Could high-flying bacteria affect the weather?

GEORGIA TECH (US) — Bacteria living four to six miles above the surface of the Earth could be playing a role in the formation of ice that affects weather and climate.

In what is believed to be the first study of its kind, researchers used genomic techniques to document the presence of significant numbers of living microorganisms in the middle and upper troposphere.

Whether the microorganisms routinely inhabit this portion of the atmosphere—perhaps living on carbon compounds also found there—or whether they were simply lofted there from the Earth’s surface isn’t yet known. Researchers say long-distance transport of the bacteria could also be of interest for disease transmission models.

The eye of Hurricane Earl is shown outside the window of a NASA DC-8 aircraft gathering air samples for a study of low- and high-altitude air masses associated with tropical storms. (Credit: NASA)


The microorganisms were documented in air samples taken as part of NASA’s Genesis and Rapid Intensification Processes (GRIP) program 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 sampling 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, assistant professor in the School of Civil and Environmental Engineering at the Georgia Institute of Technology (Georgia Tech). “There seems to be quite a diversity of species, but not all bacteria make it into the upper troposphere.”

As reported in Proceedings of the National Academy of Sciences, a filter system aboard the aircraft collected particles—including the microorganisms—from outside air entering the aircraft’s sampling probes. The filters were analyzed 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.

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 effect on the distribution and dynamics of microorganism populations.

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. By at least one order of magnitude, bacteria outnumbered fungi in the samples, and the researchers detected 17 different bacteria taxa—including some that are capable of metabolizing the carbon compounds that are ubiquitous in the atmosphere—such as oxalic acid.

The microorganisms could have an affect on cloud formation by supplementing (or replacing) the abiotic particles that normally serve as nuclei for forming ice crystals, says Athanasios Nenes, a professor in the School of Earth and Atmospheric Sciences and School of Chemical and Biomolecular Engineering.

“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,” Nenes says. “If they are the right size for forming ice, they could affect the clouds around them.”

The microorganisms 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,” he says.

The research brought together microbiologists, atmospheric modelers, and environmental researchers using the latest technologies for studying DNA. For the future, the researchers would like to know if certain types of bacteria are more suited than others for surviving at these altitudes. The researchers also want to understand the role played by the microorganisms—and determine whether or not they are carrying on metabolic functions in the troposphere.

“For these organisms, perhaps, the conditions may not be that harsh,” says 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.”

Other researchers have gathered biological samples from atop mountains or from snow samples, but gathering biological material from a jet aircraft required a novel experimental setup. The researchers also had to optimize protocols for extracting DNA from levels of biomass far lower than what they typically study in soils or lakes.

“We have demonstrated that our technique works, and that we can get some interesting information,” Nenes says. “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.”

NASA and the National Science Foundation supported the research.

Source: Georgia Tech