Continental-scale distributions of dust-associated bacteria and fungi

It has been known for centuries that microorganisms are ubiquitous in the atmosphere, where they are capable of long-distance dispersal. Likewise, it is well-established that these airborne bacteria and fungi can have myriad effects on human health, as well as the health of plants and livestock. However, we have a limited understanding of how these airborne communities vary across different geographic regions or the factors that structure the geographic patterns of near-surface microbes across large spatial scales. We collected dust samples from the external surfaces of ∼1,200 households located across the United States to understand the continental-scale distributions of bacteria and fungi in the near-surface atmosphere. The microbial communities were highly variable in composition across the United States, but the geographic patterns could be explained by climatic and soil variables, with coastal regions of the United States sharing similar airborne microbial communities. Although people living in more urbanized areas were not found to be exposed to distinct outdoor air microbial communities compared with those living in more rural areas, our results do suggest that urbanization leads to homogenization of the airborne microbiota, with more urban communities exhibiting less continental-scale geographic variability than more rural areas. These results provide our first insight into the continental-scale distributions of airborne microbes, which is information that could be used to identify likely associations between microbial exposures in outdoor air and incidences of disease in crops, livestock, and humans.For nearly 2 centuries, we have known that microbes are ubiquitous in dust and outdoor air (1–3). In the near-surface atmosphere, microbial cells likely account for a significant fraction of aerosolized organic carbon (4), with microbial cell numbers typically ranging from 10⁴ to 10⁶ cells·m⁻³ over land (5). This means we inhale thousands of microbial cells every hour spent outdoors, and the potential effects of these airborne microbes on human health and the health of plants and animals are well recognized. As just one example, there are currently ∼16 million people living in the United States suffering from allergic asthma (6), and there has been considerable attention focused on understanding the airborne and dust-associated microbes influencing asthma and how those microbial triggers vary across space and time (6–9). In addition, the number of virulent fungal infections affecting human populations, wildlife, and plant crops is increasing (10, 11). The effects of those microbes capable of atmospheric transport can even extend to entire ecosystems: Microbes in Saharan dust clouds, for example, have been shown to affect the ecology of alpine lakes in Spain (12) and coral reefs in the Caribbean Sea (13).Despite the well-recognized importance of airborne microbes and the long history of research on microbial transport through the atmosphere (3), we have only recently been able to describe the full extent of microbial diversity found in the atmosphere by using molecular approaches to characterize the microbial taxa that are difficult to identify via cultivation-dependent or microscopy-based surveys. Such molecular approaches not only have yielded new insight into the enormous diversity of airborne microbes (14, 15) but also have been instrumental in helping us understand how the composition of the airborne microbial communities varies across time and space (16, 17). However, nearly all of this work has focused on local-scale variability, examining the bacterial or fungal communities found in outdoor air at selected sites. What is missing is a continental-scale understanding of microbial diversity in the near-surface atmosphere and its deposition patterns over land. We do not know whether there are distinct microbial taxa found in the outdoor air from different geographic regions, nor do we know what biotic and abiotic factors may be driving the geographic patterns of dust-associated microbial communities across larger spatial scales. There is some evidence to suggest there are regional differences in exposures to specific bacterial and fungal taxa (18) that may be associated with geographic patterns in allergenic disease (19), but the continental-scale biogeographic patterns exhibited by the broad range of microbes that can be found in outdoor air remain undetermined.Here we used dust samples collected from the external surfaces of ∼1,200 homes located across the continental United States to gain our first insight into the continental-scale patterns of bacterial and fungal diversity in the near-surface atmosphere and the factors driving the distributions of these airborne microbial taxa. Specifically, we investigated two questions: First, to what extent are the diversity and types of bacteria and fungi found in settled dust collected from outside homes a function of geographic location, climatic variables, soil characteristics, crop production, and land use type? Second, we sought to determine whether urbanization has a significant effect on bacterial and fungal exposures, given that it has long been assumed that airborne microbial exposures differ across urban and rural areas (3), with recent work suggesting that these differential microbial exposures may be one explanation for the higher rates of allergic disorders often observed in more urbanized areas (7, 8).