title: Multiplication of microbes below 0.690 water activity: implications for terrestrial and extraterrestrial life.
abstract: Since a key requirement of known life forms is available water (water activity; aw ), 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 Earth 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 Xeromyces bisporus. The first forms of life on Earth were, though, prokaryotic. Recent evidence now indicates that some halophilic Archaea and Bacteria have water-activity limits more or less equal to those of X. bisporus. We discuss water activity in relation to the limits of Earth'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-aw limit; and whether extraterrestrial aqueous milieux of ≥ 0.605 aw can resemble fertile microbial habitats found on Earth.
authors.affiliation: Institute for Global Food Security, School of Biological Sciences, MBC, Queen's University Belfast, Belfast, BT9 7BL, Northern Ireland.
authors.forename: Andrew
authors.initials: A
authors.lastname: Stevenson
authors.forename: Jürgen
authors.initials: J
authors.lastname: Burkhardt
authors.forename: Charles S
authors.initials: CS
authors.lastname: Cockell
authors.forename: Jonathan A
authors.initials: JA
authors.lastname: Cray
authors.forename: Jan
authors.initials: J
authors.lastname: Dijksterhuis
authors.forename: Mark
authors.initials: M
authors.lastname: Fox-Powell
authors.forename: Terence P
authors.initials: TP
authors.lastname: Kee
authors.forename: Gerhard
authors.initials: G
authors.lastname: Kminek
authors.forename: Terry J
authors.initials: TJ
authors.lastname: McGenity
authors.forename: Kenneth N
authors.initials: KN
authors.lastname: Timmis
authors.forename: David J
authors.initials: DJ
authors.lastname: Timson
authors.forename: Mary A
authors.initials: MA
authors.lastname: Voytek
authors.forename: Frances
authors.initials: F
authors.lastname: Westall
authors.forename: Michail M
authors.initials: MM
authors.lastname: Yakimov
authors.forename: John E
authors.initials: JE
authors.lastname: Hallsworth
mesh_terms: D001105:Archaea / Q000166:cytology; D001203:Ascomycota / Q000166:cytology / Q000502:physiology; D001419:Bacteria / Q000166:cytology; D002455:Cell Division; D017753:Ecosystem; D018559:Exobiology; D005118:Extraterrestrial Environment; D011387:Prokaryotic Cells / Q000166:cytology / Q000502:physiology*; D054712:Salinity; D012965:Sodium Chloride; D014867:Water; D014871:Water Microbiology
chemical_list: D014867:Water; D012965:Sodium Chloride