Multiplication of microbes below 0.690 water activity: implications for terrestrial and extraterrestrial life

<jats:title>Summary</jats:title><jats:p>Since a key requirement of known life forms is available water (water activity; <jats:italic>a</jats:italic><jats:sub>w</jats:sub>), recent searches for signatures of past life in terrestrial and extraterrestrial environments have targeted places known to have contained significant quantities of biologically available water. However, early life on <jats:styled-content style="fixed-case">E</jats:styled-content>arth inhabited high‐salt environments, suggesting an ability to withstand low water‐activity. The lower limit of water activity that enables cell division appears to be ∼ 0.605 which, until now, was only known to be exhibited by a single eukaryote, the sugar‐tolerant, fungal xerophile <jats:italic><jats:styled-content style="fixed-case">X</jats:styled-content>eromyces bisporus</jats:italic>. The first forms of life on <jats:styled-content style="fixed-case">E</jats:styled-content>arth were, though, prokaryotic. Recent evidence now indicates that some halophilic <jats:styled-content style="fixed-case">A</jats:styled-content>rchaea and <jats:styled-content style="fixed-case">B</jats:styled-content>acteria have water‐activity limits more or less equal to those of <jats:italic><jats:styled-content style="fixed-case">X</jats:styled-content>. bisporus</jats:italic>. We discuss water activity in relation to the limits of <jats:styled-content style="fixed-case">E</jats:styled-content>arth's present‐day biosphere; the possibility of microbial multiplication by utilizing water from thin, aqueous films or non‐liquid sources; whether prokaryotes were the first organisms able to multiply close to the 0.605‐a<jats:sub>w</jats:sub> limit; and whether extraterrestrial aqueous milieux of ≥ 0.605 <jats:italic>a</jats:italic><jats:sub>w</jats:sub> can resemble fertile microbial habitats found on Earth.</jats:p>