Flying microbes – survival in the extreme conditions of the stratosphere during a stratospheric balloon flight experiment
Earth's stratosphere is characterized by hypobaric conditions, low temperatures, high intensities of UV and cosmic radiation as well as low water and nutrient availability. While it is not considered a permanent habitat for microorganisms, these can be transported to the stratosphere by storms, volcanic action or human activity.  The impact of those extreme conditions on microorganisms and their survival were tested by sending a sample gondola to the stratosphere. The sample gondola was built to allow exposure of Bacillus subtilis endospores at different angles to the sun. It moreover had holders for three environmental samples to test the effect of stratospheric conditions on complex microbial communities. The gondola attached to a stratospheric balloon was launched near Kiruna, Sweden, ascended to ~25 km and drifted eastwards for ~200 km. Samples were exposed to pressures as low as 2 kPa,temperatures as low as -50ºC as well as high UV radiation. Survival rates of B. subtilis were determined by comparing the numbers of colony forming units for the different exposure angles. Survival was negatively correlated with exposure angle, indicating significant impact of UV radiation. The effect of stratospheric conditions on environmental samples was assessed by comparing most probable numbers, microbial community composition and substrate-use profiles to controls that had stayed on the ground. Cultivation was possible from all samples with survival rates of at least 1% and differences in community composition were observed. Survival of environmental microorganisms might have been supported by the sample matrix which provided protection from radiation and desiccation.