Stevens Adam H, Cockell Charles S
UK Centre for Astrobiology, School of Physics and Astronomy, The University of Edinburgh, Edinburgh, United Kingdom.
Front Microbiol. 2020 Jun 26;11:1478. doi: 10.3389/fmicb.2020.01478. eCollection 2020.
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 , 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, SO, and ClO 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.
本研究调查了在混合离子溶液中生命极限的界定因素。更好地理解这些极限应能使我们更准确地预测地球极端环境和外星环境的宜居性。我们系统地研究了一种特征明确的非嗜盐模式生物对一系列由单一盐类和混合盐类制成的溶液（直至其溶解度极限）的反应，并测量了生长停止时的浓度，特别研究了钠、镁和钙阳离子以及氯、硫酸根和高氯酸根阴离子。我们测量了溶液的物理化学性质，以确定哪些性质与生长极限相关。尽管我们通常观察到氯化物盐类在比硫酸盐类更低的水活度下允许生长，而高氯酸盐即使在测量到的最高水活度下也会限制生长，但单个盐类对生长的限制特定于阳离子和阴离子的组合。生长在广泛的离子强度范围内受到限制，且无明显相关性。尽管从理论上讲，高氯酸盐的无序效应（离液序列高的）和硫酸盐的有序效应（离液序列低的）相互抵消，但当这些盐类混合时，它们反而以累加的方式缩小了钠和镁阳离子系统中的生长窗口，与两种离液序列高的钙盐的联合效应方式相同。我们的结果表明，在高浓度盐水中，除了可能由水活度、离子强度或离液序列高的性质等物理化学性质所施加的严格限制外，这些性质并不会设定生命的极限。相反这些极限对于所涉及的盐类和生物体具有高度特异性。这种特异性意味着，即使准确测量了当前的物理化学条件，极端环境的宜居性也无法预测。
