Atlas of the atmosphere

Every cubic meter of air holds up to 100 million microorganisms, but the diversity and behavior of these microbes remains masked to microbiologists — until recently, that is.

Photo by Vanessa Schipani

Thanks to next-generation sequencing techniques, scientists are finally uncovering the details of the biodiversity and biogeography of this largely unknown ecosystem. They are discovering airborne microbes do much more than just ride the wind transmitting disease — microbes also help create the intricately beautiful designs in snowflakes and facilitate the formation of clouds, for example. Studying them, researchers say, could give insight into how to better monitor global climate change, as well as predict and track weather cycles and disease and allergen outbreaks.

"There's going to be an explosion of studies using these new techniques," said Jessica Green, microbial ecologist at the University of Oregon.

Photo by Vanessa Schipani

Recent research published in PNAS suggests that the diversity of microbial life in the air is on par with the soil, at least in urban areas, yet the air remains vastly understudied in comparison.

"Just seven or ten years ago we didn't realize bacteria existed in clouds," said Anne-Marie Delort, professor of microbiology and organic chemistry at Université Blaise Pascal in France. Now researchers know microbes act as a surface for the condensation of water vapor in the atmosphere, thus forming clouds. Recent research publish in Science shows microbes also play the same role during snowflake formation and other types of precipitation. The next step, Delort said, is to uncover their metabolic activity in clouds and influence on atmospheric processes. If they are metabolically active, she added, microbes could not only be acting as cloud condensers, but affecting the carbon and nitrogen cycles as well.

Airborne microbes could even play a role in the impact of climate change. According to Christine Rogers, aerobiologist at the University of Massachusetts, the increase of carbon dioxide in the atmosphere could be creating more and larger plants which are food for microscopic fungi. More airborne fungi could affect people with allergies and asthma — plus, added Green, create a feedback loop by providing more surface area for cloud condensation, helping to create more clouds. The number of clouds in the air affects how much heat is trapped in the atmosphere as well as how much heat is reflected from the sun, said Delort. However, researchers aren't sure whether more clouds in the sky will heat or cool the atmosphere, but they are positive it will affect climate change one way or the other.

One issue that has set aerobiology behind its sister fields of soil and water microbiology is that it needs an intimate integration of both the physical and biological sciences to progress — two fields that often don't consider each other. "Physicists don't think about the possibility of life [in the air]," said Delort. "They study the particles, but ignore the biology."

Aerobiologists, on the other hand, often ignore the important physical and chemical characteristics of microbes, said Jordan Peccia, environmental engineer at Yale University. In order for the field of air microbiology to blossom, the two disciplines need to work together.

Quantifying the diversity of microbes in the air has also proven difficult until relatively recently, said Noah Fierer, microbial ecologist at the University of Colorado, Boulder. After the development of high-throughput pyrosequencing in 2005, however, researchers began to infiltrate the secret lives of airborne microbes. Now, scientists can "describe the spatial and temporal variability in these communities without relying on culture-based techniques that miss the majority of bacteria living in the atmosphere," he said.

Fierer has a simple, yet ambitious plan to uncloak the microorganisms of the atmosphere around the globe: map them. Starting with the continental United States, he intends to survey every state in more than 200 different urban and rural locations by developing a low-cost sampling device that can be sent to volunteers to sample the airborne bacteria outside their homes.

Fierer said he believes that the map will help determine the effects of land-use (e.g. agricultural, urban, suburban) and season on bacterial proliferation and distribution. It will also help decipher the influence of airborne bacteria on weather, climate change, and plant and human health.

Fierer's team also plans to collect samples of fungi and viruses in the air, and is currently working out the techniques for analyzing viral communities.

Photo by Vanessa Schipani

Researchers also need to pin down how different types of microbes get into the atmosphere in the first place. One hypothesis is that some microbes are released into the atmosphere from the popping bubbles of crashing ocean waves. In most cases, however, wind is the likely culprit for spreading microbes from solid surfaces like leaves and soil into the air. Wind can be a powerful transporter for microbes — microbes riding on airborne desert dust travel freely between Africa, Europe, and the Caribbean.

Many researchers still view the atmosphere solely as a conduit for the transportation of microbes by wind, but not a habitat in and of itself. Green, however, said she believes that the atmosphere has everything a microbe would need to survive: tolerable temperatures, reasonable pH levels, and sources of organic carbon that are on par with soil and water. It's just a matter of surveying the air to discover whether the microbes are metabolically active for extended periods of time while suspended in cloud water and in the air, she said, which would suggest that they are inhabitants of the atmosphere rather than passengers on the wind. The microbes could potentially sustain populations though 50 generations, said Green. Assuming a four-day generation time, microbes could be suspended in the atmosphere for up to 200 days.

"If the atmosphere is a habitat where microbes live, this will fundamentally change our conceptions of atmospheric processes," Green added.

Scientists studying the life of the air agree: They aren't going to get bored studying airborne microbes for a long while. "We don't need to travel to deep sea vents [to study microbes]," said Fierer. "The air right outside our door is teeming with things to explore."


A. Womack et al., "Biodiversity and biogeography of the atmosphere," Philosophical Transactions of the Royal Society: B, 365: 3645-53, 2010.

J Wolf, et al., "Elevated Atmospheric Carbon Dioxide Concentrations Amplify Alternaria alternate Sporulation and Total Antigen Production," Environmental Health Perspectives, 118: 1223-28, 2010.

A. Delort et al., "A short overview of the microbial populations in clouds: Potential roles in atmospheric chemistry and nucleation processes," Atmospheric Research, AOP, doi: 10.1016/jatmosres.2010.07.004, 2010.

R. Bowers et al., "Spatial variability in airborne bacterial communities across land-use types and their relationships to the bacterial communities of potential source environments," The International Society for Microbiology Ecology Journal, AOP, doi:10.1038/ismej.2010.167, 2010.

B. Christner et al., "Ubiquity of Biological Ice Nucleators in Snowfall," Science, 319: 1214, 2008.

E. Brodie et al., "Urban aerosols harbor diverse and dynamic bacterial populations," PNAS, 104:299-304, 2006.

Editor's note (December 1): When originally posted, the article stated that every cubic meter of air holds upwards of 100 million microorganisms. We meant to say "up to," meaning a potential maximum of 100 million. The Scientist regrets the error.


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