News•January 28, 2013

Scientists Find Bacteria Survive at High Altitudes

Next time you're flying somewhere and the plane knives through a cloud, consider this from a new study published on Monday: that cloud is teeming with microscopic particles, including some of the same bacteria you encounter regularly on the ground.

According to that new study, published in the journal Proceedings of the National Academy of Sciences, those microorganisms likely play a key role in forming clouds and precipitation The study alters scientists’ understanding of how microorganisms spread around the world, and comes on the heels of other studies that have found bacteria to be widespread in the lower atmosphere.

Terry Lathem, a graduate student in Georgia Tech's School of Earth and Atmospheric Sciences, takes notes aboard a NASA DC-8 gathering samples of microorganisms in the atmosphere.
Credit: Georgia Institute of Technology.

This is important for three main reasons. First, particles such as dust and microorganisms play a role as the building blocks of clouds by serving as what scientists refer to as condensation nuclei. Basically, they are the particles around which drops of water or crystals of ice form and congeal to eventually form a cloud, and eventually, precipitation.

Second, the study implies that bacteria can travel intact for great distances via high-altitude winds, which means that bacteria swept up in an African dust storm could conceivably make it across the Atlantic Ocean with the help of a hurricane or two, before being deposited on U.S. soil. This has significant implications for how bacteria are distributed worldwide, and may hold clues to how bacteria-caused illnesses spread as well.

Third, one of the largest sources of uncertainty in climate change projections is how clouds will change in composition and abundance as the world warms. Some types of clouds help warm the planet, while others can help cool it by reflecting solar radiation away from the planet. This study suggests that the effects of bacteria may also need to be considered in climate scientists' calculations.

“The million-dollar question in the field [right now] is how much living things can impact clouds, the hydrological cycle, and the climate overall,” said Athanasios Nenes, a professor in Earth and Atmospheric Sciences at Georgia Tech, in an interview.

The study is the first to directly measure the composition and prevalence of microorganisms in the middle to upper troposphere above the oceans, at altitudes of between 5 to 9 miles. The particles studied have a width of one half to 1 micron, which is about the size of a dust particle. To put that in perspective, there are about 25,400 microns in one inch. Previous studies had relied on observations in the lower atmosphere, on mountaintops, and analyses of snow cover for clues into the role of bacteria and fungi in the atmosphere.

The study was conducted by an interdisciplinary team of researchers, including microbiologists and atmospheric scientists, who took air samples using a filter system onboard a NASA DC-8 jet during a 2010 field campaign to study air masses associated with tropical storms. The high-altitude microorganism hunt included observations of several different environments, which allowed the researchers to gain a better understanding of which microorganisms tend to be present under different circumstances.

Scanned electron microscope image of a type of streptococcus bacteria.
Credit: Microbewiki.

The high-altitude hunt for bacteria and fungi took place over both land and ocean, including the Caribbean Sea and portions of the Atlantic Ocean, as well as the U.S. mainland. The sampling also took place before, during and after two significant 2010 hurricanes — Earl and Karl.

The filters were analyzed using genomic techniques, including polymerase chain reaction (PCR) and gene sequencing, which allowed the scientists to identify the microorganisms and estimate their quantities, without using more conventional culture methods.

The findings came as a surprise to some, since the assumption had been that most particles in the mid to upper atmosphere are dust or sea salt.

“We did not expect to find so many microorganisms in the troposphere, which is considered a difficult environment for life,” said Kostas Konstantinidis, an assistant professor in the School of Civil and Environmental Engineering at the Georgia Institute of Technology, in a press release. “There seems to be quite a diversity of species, but not all bacteria make it into the upper troposphere.”

The presence of microorganisms such as bacteria in the upper atmosphere is important because the concentration of microbial cells is known to affect the formation of clouds, whether they be comprised of water droplets, ice crystals, or both.

“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 said in a press release. “If they are the right size for forming ice, they could affect the clouds around them.”

Instruments mounted on the the NASA DC-8 jet, seen here during one of the GRIP field campaign flights in 2010, filtered outside air for the researchers to analyze.
Credit: NASA.

The researchers looked at where air masses originated from in order to get clues about bacterial composition. They found that when air masses originated over land, bacteria typically found in soils were more commonly found at high altitudes as well. And when the air masses originated over the oceans, mainly marine bacteria were present in the upper atmosphere.

Samples taken during hurricanes had higher numbers of total and viable bacterial cells compared to flights taken during clear conditions, indicating that hurricanes are responsible for dispersing large amounts of microbial cells, most of which are bacterial, into the upper atmosphere, where they can survive there for at least few days and influence cloud and precipitation formation. The study found that air masses, along with their microbial cells, that originated at lower altitudes were lifted to higher altitudes during hurricanes.

Interestingly, after Hurricanes Earl and Karl passed over populated areas, the researchers detected a “substantial signal” in the upper atmosphere of bacteria known to be associated with human and animal feces, including Streptococcus and Escherichia, also known as E. Coli. However, the analysis could not confirm if such bacteria were pathogens, meaning that it is not clear that they could have caused human illness.

However, it is clear from the study that long-range transport of viable bacteria does occur, which has implications for the global geography of bacteria.

“There are implications, actually, for the spread of diseases,” Nenes said, but the study does not prove that there is a spread of diseases via high altitude transport, he said. Further research to answer that question is underway, Nenes said.

Some bacteria can stay aloft and remain viable for longer, on the order of at least several days, the study said. To do so, these bacteria must have found ways to cope with high amounts of ultraviolet radiation from the sun, extraordinarily dry conditions, and other factors that would typically destroy living cells. “For these organisms, perhaps, the conditions may not be that harsh,” Konstantinidis said.

One possibility is that the the microorganisms are metabolizing, or living on, the carbon compounds also found at high altitudes, the study said.