Large numbers of microorganisms, mostly bacteria, have been discovered five miles up in the sky.
It's not known whether they were simply carried there by air currents, or whether that's where they routinely live, perhaps consuming the carbon compounds that are also to be found in the troposphere.
But the finding is of interest to atmospheric scientists, as the microorganisms could play a role in forming ice, which can affect the weather and climate.
The microorganisms were documented in air samples taken as part of NASA's Genesis and Rapid Intensification Processes (GRIP) program, which studies the low- and high-altitude air masses associated with tropical storms. The sampling was carried out over both land and ocean, and took place before, during and after two major tropical hurricanes - Earl and Karl - in 2010.
"We did not expect to find so many microorganisms in the troposphere, which is considered a difficult environment for life," says Kostas Konstantinidis of the Georgia Institute of Technology. "There seems to be quite a diversity of species, but not all bacteria make it into the upper troposphere."
Aboard the aircraft, a filter system collected particles, including the microorganisms, and analyzed them using genomic techniques including polymerase chain reaction (PCR) and gene sequencing. This allowed the researchers to detect the microorganisms and estimate their quantities without using conventional cell-culture techniques.
When the samples were taken over the ocean, the team found mostly marine bacteria, while the air over land had mostly terrestrial bacteria. The hurricanes had a significant impact on their distribution and dynamics.
The study showed that viable bacterial cells represented, on average, around 20 percent of the total particles detected in the size range of 0.25 to 1 microns in diameter. This was at least ten times the amount of fungi in the samples, and the researchers detected 17 different bacteria taxa - including some that can metabolize the carbon compounds that are everywhere in the atmosphere, such as oxalic acid.
The microorganisms could have a previously-unidentified impact on cloud formation by supplementing or replacing the non-living particles 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," says Athanasios Nenes of Georgia Tech. "If they are the right size for forming ice, they could affect the clouds around them."
The microorganisms likely reach the troposphere in the same way as dust and sea salt. "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 Nenes.
"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."