Urban Green Space Aerobiomes: Exposure to Airborne Bacteria Depends upon Vertical Stratification and Vegetation Complexity

Exposure to diverse environmental microbiota is thought to have an important role in “training” the immune system and maintaining human health and wellbeing. Vegetation and soil are both known to be important sources of airborne microbiota, i.e., constituents of the aerobiome. A limited number of studies have attempted to characterise the spatiotemporal dynamics of the aerobiome; however, no known studies have investigated these dynamics from a vertical perspective. Support for its existence can be drawn from studies of pollution and allergenic pollen where vertical stratification occurs at various scales. The existence of aerobiome vertical stratification could have important implications for public health and for the design and management of urban green spaces. For example, do children receive the same exposure to airborne microbiota as adults (or taller humans), and is this influenced by vegetation composition, structure and surrounding land-use?

Furthermore, the potential differences in aerobiome composition based on different land cover types are still poorly understood. In this study we developed and combined an innovative columnar sampling method (using passive petri dish sampling stations), remote sensing techniques, and high-throughput sequencing of the bacterial 16S rRNA gene to assess whether significant vertical stratification of the aerobiome occurs. We also assessed whether there were differences in the aerobiome between land cover types in the Adelaide Park Lands, in South Australia. The land cover types included in the study were as follows:
1. Amenity grassland/lawn;
2. Scrub/trees;
3. Bare ground.
Site selection was determined using a combination of remote sensing approaches (such as habitat classification indices and random point algorithms), and ground-based site assessments. We also installed dataloggers to record microclimate data. Results presented provide evidence of vertical stratification in both alpha and beta (compositional) diversity of airborne bacterial communities, with diversity decreasing roughly with height. We also found significant vertical stratification in potentially pathogenic and beneficial bacterial taxa.

Although additional research is needed, these preliminary findings point to potentially different exposure attributes that may be contingent on human height and activity type. Our results lay the foundations for further research into the vertical characteristics of urban green space aerobiomes and their implications for public health and urban planning.