Wildland fire smoke alters the composition, diversity, and potential atmospheric function of microbial life in the aerobiome

Abstract

The atmosphere contains a diverse reservoir of microbes but the sources and factors contributing to microbial aerosol variability are not well constrained. To advance understanding of microbial emissions in wildfire smoke, we used unmanned aircraft systems to analyze the aerosols above high-intensity forest fires in the western United States. Our results show that samples of the smoke contained ~four-fold higher concentrations of cells (1.02 ± 0.26 × 10⁵ m^-3) compared to background air, with 78% of microbes in smoke inferred to be viable. Fivefold higher taxon richness and ~threefold enrichment of ice nucleating particle concentrations in smoke implies that wildfires are an important source of diverse bacteria and fungi as well as meteorologically relevant aerosols. We estimate that such fires emit 3.71 × 10¹⁴ microbial cells ha^-1 under typical wildfire conditions in western US forests and demonstrate that wildland biomass combustion has a large-scale influence on the local atmospheric microbial assemblages. Given the long-range transport of wildfire smoke emissions, these results expand the concept of a wildfire's perimeter of biological impact and have implications to biogeography, gene flow, the dispersal of plant, animal, and human pathogens, and meteorology.

Fig. 1. Particulates and microbial cells in smoke and ambient air across three days of high-intensity forest fire in Utah, USA.

Drawing shows scaled concentrations of cells contrasting ambient air (“A”, mean sampling height above ground level 25 m, N = 8) and smoke plumes (“S”, mean height 75 m, N = 17). Number of viable and non-viable cells differed significantly between air types (K–S test; P < 0.01). Particulate matter concentrations (µg m⁻³) in smoke were nearly three orders of magnitude higher (K–S test; P < 0.005) than in ambient conditions for PM₁₀ and significantly higher for PM_2.5 and PM_1.0 size fractions (data not shown).

Fig. 2. Species richness, diversity and taxa composition of microbial cells characterizing smoke (N = 17) and ambient air (N = 8) conditions.

Taxa composition between ambient air and smoke (A). Venn diagrams for total, unique, and shared phylotypes between ambient air and smoke Samples for each sequencing region (B). PCoA of community similarity between ambient and smoke (C). Hill diversity comparison between ambient and smoke (error bars represent the standard error) (D).

Fig. 3. Abundance of the 11 most abundant species-level ASVs, belonging to either ambient (N = 8) or smoke (N = 17) libraries, that are detected in at least 20% of the samples.

Solid bars within boxplot represent the median read abundance and colored diamonds represent the mean read abundance of each air type.

Fig. 4. Relationship between rank order and cumulative read abundance of microbial taxa in smoke (N = 17) versus ambient (N = 8) air samples.

Rank order (log10 scale) of taxa plotted against their mean cumulative read abundance for ambient and smoke for each target region: 16S, 18S, and ITS. Ribbons represent the standard deviation of taxa prevalence across sample type. The number of taxa representing 80% of the diversity within each sample type is shown.

Fig. 5. Ice nucleation activity of biological and non-biological particles in smoke and ambient air.

Cumulative INPs m^-3 in ambient air and smoke show higher percentage of INPs are biological in smoke and overall INPs are higher across all temperatures evaluated (a). Fold-change of INPs across different temperatures between smoke and ambient air trend towards increasing differences at higher temperatures (b).