Abstract
This study investigated what defines the limits of life in mixed ion solutions. Better understanding these limits should allow us to better predict the habitability of extreme environments on the Earth and extraterrestrial environments. We systematically examined the response of Bacillus subtilis, a well characterized non-halophile model organism, to a range of solutions made from single and mixed salts up to their solubility limits and measured at what concentration growth was arrested, specifically exploring Na, Mg, and Ca cations and Cl, SO4, and ClO4 anions. We measured the physicochemical properties of the solutions to identify which properties correlated with the limits of growth. Individual salts imposed a growth limit specific to the combination of cation and anion, although we generally observe that chloride salts allow growth at lower water activity than sulfate salts, with perchlorate restricting growth even at the highest measured water activity. Growth was limited at a wide range of ionic strength, with no apparently correlation. Despite the theoretically counteracting disordering effects (chaotropic) of perchlorates and ordering effects (kosmotropic) effects of sulfates, when these salts were combined they instead additively narrowed the window for growth in both the Na and Mg cation systems, in the same manner as the combined effects of two chaotropic Ca salts. Our results imply that away from hard limits that might be imposed by physicochemical properties such as water activity, ionic strength or chaotropicity in highly concentrated brines, these properties do not set the limits of life. Instead these limits are highly specific to the salts and organisms in question. This specificity means that the habitability of extreme environments cannot be predicted, even with accurate measurements of the physicochemical conditions present.
Highlights
A wide diversity of organisms have been found in highly saline terrestrial environments (Andrei et al, 2017)
Further experiments used these initial results to narrow the range of concentrations across the upper limit of growth, enabling us to build up a large database of measurements for each salt that identify the limits of growth
There is no apparent correlation (R2 = 0.05) between the water activity and ionic strength at the limits of growth, nor a particular cutoff value of either property. This is true of the calculated chaotropic activity, which showed a wide range of values with no correlation to the growth limits
Summary
A wide diversity of organisms have been found in highly saline terrestrial environments (Andrei et al, 2017). There is evidence that other factors such as ionic strength (Fox-Powell et al, 2016) and chaotropicity/kosmotropicity (Williams and Hallsworth, 2009) have important effects on life (Chin et al, 2010) These properties are not always directly correlated with each other and depend strongly on the solutes in question. The majority of known organisms are unable to grow in conditions near these measured limits, but halotolerant or halophilic organisms have evolved adaptations that enable metabolic activity at high concentrations, currently defining limits. Irrespective of these adaptations, we can hypothesize the existence of a theoretical limit for a given physical parameter, caused by thermodynamic requirements rather than metabolic considerations. Whether the limits measured so far align with these putative hard physicochemical limits is unknown
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