Roughly 100 species of bacteria and equivalent numbers of fungal species were found in each of nine air samples from Storm Peak-far fewer than in urban or rural air samples, and fewer than in one sample of snow from there, which contained 800 species, Bowers says. Airborne microbial population density values range from 9.6 x 105 to 6.6 x 106 cells per m3, with roughly equivalent numbers of bacteria and fungi. "These airborne microbes are likely to play some role in cloud formation, and can therefore be said to play a role in the global radiation budget," he says.
"Gram-negative proteobacteria were the most commonly detected bacteria," says Eoin Brodie of the Lawrence Berkeley Laboratory, who was not involved in the research. This dominance contrasts with previous findings, he points out. For example, in urban aerosols, spore-forming grampositive bacteria such as the bacilli and actinomycetes tend to be dominant. He notes that because so few species in the air samples could be grown in culture, they were identified by rapid "pyrosequencing" of ribosomal genes.
"On average, about 40% of the organic carbon in the atmosphere is probably intact cells," Fierer says. The stability of microbial populations within the troposphere, despite both changes in climate conditions and the source of the air, suggests that the atmosphere "is a tough place to live." It takes a tough but stolid set of microbes to contend with high-intensity UV radiation, lack of food, and low moisture in that setting. Further, he asks, "What are these things doing up there? Do they have any influence on atmospheric conditions? There is recent data, and anecdotal evidence from Bob [Bowers] and others showing that some of the best nucleators are bacteria and fungal cells, so what percentage of nucleation is driven by cells?"