Key questions to be addressed

• How does atmospheric microbial diversity and abundance vary spatio-temporally on a global scale?
• Are there functional traits that are associated with adaptation to the atmospheric habitat?
• What level of connectivity occurs between atmospheric and underlying soil and aquatic microbiomes?

Rationale for the Project.

Atmospheric transport of microorganisms between source and sink locations is a central assumption in microbiology. From the pioneering experiments of Louis Pasteur to the iconic hypothesis of Lourens Baas Becking that “everything is everywhere, but the environment selects” there has been a widely held assumption that microbial transport in the atmosphere is a neutral process where cells are ubiquitously dispersed.

Studying atmospheric microorganisms has historically been extremely challenging due to low biomass in air, a transient habitat and various methodological limitations. Recent advances in sampling technology and high-throughput ‘omics have allowed the acquisition of robust experimental evidence that challenges the notion of atmospheric transport as a neutral process. It is emerging that the abundance and viability of cells is influenced by atmospheric conditions and therefore biogeographic patterns are likely to occur in the atmosphere. This may be further influenced by temporal shifts in atmospheric conditions and air movements, underlying surface cover and other factors such as anthropogenic emissions.

Microbial cells and microbially-derived compounds have been implicated in important biophysical processes in the atmosphere such as ice nucleation. Consideration of the physico-chemical conditions in the atmosphere, and particularly in the lower troposphere where most airborne microorganisms occur, suggest that active microbial metabolism may also be possible under certain conditions.

The Global Atmospheric Microbiome Project was established to undertake a globally relevant interrogation of the diversity and putative functionality of microorganisms in the lower troposphere. The scale of this effort is unprecedented with every continent and climate included and sample recovery from near-ground air as well as air mass above the boundary layer for surface interactions. Concurrent sampling of underlying surface cover allows robust source tracking and assessment of connectivity between atmospheric, ocean and soil microbiomes.

The project has collected over 600 original samples of bulk-phase air from the lower troposphere above and below the boundary layer for surface interactions, as well as a comprehensive set of reference soil and water samples from underlying surface cover. A comprehensive molecular interrogation of the microbiomes is under way using a combination of targeted amplicon sequencing and shotgun metagenomics. An extensive set of metadata for all samples will be used to make ecological inference.

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