Doctor of Philosophy (PhD)


Biological Sciences

Document Type



Although bioaerosols in the lower altitudes of the atmosphere have been extensively studied, little is known about the distribution and nature of microorganisms at altitudes above the Earth’s surface. To examine the vertical distribution of microorganisms in the atmosphere, a helium balloon payload system was developed to sample bioaerosols at stratospheric altitudes of up to 38 km, where temperature, air pressure, relative humidity, and ultraviolet-C (UV-C) radiation conditions are similar to the surface of Mars. Bioaerosol concentrations between 3 and 29 km were similar to each other, ~three-fold lower than those in the convective boundary layer (CBL; 0-3 km; 1 x 106 cells m-3), and decreased to 8 x 104 cells m-3 between 35 and 38 km. From these data we estimate that Earth’s atmosphere contains at least 100-fold more microorganisms (~1024 cells) than previously thought (Burrows et al., 2009a), ~40% of which may exist of bioaerosols in the stratosphere. Isolates recovered from 6 to 29 km were evaluated for the tolerance to desiccation and UV-C in order to better constrain their ability to survive transport in the stratosphere. Based on the survival data, UV-C was determined to be the limiting factor for microbial survival in the atmosphere. Populations of L6-1, the most UV-C tolerant isolate examined, at the low end of stratospheric cell concentrations (3.8 x 105 cells m-3) would be inactivated in 43 days at 20 km, a value that decrease to ~2 hours at 30 km, above the shielding effects of the ozone layer. Based on these data, we hypothesize the ozone layer may represent the upper limit of the biosphere.



Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Christner, Brent

Included in

Life Sciences Commons