Water activity is an important characteristic for describing unusual waters and is a determinant of habitability for microorganisms. However, few empirical studies of water activity have been done for natural waters exhibiting an extreme chemistry. Here, we investigate water activity for acid brines from Western Australia and Chile with pH as low as 1.4, salinities as high as 32% total dissolved solids, and complex chemical compositions. These acid brines host diverse communities of extremophilic microorganisms, including archaea, bacteria, algae, and fungi, according to metagenomic analyses. For the most extreme brine, its water activity (0.714) was considerably lower than that of saturated (pure) NaCl brine. This study provides a thermodynamic insight into life within end-member natural waters that lie at, or possibly beyond, the very edge of habitable space on Earth.

中文翻译：

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酸卤湖水活度逼近生命极限


水活度是描述异常水域的重要特征，也是微生物可居住性的决定因素。然而，对于表现出极端化学性质的天然水体，水活度的实证研究却很少。在这里，我们研究了来自西澳大利亚和智利的酸性盐水的水活度，这些盐水的 pH 值低至 1.4，盐度高达 32% 的总溶解固体，且化学成分复杂。根据宏基因组分析，这些酸性盐水中存在着多种嗜极微生物群落，包括古细菌、细菌、藻类和真菌。对于最极端的盐水，其水分活度 (0.714) 远低于饱和（纯）氯化钠盐水。这项研究为位于或可能超出地球可居住空间边缘的端元天然水域中的生命提供了热力学见解。