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

Abstract

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.

FIG. 1.

The number of CFU and MPN of bacteria in air samples collected from April 2007 to September 2008.

FIG. 2.

Phylogenetic tree based on partial 16S rRNA gene sequences of dominant airborne bacteria isolated from the DCAs and their distribution during the study period. The tree is based on consensus sequences resulting from a contig assembly based on ≥97% similarity.

FIG. 3.

Phylogenetic tree based on partial 16S rRNA gene sequences from clone libraries constructed from air samples collected during the study period. The tree is based on consensus sequences resulting from a contig assembly based on ≥97% similarity. For a more detailed view, see Fig. S1 in the supplemental material.

FIG. 4.

Number (n) of sequences belonging to higher-level taxa (phyla) in the clone libraries. For each sample the number of OTUs represented by the sequenced clones is presented as a number inside the bars. The left-most blue gradient represents sequences belonging to Gram-negative bacteria while the right-most purple gradient represents sequences belong to Gram-positive bacteria.

FIG. 5.

Rank abundance curve based on the 169 OTUs identified from the eight clone libraries collected during the study period. The x axis displays the OTUs ranked in order of decreasing abundance. Open circles represent OTUs containing several sequences while filled circles represent OTUs containing only one sequence.

FIG. 6.

Back trajectories arriving at 25 m a.s.l. at the sampling location, corresponding to the sample dates in Table 1. The sampling location is marked in red. Each trajectory is indicated by a different symbol at 12-h intervals up to 120 h back in time (of trajectories totaling 240 h). Solid lines indicate that the trajectory moved within the atmospheric boundary layer, and dashed lines indicate that it moved above the boundary layer in the free troposphere. Blue indicates that the trajectory moved over ocean while gray indicates that it moved over land. The solid blue lines therefore indicate the substantial possibility of a marine source.