Interaction of microbial biomass with cryogenic mineral phases during freezing of Europa-relevant brines 

IntroductionIcy moons of the giant planets contain liquid water oceans where habitability is feasible. A prime example is Jupiter’s moon, Europa, one of Jupiter’s moons, which could contain multiple regions where life might be sustained under its thick ice layer. In previous work, we obtained a microbial community capable of growing under simulated sub-surface European ocean conditions (up to 30 MPa) [1]. The community was dominated by sulphate reducers and was able to grow in a medium based on one of the most recent models for Europa’s ocean composition [2]. Due to the observation of salt minerals on Europa’s surface that likely originate from the ocean, it has been hypothesised that, if life existed in the sub-surface, it could become trapped within the ice along with ocean-derived brines, where it would be frozen and eventually transported to the surface [3,4], leading to close associations between microbial biomass and salts. The aim of this study was to investigate 1) the feasibility of preservation of microbial material in frozen salt solutions relevant to Europa’s ocean; and 2) the potential for biomass to influence the formation of ice-bound mineral assemblages.MethodsSalt solutions based on models of Europa ocean chemistry were seeded with cells of our Europa analogue community and a pure strain isolated from a Europa analogue environment, and frozen at three different rates: gradual (~0.01 K s-1), rapid (~10-100 K s-1) and flash (>>100 K s-1) freezing, allowing us to explore implications of a range of cryovolcanic and exhumation processes at Europa. Salt assemblages were extracted from ice through sublimation. Scanning and transmission electron microscopy were used to analyse ice-templated salt structures and microbial cells, respectively (Figure 1.A). The identity and abundance of mineral phases was determined using X-ray diffraction (Figure 1.B) and spacecraft relevant techniques such as Raman and near-infrared spectroscopy.ResultsOur results demonstrate that freezing at all rates leads to a decrease in the number of intact cells, but that the fractions of intact vs. damaged or destroyed cells showed opposite correlations with freezing rate for our community vs. the pure strain. While the spatial distribution of salts and structure of the salt lattice was controlled primarily by freezing rate, we observed close associations between cryogenic salt phases and microbial biomass. We also found that the occurrence of specific cryogenic mineral phases, including sodium carbonates and sulfates, depended on the presence or absence of cells, providing evidence that biomass can influence salt precipitation during freezing. Furthermore, these mineralogical differences were detectable using mission-relevant techniques such as Raman spectroscopy. Our findings have implications for spacecraft visiting the Jovian system in the near future, which aim to measure the abundance of oceanic salt minerals on Europa’s surface, and for the development of landed missions targeting the most promising locations to search for evidence of life.Figure 1 A) False colour SEM image representative of the mineral structures formed in the brine veins of frozen material in between ice crystals. The image shows close association between intact bacterial cells (green) and cryogenic salts. B) Stacked barplot of the weight percent for each of the mineral phases identified through XRD. Halite has been eliminated from these plots as it corresponds to 74-94 wt% in all samples.ReferencesDel Moral, A., Siggs, D., Fox-Powell, M. G., Pearson, V. K. & Olsson-Francis, K. Exploring Europa’s biological potential using machine learning and laboratory simulations, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1227. Melwani Daswani, M., Vance, S. D., Mayne, M. J. & Glein, C. R. A Metamorphic Origin for Europa’s Ocean. Geophysical Research Letters 48, (2021). Santibáñez, P. A. et al. Differential Incorporation of Bacteria, Organic Matter, and Inorganic Ions Into Lake Ice During Ice Formation. JGR Biogeosciences 124, 585–600 (2019). Buffo, J. J. et al. The Bioburden and Ionic Composition of Hypersaline Lake Ices: Novel Habitats on Earth and Their Astrobiological Implications. 19 (2022).