Vertical distribution of airborne microorganisms over forest environments: Estimation of microbial ice nucleation in forest atmosphere

Airborne microorganisms transported from forest areas can influence the cloud formation by forming ice nuclei. However, the vertical transportation of airborne microorganisms over forest areas is not well understood. We collected aerosols at the three heights, namely, floor (2 m) and canopy top (20 m) and above canopy (500 m), during summer, fall and winter, to analyze the airborne microbial communities that were distributed vertically over the forest. During summer and fall, the microbial particles maintained similar concentrations in the forest zone (canopy top and floor) and decreased to 1/10th of the concentration at the above canopy, while the particle concentrations in winter indicated vertical mixing below 500 m. High-throughput DNA sequencing revealed that the airborne microbial communities were composed of terrestrial and phyllospheric species associated with the degradation of decaying plan litters. Regardless of the three seasons, the above canopy is dominated by atmospheric-stress resistant bacteria from the phyla Actinobacteria and Firmicutes. Unlike bacteria, mushroom-type Agaricomycetes fungal members grew in relative abundance at the above canopy, primarily throughout summer and winter, while mold-type fungal Dothideomycetes species were often found at all three heights during fall. Some microbial isolates of the genera Fusarium, Pseudomonas and Bacillus indicated high activities of ice nucleation in the water-drop freezing assay. Therefore, ice-nucleating microbial taxa likely originated from fungal and bacterial communities in floor-level litter and plant surfaces that were transported towards beyond the tree canopy.