Abstract EANA2025-18

Astrobiology research with extremophilic microorganisms from Earth can help us evaluate the limits of life as well as guiding our understanding of solar system habitability. The icy moons Enceladus and Europa are two promising targets for such investigations. Past orbital measurements have demonstrated how these locations might be habitable for terrestrial microbial life. Especially the subsurface ocean conditions which resemble Earth’s oceans might support life. Given the renewed interest in these locations such as by ESA’s JUICE and NASA’s Europa Clipper missions, we wanted to investigate which organism from Earth could survive on such locations. Pro- and eukaryotic psychrophilic and/or psychrotolerant microorganisms from analogue environments on Earth can be the ideal candidates for such research. For this reason, we have selected organisms from such locations, including six bacteria, two archaea and two yeasts, to understand if a specific organism would be suitable to survive on the icy moons. We have also included a yeast isolated from a spacecraft assembly cleanroom in order to adress forward planetary protection concerns.

In order to determine the survival limits, our experiments consisted in the exposure of the microorganism to extreme conditions similar to the icy moons. To replicate the oligotrophic nature of the icy moons, the microbial species capable of growth in minimal media supplemented with a single carbon source different from glucose were selected for exposure. After growth the microorganisms were exposed to: desiccation at varying temperatures, UV-C (254 nm) and polychromatic UV (200-400 nm) and X-ray  radiation as well as to freezing and thawing cycles at ΔT 105°C without supplemented cryoprotectant.

Despite all the species tolerating all the exposure conditions, our results showed how the Rhodotorula yeast species were the most tolerant ones. We therefore selected R. frigidalcoholis as a candidate for further investigations into combined conditions. This would reflect more closely the environment of the icy moons and provide us with an improved understanding of its survival in these locations. Exposure to combined conditions revealed that the yeast is capable of surviving to 100 days of desiccation, 1500 Gy of X-ray radiation and 15000 J/m2 of polychromatic UV radiation. Furthermore, to identify survival traits, transcriptomic analyses of RNA isolated from the yeast exposed to the combined conditions revealed the upregulation of metabolic pathways during exposure. These include DNA repair mechanisms, the halting of cell cycle progression and other structural cell repair mechanisms. The RNA of cells surviving the combined conditions and re-grown in minimal media revealed that the metabolic functions of the cells return to baseline within five hours after successful repair.

The results we have produced revealed interesting relationships and differences between the survival of pro- and eukaryotic organisms as well as the feasibility of multifactorial studies in the context of icy moon research. We also believe our results to be valuable to the planetary protection community, which has highlighted knowledge gaps in the forward planetary protection of the icy moons.