Vitreous Magnesium Sulfate Hydrate as a Potential Mechanism for Preservation of Microbial Viability on Europa

Europa's subsurface ocean is postulated to contain appreciable amounts of Mg ^2+ and ${{\mathrm{SO}}_{4}}^{2-}$ ions, among other species. Recent laboratory experiments have shown that when solutions containing these species freeze to Europa-relevant temperatures, they can form vitreous MgSO _4 hydrate, which can remain stable at these temperatures. Since vitreous phases can protect cells from physical damage that can occur during crystallization, their presence on Europa could potentially preserve entrained microorganisms from the ocean below. However, to date, it remains unclear whether such materials actually impact microbial survival. In this work, experiments were performed in which the motile nonspore-forming Antarctic isolate Pseudoalteromonas haloplanktis in solutions of 0.1 M MgSO _4 were frozen to Europa surface temperatures (100 K) under conditions that resulted in the formation of either vitreous or crystalline salt hydrates. We found that cells survived in both cases, exhibiting a 3-log reduction in viable cells in the crystalline salt hydrate case while only a 1.5-log reduction in the vitreous salt hydrate case. Scanning electron microscopy accordingly showed much higher degrees of membrane lysis and cellular damage in the crystalline salt hydrate than the vitreous case. Our results demonstrate the ability of a terrestrial oceanic microorganism to survive in MgSO _4 solutions frozen to Europa surface temperatures, with enhanced viability in vitreous salt-hydrate-producing conditions versus crystalline. These findings suggest that future missions should target vitrified salt-rich environments for life detection due to this potential for preserving viable microorganisms that may be present and trapped in ocean world ices.