Abstract
Greater exposure to environmental microorganisms has been hypothesized to reduce the likelihood of developing autoimmune disorders, and vegetation is known to be a source of diverse microbiota to the air. However, the spatiotemporal dynamics of airborne microbial communities in urban environments with varying amounts and types of vegetation are poorly understood. In this study we used high-throughput sequencing of the bacterial 16S rRNA gene to assess whether fine-scale variation in urban vegetation influences the diversity, composition, or structure of airborne bacterial communities over time. We used passive settling dishes to collect airborne bacteria from 36 sites representing three urban land cover types (forest, grassland, paved) over a 3-month period in Eugene-Springfield, Oregon, USA. We used remote sensing data (aerial 4-band orthoimagery and LiDAR) and geographic information systems (GIS) to assess detailed site characteristics (e.g., total vegetation cover and structural diversity) for each site. Our initial analysis indicated that site was the most important factor explaining variation in bacterial community structure (R2 = 0.32, p < 0.001), followed by sampling date (R2 = 0.24, p < 0.001), while land cover type was a significant but weak predictor (R2 = 0.06, p < 0.001) and other vegetation metrics were even less predictive. However, when samples were analyzed separately by date, the explanatory power of land cover type increased substantially; six of nine dates showed significant effects (p < 0.05) with R2 ranging from 0.16–0.31, indicating that land cover type had a marked influence on bacterial community structure that was obscured by the effects of site and sampling date. Despite the importance of site as a predictor of bacterial community structure, Mantel tests for spatial correlation were insignificant for most sampling dates, suggesting that localized site characteristics were driving this relationship. We use our results to propose a space-time conceptual model of the interactions between site-scale environmental features (e.g., vegetation characteristics) and regional-scale temporal processes and events (e.g., agricultural harvesting) to understand and perhaps manage intraurban airborne bacterial communities.
Highlights
Humans inhale an estimated one microgram of DNA daily, which, if it all belonged to bacteria, would represent on the order of 108 bacterial genomes (Després et al, 2007)
Two of the forest sites were located within 50 m of water bodies and have lower values for proportion of vegetation cover; one of the grass sites was later found to be artificial turf, which resulted in vegetation values similar to those of paved sites; and some of the paved sites were planted with individual landscaping planters, increasing their values to as high as 0.34
Our investigation revealed patterns of fine-scale spatial and temporal variability in the urban aerobiome and suggests potential controls over its community structure and richness that range from regional land uses and air movement, to localized vegetation and site management
Summary
Humans inhale an estimated one microgram of DNA daily, which, if it all belonged to bacteria, would represent on the order of 108 bacterial genomes (Després et al, 2007). Little is known about the composition and diversity of airborne microorganisms—the aerobiome— in outdoor urban environments. Understanding how landscape features, such as vegetation, influence the urban aerobiome may represent a new approach for improving public health. Recent research suggests that microbes commonly associated with vegetation, soil, water, and wildlife are critical to human immune function (Sorci et al, 2016; Rook et al, 2017). A lack of adequate childhood immune priming through exposure to environmental microbes has been implicated in a variety of autoimmune disorders later in life, including allergies, asthma, and mental disorders (Haahtela et al, 2013; Raison and Miller, 2013; Rook, 2013)
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