Survival of microorganisms in Europa-relevant brines and conditions

Abstract

IntroductionInterest in the icy moons of our Solar System has grown since the discovery of liquid water oceans underneath their surface. Of particular interest is Europa, which is the target of two upcoming missions (Europa Clipper, NASA and JUICE, ESA). Geochemical models suggest the conditions in the sub-surface oceans are habitable and brine veins may exist within the surface ice, which may mediate the transport of microorganisms between the ice shell, water reservoirs and the subsurface ocean [1,2]. The brines are thought to occur during freezing of Europa’s ocean, mineral precipitates can occur as ice crystals form and absorb water as the ice grains grow.Recent observations of Europa’s newly disrupted geological regions, such as chaos terrains, suggest a predominance of sodium and magnesium chloride salts [3,4]. Models of the freezing of Europa’s ocean result in MgCl2 (a chaotropic agent) dominated brine veins in the ice shell, from different starting compositions, and in some cases, even sulfate-rich oceans [5].  These cold and concentrated MgCl2 brines are likely to be the closest remnants of liquid water to the surface of the ocean of Europa. Solutions of MgCl2 can reach eutectic temperatures of -33°C expanding the regions of habitability in the ice shell into colder regimes. The aim of this study was to investigate the effect of simulated Europa brines (0˚C to 33˚C, 0.1 MPa to 300 MPa of pressure) on microbial survival and growth.MethodsIn ongoing experiments, microorganisms isolated from extreme environments (Planococcus halocryophilus, Sphingopyxis alaskensis, Halobacillus basquensis and Bacillus subtilis), were exposed to MgCl2 (0 to 3.15 M), temperatures (room temperature, 4˚C and -33˚C), and pressures (10 to 30 MPa). The microorganisms were grown in a defined medium (based on a modified marine broth recipe), and their growth rates were monitored. New cultures were set up at the maximum tolerated MgCl2 concentrations (approximately 1M). They were then washed at increasing MgCl2 concentrations and incubated at three different temperatures for a week. Microorganisms were washed with a phosphate-buffered saline solution to reduce osmotic shock and plated to measure their survival rate. These experiments were repeated following the same procedure, but in a high-pressure reactor at 10, 20 and 30 MPa.ResultsPreliminary results suggest that certain microorganisms were able to grow at a maximum of 1 M MgCl2. Future work will determine if temperature and/or pressure play a role in the survival rates of microorganisms exposed to MgCl2. These results will help inform the planetary protection requirements for future spacecraft and how terrestrial organisms might interfere with biosignature detection missions to icy moons.  ReferencesBuffo, J. J., Schmidt, B. E., Huber, C. & Walker, C. C. Entrainment and Dynamics of Ocean‐Derived Impurities Within Europa’s Ice Shell. J. Geophys. Res. Planets 125, (2020). Buffo, J. J. et al. The Bioburden and Ionic Composition of Hypersaline Lake Ices: Novel Habitats on Earth and Their Astrobiological Implications. Astrobiology 22, 962–980 (2022). Ligier, N., Poulet, F., Carter, J., Brunetto, R. & Gourgeot, F. VLT/SINFONI Observations of Europa: New Insights into the Surface Composition. Astron. J. 151, 163 (2016). Trumbo, S. K., Brown, M. E. & Hand, K. P. Sodium chloride on the surface of Europa. Sci. Adv. 5, (2019). Fox-Powell, M. G., Wolfenbarger, N. S., Buffo, J. J., Semprich, J. & Ramkissoon, N. K. Modelling possible chemical evolution pathways during freezing of Europa’s ice shell. in (54th Lunar and Planetary Science Conference 2023, 2023).