The atmosphere is an important route for transporting and disseminating microorganisms over short and long distances. Understanding how microorganisms are distributed in the atmosphere is critical due to their role in public health, meteorology and atmospheric chemistry. In order to determine the dominant processes that structure airborne microbial communities, we investigated the diversity and abundance of both bacteria and fungi from the PM10 particle size (particulate matter of 10 micrometers or less in diameter) as well as particulate matter chemistry and local meteorological characteristics over time at nine different meteorological stations around the world. The bacterial genera Bacillus and Sphingomonas as well as the fungal species Pseudotaeniolina globaosa and Cladophialophora proteae were the most abundant taxa of the dataset, although their relative abundances varied greatly based on sampling site. Bacterial and fungal concentration was the highest at the high-altitude and semi-arid plateau of Namco (China; 3.56 × 106 ± 3.01 × 106 cells/m3) and at the high-altitude and vegetated mountain peak Storm-Peak (Colorado, USA; 8.78 × 104 ± 6.49 × 104 cells/m3), respectively. Surrounding ecosystems, especially within a 50 km perimeter of our sampling stations, were the main contributors to the composition of airborne microbial communities. Temporal stability in the composition of airborne microbial communities was mainly explained by the diversity and evenness of the surrounding landscapes and the wind direction variability over time. Airborne microbial communities appear to be the result of large inputs from nearby sources with possible low and diluted inputs from distant sources.

Primary biological aerosols are transported over large distances, are traveling in various media such as dry air masses, clouds or fog, and eventually deposited with dry deposition, especially for larger particles, or precipitation like rain, hail or snow. To investigate relative abundance and diversity of airborne bacterial and fungal communities, samples have been collected with a liquid impinger (Coriolis l) from the top of Mount Sonnblick (3106 m asl, Austrian Alps) from the respective sources under a temporal aspect over four seasons over the year to include all climatic conditions. Bacterial and fungal