The effects of brines relevant to Mars and the ocean worlds on bacterial growth reflect salt-specific responses across water activity

Freezing point depression due to high salt concentration is crucial for liquid water to exist on cold worlds, expanding special regions where habitats are plausible. Determination of the growth tolerances of terrestrial microbes in analog systems impacts planetary protection protocols aimed at preventing interference with life detection missions or potential native ecosystems on celestial bodies. We measured the salinity tolerances of 18 salinotolerant bacteria (Bacillus, Halomonas, Marinococcus, Nesterenkonia, Planococcus, Salibacillus, and Terribacillus). The salts tested to high concentrations included the anions, Cl, NO₃, and SO₄, paired in every combination with the cations, Mg, K, and Na. Certain salts of NH₄, borate, Ca, Cs, ClO₃, Fe, ClO₄, and PO₄ also were examined. Strong growth was observed among the isolates in Salt Plains media supplemented with 2 M MgCl₂, 1 M Mg(NO₃)₂, 2 M MgSO₄, 0.6 M K chlorate, 4 M KCl, 3 M KNO₃, 0.8 M K₂SO₄, 3 M NaCl, 2 M NaNO₃, 2 M NaH₂PO₄, and 1.5 M Na₂SO₄, often with modest growth at higher concentrations. Logistic growth curves were fitted to estimate the rate of population increase (r) and carrying capacity (K) for each replicate. Effects on bacterial growth were best explained by statistical models that included anion, cation, water activity and all their 2-way and 3-way interactions. Salt-specific effects suggest that the concentration of any single ion is not consistently the reason behind the inhibition of microbial growth. Knowledge of the tolerances of terrestrial microbes to salts at extremely high concentrations sets limits for the permissible conditions of extraterrestrial habitable regions and for the risks of forward planetary contamination.