Bacterial Growth in Brines Formed by the Deliquescence of Salts Relevant to Cold Arid Worlds

Hygroscopic salts at Mars' near-surface (MgSO₄, (per)chlorates, NaCl) may form brines by absorbing moisture from the atmosphere at certain times through the process of deliquescence. We have previously shown strong bacterial growth in saturated MgSO₄ (∼67% w/v as epsomite) at room temperature, and growth was observed at the MgSO₄ eutectic point (43% w/v at -4°C). Here, we have investigated the growth of salinotolerant microbes (Halomonas, Marinococcus, Planococcus) from Hot Lake, Washington; Basque Lake, British Columbia; and Great Salt Plains, Oklahoma under deliquescing conditions. Bacterial cultures were grown to mid-log phase in SP medium supplemented with 50% MgSO₄ (as epsomite), 20% NaClO₃, or 10% NaCl (w/v), and small aliquots in cups were dried by vacuum desiccation. When the dried culture was rehydrated by the manual addition of water, the culture resumed growth in the reconstituted brine. When desiccated cultures were maintained in a sealed container with a brine reservoir of the matching growth medium controlling the humidity of the headspace, the desiccated microbial culture evaporites formed brine by deliquescence using humidity alone. Bacterial cultures resumed growth in all three salts once rehydrated by deliquescence. Cultures of Halomonas sp. str. HL12 showed robust survival and growth when subjected to several cycles of desiccation and deliquescent or manual rehydration. Our laboratory demonstrations of microbial growth in deliquescent brines are relevant to the surface and near-subsurface of cold arid worlds like Mars. When conditions become wetter, hygroscopic evaporite minerals can deliquesce to produce the earliest habitable brines. Survival after desiccation and growth in deliquescent brines increases the likelihood that microbes from Earth, carried on spacecraft, pose a contamination risk to Mars.