Abundance and survival of microbial aerosols in the troposphere and stratosphere

Abstract

Bioaerosol transport in the atmosphere disperses microbial species between continents, affects human and plant health, and may influence hydrologic cycling. However, there have been few quantitative observations of bioaerosols at altitudes more than a few kilometers above the surface. Lack of data on bioaerosol distributions in the atmosphere has impeded efforts to assess the aerial dissemination of microbes and their vertical extent in the biosphere. In this study, a helium balloon payload system was used to sample microbial cells and dust particles in air masses as high as 38 km above sea level over three locations in the southwestern United States. The cell concentrations at altitudes between 3 and 29 km were highly similar (2-5 × 10⁵ cells m^-3) and approximately threefold lower than those observed in the convective boundary layer (CBL; 1 × 10⁶ cells m^-3), decreasing to 8 × 10⁴ cells m^-3 at 35-38 km. The detection of adenosine triphosphate (ATP) and recovery of bacteria possessing extreme tolerance to desiccation and shortwave ultraviolet radiation confirmed that certain microorganisms have the capacity to persist at lower altitudes of the stratosphere. Our data and related calculations provide constraints on the upper altitudinal boundary for microbial habitability in the biosphere.

Fig. 1

Temperature (a), microbiological (b), and particle (c) data with altitude. a Temperature profiles from Amarillo, TX were used to identify the CBL, free troposphere, tropopause, and stratosphere, the approximate boundaries of which are demarcated by the gray horizontal lines. b Cell (black circles) and cell carbon (gray squares) concentration estimates for the flights listed in Table 1. The values for “viable” cells (black triangles) were derived from the ATP data (Table S4). Open symbols are samples that did not exceed the procedural limits of detection. c Particle concentration (black circles) and mass (gray squares) for each flight in Table 1. Error values are obscured by symbols and provided in Table 1

Fig. 2

Microbial cells collected from tropospheric and stratospheric air masses. Epifluorescence micrographs of SYBR® Gold-stained cells observed at 2.2 km (a), 26 km (b–e), and 36 km (f). The scale bar is 2.5 µm

Fig. 3

Extreme tolerance to desiccation and UVC radiation in bacteria isolated from the stratosphere. Survival of populations during desiccation (a; Table S6) and UVC radiation exposure at 254 nm (b; Table S7). The isolates are labeled as follows: L6-1 (Curtobacterium sp., black circles), L7-7A (Noviherbaspirillum sp., white circles), L9-4 (Modestobacter sp., gray squares), and D. radiodurans (gray triangles). c Conservative estimated timeframe to inactivate populations of L6-1 and L7-7A based on the combined effect of desiccation and the estimated flux of  UVC radiation (data from panels a and b) with altitude in the stratosphere [13]. Some error bars in panel c are obscured by the symbols and provided in Table S8. *Assuming 12 h of sun day⁻¹