Psychrophilic limits of survival – what are the implications for icy moon exploration?

Kurzfassung

Exposing microorganisms to extreme environmental conditions can help us understand what their survival limits are. On Earth, microorganisms can be found in almost all environments, and are capable of tolerating extreme physical and chemical stressors. Investigating how these microbial extremophiles tolerate and thrive with these stressors might give us an indication of how life might survive on other planets. Several environments on Earth classified as extreme are valuable analogues which replicate the locations found on other bodies of the solar system. An interesting astrobiology location in the solar system are the icy moons Europa and Enceladus. These moons could support life due to their peculiar environment having a subsurface ocean covered by a thick ice layer. By making use of psychrophilic microorganisms which are already adapted to grow at low temperatures we can investigate whether they could hypothetical endure on the icy moons. This information might also shed light on how life could develop on the icy moons. Additionally, making use of Earth extremophiles in laboratory simulations, we can evaluate which ones might pose a threat of contamination if found to be hitchhiking on a spacecraft bound for the icy moons. The experiments initially focused on selecting four psychrophilic microbial extremophiles candidates from icy moon-analogue environments on Earth. The selected organisms Chromohalobacter sarecensis, Planococcus halocryophilus, Rhodotorula frigidalcoholis and Rhodotorula mucilaginosa, and were grown in minimal media supplemented with a single carbon source in order to represent more accurately the nutrient-limiting conditions in the subsurface ocean of the icy-moons. Following growth in minimal media we exposed the organisms to laboratory simulated extreme conditions in order to evaluate their survival limits. These conditions include: desiccation at subzero temperatures, freezing at -80°C and freezing and thawing, exposure to UV-C and polychromatic UV radiation as well as exposure to X-ray radiation. Our results highlight how all organisms can tolerate to an extent the extreme conditions, however, the yeasts are the ones which tolerate the most the conditions of desiccation and freezing and thawing. Additionally, current experiments also include the analysis of RNA extracted from yeast samples after exposure to extreme conditions to evaluate the mechanisms of repair.