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.
Keywords: Acid brines; Extremophiles; Habitability; Halophile ecology; Water activity.