and the community has
been growing since then, Morris says. She
estimates there are between 30 and 50 labs
around the world investigating this topic.
These researchers are providing growing
evidence that in the wet oasis of the
clouds, where bacteria can at least remain
hydrated, the microbes aren’t just loitering—they’re
getting down to business.
For example, bacteria in clouds are using
the energy stored in adenosine triphosphate
to take care of their biochemical
needs. Bacteria have been caught breaking
down airborne carbon compounds, including
organic aerosols. This observation
begs the questions: Is the metabolic life
of bacteria playing a role in atmospheric
WWW.CEN-ONLINE.ORG 40 APRIL 14, 2008
chemistry? Can this airborne microbial
population act as a carbon sink or source?
Do the bacteria degrade pollution?
EVEN MORE TANTALIZING, the bacterium
called Pseudomonas syringae, which is
found regularly in clouds, has proteins on
its cell surface that can nucleate the formation
of ice, the precursor to most forms of
precipitation. Some atmospheric scientists
are starting to wonder if P. syringae and other
bacteria in the air may influence weather
by initiating rain and snow.
A recent Science paper whetted the palates
of many cloud microbiologists by revealing
that ice-nucleating biological particles such
as bacteria are ubiquitous in snowfalls from
all around the world (2008, 319, 1214).
One of the authors, Brent C. Christner,
a microbiologist at Louisiana State University,
says he’s excited about the data.
But he acknowledges that it’s one thing to
find cloud-living, ice-nucleating bacteria in
snowfalls; it’s another thing to say conclusively
that the bugs initiated the snowfall.
“Just because you can culture bacteria
from snow doesn’t mean they actually
caused the snowfall,” he adds. “I’d like to
think so, but the proof isn’t there yet.
“It’s like sifting through the ashes of a
major blaze, finding a lighter in the rubble,
and saying, ‘Aha! This is the cause of the
blaze.’ ”
But catching bacteria in the act of icenucleation,
at altitude, in situ, is exactly
what Christner, Morris, and others in the
field are hoping to do.
On Earth P. syringae is a plant pathogen
that lives on the surfaces of a wide variety
of foliage—from tea plant leaves in Zimbabwe
to corn stalks in the U.S. The microbe
uses its ice-nucleation skills to freeze
water on these leaves. Specifically, P. syringae
can catalyze the formation of frost at
−2 °C, whereas pure water on a leaf surface
doesn’t spontaneously freeze before the
temperature drops to several tens of degrees
below 0 °C.
The frost causes the plant cells to burst,
allowing the pathogenic bacterium to gain
entry and infect the plant or its fruit, explains
Steve Lindow, a plant microbiologist
at the University of California, Berkeley.
“Ice nucleation leading to precipitation
could be a ‘do-it-yourself’ landing strategy
for bacteria in the atmosphere.”