Annual variations in the diversity, viability, and origin of airborne bacteria.

The presence of bacteria in aerosols has been known for centuries, but information on their identity and role in dispersing microbial traits is still limited. This study monitored the airborne bacterial community during an annual survey using samples collected from a 25-m tower near the Baltic Sea coast. The number of CFU was estimated using agar plates while the most probable number (MPN) of viable bacteria was estimated using dilution-to-extinction culturing assays (DCAs). The MPN and CFU values produced quantitatively similar results, displaying a pronounced seasonal pattern, with the highest numbers in winter. The most dominant bacteria growing in the DCAs all formed colonies on agar plates, were mostly pigmented (80%), and closely resembled (>97%) previously cultured bacteria based on their 16S rRNA gene sequences. 16S rRNA gene clone libraries were constructed on eight occasions during the survey; these revealed a highly diverse community with a few abundant operational taxonomic units (OTUs) and a long tail of rare OTUs. A majority of the cloned sequences (60%) were also most closely related to previously "cultured" bacteria. Thus, both culture-dependent and culture-independent techniques indicated that bacteria able to form colonies on agar plates predominate in the atmosphere. Both the DCAs and clone libraries indicated the dominance of bacteria belonging to the genera Sphingomonas sp. and Pseudomonas sp. on several sampling occasions. Potentially pathogenic strains as well as sequences closely resembling bacteria known to act as ice nuclei were found using both approaches. The origin of the sampled air mass was estimated using backward trajectories, indicating a predominant marine source.

Seawater mesocosm experiments in the Arctic uncover differential transfer of marine bacteria to aerosols.

Biogenic aerosols critically control atmospheric processes. However, although bacteria constitute major portions of living matter in seawater, bacterial aerosolization from oceanic surface layers remains poorly understood. We analysed bacterial diversity in seawater and experimentally generated aerosols from three Kongsfjorden sites, Svalbard. Construction of 16S rRNA gene clone libraries from paired seawater and aerosol samples resulted in 1294 sequences clustering into 149 bacterial and 34 phytoplankton operational taxonomic units (OTUs). Bacterial communities in aerosols differed greatly from corresponding seawater communities in three out of four experiments. Dominant populations of both seawater and aerosols were Flavobacteriia, Alphaproteobacteria and Gammaproteobacteria. Across the entire dataset, most OTUs from seawater could also be found in aerosols; in each experiment, however, several OTUs were either selectively enriched in aerosols or little aerosolized. Notably, a SAR11 clade OTU was consistently abundant in the seawater, but was recorded in significantly lower proportions in aerosols. A strikingly high proportion of colony-forming bacteria were pigmented in aerosols compared with seawater, suggesting that selection during aerosolization contributes to explaining elevated proportions of pigmented bacteria frequently observed in atmospheric samples. Our findings imply that atmospheric processes could be considerably influenced by spatiotemporal variations in the aerosolization efficiency of different marine bacteria.