Interplanetary survival probability of Aspergillus terreus spores under simulated solar vacuum ultraviolet irradiation

This work is a part of ESA/EU SURE project aiming to quantify the survival probability of fungal spores in space under solar irradiation in the vacuum ultraviolet (VUV) (110–180 nm) spectral region. The contribution and impact of VUV photons, vacuum, low temperature and their synergies on the survival probability of Aspergillus terreus spores is measured at simulated space conditions on Earth. To simulate the solar VUV irradiation, the spores are irradiated with a continuous discharge VUV hydrogen photon source and a molecular fluorine laser, at low and high photon intensities at 1015 photon m−2 s−1 and 3.9×1027 photons pulse−1 m−2 s−1, respectively. The survival probability of spores is independent from the intensity and the fluence of photons, within certain limits, in agreement with previous studies. The spores are shielded from a thin carbon layer, which is formed quickly on the external surface of the proteinaceous membrane at higher photon intensities at the start of the VUV irradiation. Extrapolating the results in space conditions, for an interplanetary direct transfer orbit from Mars to Earth, the spores will be irradiated with 3.3×1021 solar VUV photons m−2. This photon fluence is equivalent to the irradiation of spores on Earth with 54 laser pulses with an experimental ∼92% survival probability, disregarding the contribution of space vacuum and low temperature, or to continuous solar VUV irradiation for 38 days in space near the Earth with an extrapolated ∼61% survival probability. The experimental results indicate that the damage of spores is mainly from the dehydration stress in vacuum. The high survival probability after 4 days in vacuum (∼34%) is due to the exudation of proteins on the external membrane, thus preventing further dehydration of spores. In addition, the survival probability is increasing to ∼54% at 10 K with 0.12 K/s cooling and heating rates.