Life in the upper troposphere

Sci-fi writers from Arthur Conan Doyle to Ray Bradbury have long speculated about the invisible lighter-than-air creatures that must surely inhabit the realms above the ground. After all, just about every other niche on the planet has life. Turns out they were right – there are skybugs up there.

Genomic techniques to document microorganisms in troposphere

Scientists have long known there are airborne microorganisms living near the Earth’s surface. However, in a recent study believed to be the first of its kind, researchers used genomic techniques to document the presence of significant numbers of living microorganisms – principally bacteria – in the middle and upper troposphere – that section of the atmosphere approximately 8–15 km above the Earth’s surface.

Most of the mass of the atmosphere is in the troposphere, and almost all weather (clouds, rain, storms, winds and so on) happens within this layer. Whether the microorganisms the researchers found usually live in this portion of the atmosphere – perhaps living on atmospheric carbon compounds such as oxalic acid – or whether they lofted there on air currents from the Earth’s surface isn’t yet known.

Possible role of microorganisms in forming ice

Though more research is needed, discovering this host of life is of interest to atmospheric scientists because the microorganisms could play a role in forming ice, which in turn can affect the hydrological cycle, clouds and climate. Also, if the microorganisms were once surface dwellers, then their long-distance transport through the troposphere could have implications for disease transmission models.

In a press release from the Georgia Institute of Technology, the researchers explain that the microorganisms could have an impact on cloud formation by supplementing (or replacing) the abiotic (non-living) particles, such as dust, that normally serve as nuclei for forming ice crystals.

“In the absence of dust or other materials that could provide a good nucleus for ice formation, just having a small number of these microorganisms around could facilitate the formation of ice at these altitudes and attract surrounding moisture. If they are the right size for forming ice, they could affect the clouds around them,” says Professor Athanasios Nenes from the School of Earth and Atmospheric Sciences and School of Chemical and Biomolecular Engineering at Georgia Tech.

Specially designed filter system for collecting microorganisms

The microorganisms were documented in air samples taken as part of NASA’s Genesis and Rapid Intensification Processes (GRIP) programme to study low- and high-altitude air masses associated with tropical storms. The sampling was done from a DC-8 aircraft over both land and ocean, including the Caribbean Sea and portions of the Atlantic Ocean. The samples were collected in cloudy and cloud-free air masses before, during and after two major tropical hurricanes – Earl and Karl – in 2010.

Aboard the aircraft, a filter system designed by the research team collected particles – including the microorganisms – from outside air entering the aircraft’s sampling probes. The filters were analysed using genomic techniques including polymerase chain reaction (PCR) and gene sequencing, which allowed the researchers to detect the microorganisms and estimate their quantities without using conventional cell-culture techniques.

Diversity of species among microorganisms collected

Professor Kostas Konstantinidis, School of Civil and Environmental Engineering at Georgia Tech, said they did not expect to find so many microorganisms in the troposphere, which is considered a difficult environment for life. However, he said there seemed to be quite a diversity of species.

When the air masses studied originated over the ocean, the sampling found mostly marine bacteria. Air masses that originated over land had mostly terrestrial bacteria. The researchers also saw strong evidence that the hurricanes had a significant impact on the distribution and dynamics of microorganism populations.

The study showed that viable bacterial cells represented, on average, around 20% of the total particles detected in the size range of 0.25–1 microns in diameter. Bacteria outnumbered fungi by at least 10 times in the samples, and the researchers detected 17 different bacterial taxa – including some that are capable of living off the carbon compounds that are everywhere in the atmosphere.

If the microorganisms originated on the Earth’s surface, they likely reach the troposphere through the same processes that launch dust and sea salt skyward. “When sea spray is generated, it can carry bacteria because there are a lot of bacteria and organic materials on the surface of the ocean,” says Professor Nenes.

Do microorganisms carry out metabolic functions in the troposphere?

The researchers want to understand the role played by the microorganisms – and determine whether or not they are carrying out metabolic functions in the troposphere.

“For these organisms, perhaps the conditions may not be that harsh,” says Professor Konstantinidis. “I wouldn’t be surprised if there is active life and growth in clouds, but this is something we cannot say for sure now.”

“A big fraction of the atmospheric particles that traditionally would have been expected to be dust or sea salt may actually be bacteria. At this point, we are just seeing what’s up there, so this is just the beginning of what we hope to do,” says Professor Nenes.

Future research will aim to figure out how the microorganisms get there and then survive at such heights.

The research, which has been supported by NASA and the National Science Foundation, was published online on 28 January 2013 in the Proceedings of the National Academy of Sciences (PNAS).