| Abstract: | Bacteria and fungi are ubiquitous in the atmosphere. The diversity and abundance of airborne microbes may be strongly influenced by atmospheric conditions or even influence atmospheric conditions themselves by acting as ice nucleators. However, few comprehensive studies have described the diversity and dynamics of airborne bacteria and fungi based on culture-independent techniques. We document atmospheric microbial abundance, community composition, and ice nucleation at a high-elevation site in northwestern Colorado. We used a standard small-subunit rRNA gene Sanger sequencing approach for total microbial community analysis and a bacteria-specific 16S rRNA bar-coded pyrosequencing approach (4,864 sequences total). During the 2-week collection period, total microbial abundances were relatively constant, ranging from 9.6 × 105 to 6.6 × 106 cells m−3 of air, and the diversity and composition of the airborne microbial communities were also relatively static. Bacteria and fungi were nearly equivalent, and members of the proteobacterial groups Burkholderiales and Moraxellaceae (particularly the genus Psychrobacter) were dominant. These taxa were not always the most abundant in freshly fallen snow samples collected at this site. Although there was minimal variability in microbial abundances and composition within the atmosphere, the number of biological ice nuclei increased significantly during periods of high relative humidity. However, these changes in ice nuclei numbers were not associated with changes in the relative abundances of the most commonly studied ice-nucleating bacteria.Microbes are abundant in the atmosphere, with both cultivation-dependent and molecular approaches showing that the atmosphere harbors a diverse assemblage of bacteria and fungi, including taxa also commonly found on leaf surfaces (5, 49) and in soil habitats (30). The abundance and composition of airborne microbial communities are variable across time and space (14, 24, 27, 33, 47, 48, 69). However, the atmospheric conditions responsible for driving the observed changes in microbial abundances are unknown. The diversity of airborne microorganisms, and the factors influencing diversity levels, also remains poorly characterized. One reason for these limitations in knowledge is that until recently, culture-based microbiological methods have been the standard, and it is well-recognized that such methods capture only a small portion of the total microbial diversity (59). As demonstrated in a number of recent studies (6, 13, 22, 23, 33, 52, 59, 63, 73), advances in culture-independent techniques allow far more of the microbial diversity present in the atmosphere to be surveyed and the spatiotemporal variability in microbial communities to be examined.Microbes are often considered passive inhabitants of the atmosphere, dispersing via airborne dust particles. However, recent studies suggest that many atmospheric microbes may be metabolically active (3, 4, 64), even up to altitudes of 20,000 m (34). Some airborne microbes may alter atmospheric conditions directly by acting as cloud condensation nuclei (7, 25, 56) and/or ice nuclei (IN) (19, 41, 56, 57, 61); this hypothesis is supported by the observation that most ice nuclei in snow samples are inactivated by a 95°C heat treatment (16, 17). However, the overall contribution of airborne microbes to atmospheric processes such as ice nucleation remains unclear.The best-studied ice-nucleating microbes are gram-negative bacteria that have also been isolated from leaf surfaces, including Pseudomonas syringae, Pseudomonas fluorescens, Erwinia herbicola, Xanthomonas campestri, and Sphingomonas spp. (45). These bacteria have been cultured extensively, and their ice-nucleating activity has been traced to a membrane-bound glycoprotein (40, 42, 70). However, their specific influence on atmospheric processes remains, at this point, largely anecdotal. Less is known about the ice-nucleating activities of fungi, but a few studies have shown that fungi can be effective ice nucleators, capable of initiating ice nucleation at temperatures as high as −2°C (41, 61). At this point, all known ice-nucleating microorganisms are amenable to culture-based studies, but given that the vast majority of microorganisms have yet to be cultured, it is likely that other ice-nucleating microbes remain undiscovered.The work presented here addresses three overarching questions. (i) Are microbial abundances altered by changes in atmospheric conditions? (ii) How is the diversity and composition of airborne microbial communities influenced by changes in atmospheric conditions? (iii) Can we identify known and novel ice-nucleating microbes in the atmosphere by testing for correlations between taxa abundances and the concentrations of biological ice nuclei? To address these questions, we combined epifluorescence microscopy, tagged pyrosequencing, Sanger sequencing, and an ice nucleation assay with atmospheric measurements to characterize the microbial communities at a high-elevation research site. |
