Science knows a fair amount about the parts of Earth that can be touched by people. However, not very much is known about microbiology in and above the clouds. A few experiments done earlier suggested that there are microbes in clouds since freshly-fallen snow will “grow’ in a petri dish, clouds have enough of the right chemicals to support microbial life, and the clouds drifting past mountaintops have microbes from lower elevations in them. When cooled to freezing, water needs a small particle to begin crystallizing on, known as an ice nucleus. Some bacteria have special proteins coating them that help water crystallize into ice more easily, and since there were bacteria growing from the snow, it would make sense that the bacteria were serving as ice nuclei. If this is the case, then most of the ice nuclei high in the atmosphere may in fact, be bacteria.
In this experiment, the scientists rode along on a NASA airplane while it was flying ahead of and behind hurricanes Earl and Karl in 2013 doing many other hurricane- and atmosphere-related experiments. The experimenters used a sensor in the airplane that measured how common particles of different sizes were in the air. They then ran the air through a filter that would catch bacteria and took them back to a lab to analyze. In the lab, they examined SSU rRNA, a gene found in all living things to count the bacteria and to identify what the organisms were.
In the air before the hurricanes passed through, most of the bacteria were hardy varieties that could thrive on the chemicals that clouds brought up from the surface. They largely belonged to the Proteobacteria, a group which has a few members known for whacky lifestyles (such as living in frozen ground or inside other cells). After the hurricanes swept through, they found most of the bacteria were from the ocean, except when the hurricanes went over a city, when the bacteria were mostly human fecal bacteria. Based on these findings, the scientists concluded that storms are able to pick up bacteria from near the ground and carry them up into the upper atmosphere where they can help the next generation of clouds condense and fall back down to Earth.
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.