Microorganisms are ubiquitous and diverse in the atmosphere, and the air has long been recognized as an important dispersal conduit for microbial life. Recent studies have underscored the important role that airborne microbes play in atmospheric processes, by affecting climate (e.g. as ice nucleators) and atmospheric chemistry (e.g. through biological transformations of atmospheric compounds). Despite the importance of airborne microbes, we do not know the extent of microbial biodiversity in the atmosphere or what proportion of this biodiversity is metabolically active, and we understand very little about how microbial biodiversity affects the functioning of the atmosphere. Here, we bridge this knowledge gap by applying RNA-based analyses to atmospheric bacterial communities. RNA-based approaches can be used to differentiate the metabolically active portion of a sampled community from the total community, which includes dormant, dead, or otherwise inactive cells. Total DNA and RNA were extracted from air samples collected over six-hour intervals. Total community diversity based on 16S rDNA was compared to active community diversity based on 16S rRNA.
Significant differences were observed between RNA-based and DNA-based community composition, indicating that a significant portion of atmospheric microbial communities were metabolically active. Cyanobacteria were numerically dominant in RNA-based communities and nearly absent in DNA-based communities. Firmicutes, many of which produce endospores, were more abundant in DNA-based communities. These results suggest that active airborne communities may be a specialized subset of the total community better adapted for life in the atmosphere and that the total community is at least partially dominated by dormant cells.