From ocean floors to office desks, bacteria coat nearly every inch of the Earth. Scientists have recently discovered bacteria are present high above the Earth, as well. Ten kilometers up, to be exact, in a region of our atmosphere known as the upper troposphere. In a place where freezing temperatures, high ozone concentrations and intense UV radiation are toxic for most life, to say nothing of the gravity issue, communities of microorganisms make a home. Furthermore, the “microbiome” of the upper troposphere may play an important role in the formation of clouds and precipitation, according to a study published in the Proceedings of the National Academies of Science.
The NASA-led study began in 2010 as part of an extensive field campaign to better understand how tropical storms develop. Microbiologists worked onboard NASA’s “flying science laboratory”, a renovated Douglas DC-8 jetliner, collecting air samples across the continental US and Caribbean during and after tropical hurricanes Earl and Karl. Collecting each “sample” involved vacuum-pumping six cubic meters of air- the volume of a large van – through specialized filters that trap bacteria-sized particles. The microbes collected on the filters were then studied using high-powered microscopes and genomic techniques to determine their abundance, composition and origin.
While not teeming with life, the upper troposphere is hardly a cold, dead wasteland. The scientists estimated roughly 150,000 living microbial cells per cubic meter of air. (For comparison, there can be billions of microbes in a cubic centimeter of soil). The air samples collected inside Earl and Karl had greater numbers of microbes, many of which are “transplants”, swept out of oceans and soils by powerful hurricane winds. Bacteria are roughly a hundred times more abundant than fungi in the upper troposphere, probably due to their smaller size. Weighing a trillionth of a gram means it can take weeks to fall out of the atmosphere.
After establishing that bacteria live in the upper troposphere, the scientists set out to determine exactly who’s up there. They did so by analyzing bacterial community DNA using a technique called pyrosequencing. The team uncovered several striking patterns. Perhaps unsurprisingly, hurricanes exhibited the most diverse microbiomes. Post-hurricane, however, there remained dramatic differences in community composition as compared to regions that never experienced hurricanes. Tropical storms appear to leave a lasting legacy on the upper troposphere microbiome, due to the influx of new bacteria and “precipitation scavenging”, or raining out, of preexisting bacteria. In spite of these differences, across all air samples most bacteria fell into two broad groups, the Alpha and Betaproteobacteria. Even more remarkably, seventeen distinct taxa, dubbed the “core microbiome” were present in all samples, from the California coast to the central Caribbean.
Why are the seventeen taxa of the core microbiome so persistent in upper troposphere? In all likelihood, a number of unique characteristics promote the survival of these hardy bacteria. For one, they may have adaptations for coping with UV radiation and high concentrations of oxidants (chemicals like ozone and hydrogen peroxide). Food is another big issue. There is evidence the core seventeen may have more lunch options up high, using the limited carbon sources present in cloud water. For example, Afipia, the most common bacteria in all samples, thrives on a simple carbon compound known as dimethyl sulfone (DMSO2). Compounds similar to DMSO2 are common in marine atmospheres. Two of the other core families, Methylobacteriaceae and Oxalobacteraceae, metabolize oxalic acid, another carbon compound abundant in cloud water. Finally, due to their weight and shape, the core seventeen may simply be harder to rain out.
Microbial life in the upper troposphere is more than just a curious discovery. These microbes may play an important and unexplored role in cloud formation. Tiny suspended aerosols, previously thought to be non-living, serve as “cloud condensation nuclei” – surfaces that water vapor can condense onto. This study suggests living bacteria may account for 20% of the particles serving as cloud condensation nuclei. And where life is involved, things start to get more complicated. What if different bacteria promote cloud condensation at different rates? Certain plant-pathogenic bacteria actively promote ice condensation on leaf tissue, rendering the plant susceptible to infection. Much as gut microbiomes are now believed to play an important role in our health, atmospheric microbiomes may play an underappreciated role in regulating global weather patterns and climate.