Compared to the terrestrial, marine, and limnic environments, the microbial community is relatively unknown in the upper atmosphere (>8 km). Some attempts to measure the lower atmosphere (1 – 7 km) from the tops of mountains have been made, showing that airborne bacteria and fungi from near the ground are lofted by zephyrs and other types of air movement. Studies in the past have also shown that some plant pathogens carry genes for the ice nucleation protein inaZ. If these bacteria were present in the upper atmosphere, they would constitute a major portion of the high-altitude supermicron ice nuclei (particles 0.5 – 3 \( \mu \)m in size that supercooled water can freeze to). Bacteria are also thought to be meaningful condensation nuclei for cloud formation at lower altitudes.
In this paper, the authors sampled air from the platform of NASA’s aircraft while they were engaged studying the upper atmosphere studying the effects of hurricanes on the upper atmosphere. They sampled ~ 8 m\(^3\) of air with an aerosol spectrometer to estimate the size and abundance of molecules followed by filtration for the sample to perform microscopic counting and qPCR analyses of the small subunit rRNA. They found that there were on average 15,000 cells per cubic meter of air. at high altitude. Most of these were bacteria which do not fall as quickly through the air as do the larger and heavier fungal cell/spores. Analysis of the 16S rRNA gene revealed the vast majority of OTUs were Alpha– and Betaproteobacteria with genera such as Afipia (alpha) and Burkholderiales (beta) making up > 70% of the total reads. Many of the OTUs were in families known to utilize 1-4 carbon molecules in their metabolism, which exist in abundance at that altitude in cloud droplets.
Sampling in the wake of hurricanes Earl and Karl showed that a large portion of the bacterial community resulted from ocean water lofted to altitude and fecal coliforms whenever the hurricane encountered human settlement. In addition, samples taken over land in transit from California, USA to Florida, USA showed that the majority of bacteria were from limnic systems. Together, this suggests that the high altitude bacterial community is primarily supplied by specimens originating in bodies of water that evaporate and are lofted by updrafts and storms.
Paper: DeLeon-Rodriguez, N., Lathem, T.L., Rodriguez-R, L.M., Barazesh, J.M., Anderson, B.E., Beyersdorf, A.J., Ziemba, L.D., Bergin, M., Nenes, A. and Konstantinidis, K.T., 2013. Microbiome of the upper troposphere: species composition and prevalence, effects of tropical storms, and atmospheric implications. Proceedings of the National Academy of Sciences, 110(7), pp.2575-2580.