Microbial survival in the stratosphere and implications for global dispersal
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Spores of Bacillus subtilis were exposed to a series of stratosphere simulations. In total, five distinct treatments measured the effect of reduced pressure, low temperature, high desiccation, and intense ultraviolet (UV) irradiation on stratosphere-isolated and ground-isolated B. subtilis strains. Environmental conditions were based on springtime data from a mid-latitude region of the lower stratosphere (20 km). Experimentally, each treatment consisted of the following independent or combined conditions: −70°C, 56 mb, 10–12% relative humidity and 0.00421, 5.11, and 54.64 W/m2 of UVC (200–280 nm), UVB (280–315 nm), UVA (315–400 nm), respectively. Bacteria were deposited on metal coupon surfaces in monolayers of ~1 × 106 spores and prepared with palagonite (particle size < 20 μm). After 6 h of exposure to the stratosphere environment, 99.9% of B. subtilis spores were killed due to UV irradiation. In contrast, temperature, desiccation, and pressure simulations without UV had no effect on spore viability up through 96 h. There were no differences in survival between the stratosphere-isolated versus ground-isolated B. subtilis strains. Inactivation of most bacteria in our simulation indicates that the stratosphere can be a critical barrier to long-distance microbial dispersal and that survival in the upper atmosphere may be constrained by UV irradiation.
KeywordsStratosphere Natural selection Dispersal Spores Aerobiology
Our research was supported by the National Science Foundation Integrative Graduate Education and Research Traineeship (IGERT) program at the University of Washington Graduate Program in Astrobiology, and the Cooperative Education Program at NASA Kennedy Space Center (KSC). Critical resources came from the U.S. Geological Survey (USGS), NASA Goddard Space Flight Center and the University of Florida. The authors are grateful to Paul Hintze (NASA KSC) for his assistance with SEM imaging and energy-dispersive X-ray spectrometry and to Phillip Metzger and Luke Roberson (NASA KSC) who helped generate the dust analog. We also acknowledge John Frederick (University of Chicago) for guidance during our research. Any use of trade names is for descriptive purposes only and does not imply endorsement by the US Government.
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