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

Abstract

Summary Since a key requirement of known life forms is available water (water activity; a w ), 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 E 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 X eromyces bisporus . The first forms of life on E arth were, though, prokaryotic. Recent evidence now indicates that some halophilic A rchaea and B acteria have water‐activity limits more or less equal to those of X . bisporus . We discuss water activity in relation to the limits of E 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 w limit; and whether extraterrestrial aqueous milieux of ≥ 0.605 a w can resemble fertile microbial habitats found on Earth.

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