Diurnal and Inter-day Variation in Atmospheric Microbial Community Diversity and Composition at Mt. Bachelor Observatory

The atmosphere is often considered an extreme microbial environment with limited water and resource availability and high levels of UV radiation. Despite these extremes, diverse assemblages of microorganisms have been sampled from the atmosphere. The study of temporal variation of atmospheric microbial communities has led to an increased understanding of the role of dispersal from source environments in structuring these communities. Seasonal variation in overall community composition and in the relative abundances of specific taxonomic groups is thought be related to seasonal changes in local sources environments (i.e. snow cover, leaf surfaces). However, studies on shorter time scales have yielded conflicting results including communities that are highly variable from day to day and communities that are relatively stable across days. It is unknown if there are diurnal changes in airborne community composition and how changes may be related to atmospheric dynamics and inputs of microbes into the atmosphere. In this study, sampling of airborne bacterial communities was conducted at Mt. Bachelor Observatory from 8/12/13 - 8/17/13. Samples were collected onto filters at 4-hour intervals for the duration of the sampling campaign resulting in 30 total samples. DNA was extracted from the filters, and bacterial community composition was determined by sequencing partial 16S genes. Various environmental and meteorological data were also collected. Community composition varied between day and night samples but not across days. The relative abundances of five species including two members each from the phyla Actinobacteria and Chloroflexi were found to have significant diurnal periodicity. Community richness varied across days and was greater is day versus night samples. These findings demonstrate that bacterial communities in the atmosphere are variable over multiple timescales, and diurnal variation may be more pronounced than variation across days under some conditions.