Low-troposphere microbial communities differ between dry-air and rainfall but do not show strong seasonal patterns in Metro Atlanta, Southeast USA

The composition and seasonal patterns of airborne bacterial and fungal communities and how these are affected by atmospheric conditions (e.g., dry vs. rain), origin of air masses, and presence of air pollutants remain understudied, despite their obvious importance for public health. To provide insights into these questions, monthly dry air and rain samples were collected at the Environmental Science and Technology building rooftop on Georgia Techs campus (Metro Atlanta) between June 2017 and November 2019. The sampling included the remnants of Hurricane Irma and a Saharan dust event in 2020. Amplicon sequencing of the V4 region of the 16S rRNA gene and the fungal nuclear ribosomal internal transcribed spacer (ITS) region showed that spore-forming bacteria and widespread fungi were enriched in dry samples, while photosynthetic bacteria and wood-decaying fungi were more abundant in rain samples, demonstrating the effect of sample type on bioaerosol composition. Further, higher relative abundance of fungal human pathogens and allergens were identified in the dry-air and Saharan dust samples, including Alternaria alternata and Cladosporium cladosporioides. Bacterial and fungal species richness and composition appeared to be relatively consistent between seasons for both sample types. Accordingly, sample type and seasonality explained [~]14% and [~]8.5% of the microbial diversity between samples, respectively, while presence of air pollutants and three-day back trajectory data were not significant. Collectively, our data indicates that dry air might represent a higher public health risk and provides a reference point for the long-term monitoring of airborne microbial communities in an urban Southeast US setting. ImportanceIn the atmosphere, or in the air we breathe, bioaerosols are always present. Bioaerosols are biological particles (alive or dead) suspended in the air, being bacteria and fungi the most abundant. In addition, bioaerosols can potentially contribute to weather/climate patterns. Although and unfortunately, a clear biodiversity pattern from different atmospheric events (air, rain, snow, etc.) remains to be discovered, especially in urban areas, where bioaerosols can also have implications for public health. The role of airborne microbes and their diversity patterns in the atmosphere constitutes a significant gap in our understanding of their interactions with health, climate, and other ecosystems compared to other environments. Our research provides the first reference point for long-term monitoring of airborne microbial communities in an urban Southeast US setting. This research contributes novel knowledge about public health and insights for integrating biological information into weather and climate prediction models.