Effects of Ionizing Radiation and Long-TermStorage on Hydrated vs. Dried Cell Samples of ExtremophilicMicroorganisms

A main factor hampering life in space is represented by high atomic number nuclei andenergy (HZE) ions that constitute about 1% of the galactic cosmic rays. In the frame of the "STARLIFE"project, we accessed the Heavy Ion Medical Accelerator (HIMAC) facility of the National Instituteof Radiological Sciences (NIRS) in Chiba, Japan. By means of this facility, the extremophilic speciesHaloterrigena hispanica and Parageobacillus thermantarcticus were irradiated with high LET ions (i.e.,Fe, Ar, and He ions) at doses corresponding to long permanence in the space environment. Thesurvivability of HZE-treated cells depended upon either the storage time and the hydration stateduring irradiation; indeed, dry samples were shown to be more resistant than hydrated ones. Withparticular regard to spores of the species P. thermantarcticus, they were the most resistant to irradiationin a water medium: an analysis of the changes in their biochemical fingerprinting during irradiationshowed that, below the survivability threshold, the spores undergo to a germination-like process,while for higher doses, inactivation takes place as a consequence of the concomitant release of thecore's content and a loss of integrity of the main cellular components. Overall, the results reportedhere suggest that the selected extremophilic microorganisms could serve as biological model forspace simulation and/or real space condition exposure, since they showed good resistance to ionizingradiation exposure and were able to resume cellular growth after long-term storage.