Vertical stratification of the air microbiome in the lower troposphere

Abstract

The troposphere constitutes the final frontier of global ecosystem research due to technical challenges arising from its size, low biomass, and gaseous state. Using a vertical testing array comprising a meteorological tower and a research aircraft, we conducted synchronized measurements of meteorological parameters and airborne biomass (n = 480) in the vertical air column up to 3,500 m. The taxonomic analysis of metagenomic data revealed differing patterns of airborne microbial community composition with respect to time of day and height above ground. The temporal and spatial resolution of our study demonstrated that the diel cycle of airborne microorganisms is a ground-based phenomenon that is entirely absent at heights >1,000 m. In an integrated analysis combining meteorological and biological data, we demonstrate that atmospheric turbulence, identified by potential temperature and high-frequency three-component wind measurements, is the key driver of bioaerosol dynamics in the lower troposphere. Multivariate regression analysis shows that at least 50% of identified airborne microbial taxa (n = ∼10,000) are associated with either ground or height, allowing for an understanding of dispersal patterns of microbial taxa in the vertical air column. Due to the interconnectedness of atmospheric turbulence and temperature, the dynamics of microbial dispersal are likely to be impacted by rising global temperatures, thereby also affecting ecosystems on the planetary surface.

Short-range contributions of local sources to ambient air

“…Our previous air microbiome studies ( 13–16 ) revealed the temporal and spatial dynamics of atmospheric microbial communities, particularly in a tropical setting ( 14 ). In addition, the recently observed phenomenon of the microbial community composition oscillating between day and night (diel cycle) has now been confirmed to also occur in temperate climates ( 15 , 16 ). In particular, the vertical stratification of airborne microorganisms in the lower troposphere was demonstrated above and below the planetary boundary layer ( 16 ).…”

“…In addition, the recently observed phenomenon of the microbial community composition oscillating between day and night (diel cycle) has now been confirmed to also occur in temperate climates ( 15 , 16 ). In particular, the vertical stratification of airborne microorganisms in the lower troposphere was demonstrated above and below the planetary boundary layer ( 16 ). Importantly, it was shown that atmospheric turbulences cause strong mixing of air masses during daytime below the boundary layer, resulting in largely homogeneous airborne microbial communities.…”

“…Based on the results of our previous studies on temporal and spatial dynamics of airborne microbial communities, we hypothesize that contributions from local sources to ambient air can be qualitatively and quantitatively identified by high-volumetric air sampling and metagenomics analysis. Adopting the concept of atmospheric mixing ( 16 ), which results in a largely homogeneous ambient air background in the near-surface atmosphere, our experimental design investigated the air microbiomes of six outdoor locations and one indoor site at identical time points. Despite their proximity, these sites likely represent divergent microhabitats, due to their engineered landscaping and high occupancy rate of various avian species, as well as human visitors to the AP.…”

“…This mechanism is particularly pronounced in the tropics, where upward convection is strong due to the high prevailing daytime temperatures. The resulting mixing of ambient air in a tropical urban setting demonstrated the absence of daytime vertical stratification on a 50-floor high-rise building (1 to 156 m) ( 16 ).…”

Microbes and Climate Change: a Research Prospectus for the Future

“…Warmer temperatures affect the densities of airborne microbes and can accelerate their long-distance transport ( 33 ). Higher temperatures and environmental stresses can also alter human and animal physiologies and defenses against pathogens.…”