NaCl-saturated brines are thermodynamically moderate, rather than extreme, microbial habitats

Callum J.D. Lee, Phillip E. McMullan, Callum J. O'Kane, Andrew Stevenson, Inês C. Santos, Chayan Roy, Wriddhiman Ghosh, Rocco L. Mancinelli, Melanie R. Mormile, Geoffrey McMullan, Horia L. Banciu, Mario A. Fares, Kathleen C. Benison, Aharon Oren, Mike L. Dyall-Smith, John E. Hallsworth*

*Corresponding author for this work

Research output: Contribution to journalReview articlepeer-review

53 Scopus citations

Abstract

NaCl-saturated brines such as saltern crystalliser ponds, inland salt lakes, deep-sea brines and liquids-of-deliquescence on halite are commonly regarded as a paradigm for the limit of life on Earth. There are, however, other habitats that are thermodynamically more extreme. Typically, NaCl-saturated environments contain all domains of life and perform complete biogeochemical cycling. Despite their reduced water activity, ∼0.755 at 5 M NaCl, some halophiles belonging to the Archaea and Bacteria exhibit optimum growth/metabolism in these brines. Furthermore, the recognised water-activity limit for microbial function, ∼0.585 for some strains of fungi, lies far below 0.755. Other biophysical constraints on the microbial biosphere (temperatures of >121C; pH > 12; and high chaotropicity; e.g. ethanol at >18.9% w/v (24% v/v) and MgCl2 at >3.03 M) can prevent any cellular metabolism or ecosystem function. By contrast, NaCl-saturated environments contain biomass-dense, metabolically diverse, highly active and complex microbial ecosystems; and this underscores their moderate character. Here, we survey the evidence that NaCl-saturated brines are biologically permissive, fertile habitats that are thermodynamically mid-range rather than extreme. Indeed, were NaCl sufficiently soluble, some halophiles might grow at concentrations of up to 8 M. It may be that the finite solubility of NaCl has stabilised the genetic composition of halophile populations and limited the action of natural selection in driving halophile evolution towards greater xerophilicity. Further implications are considered for the origin(s) of life and other aspects of astrobiology.

Original languageAmerican English
Pages (from-to)672-693
Number of pages22
JournalFEMS Microbiology Reviews
Volume42
Issue number5
DOIs
StatePublished - 1 Sep 2018

Bibliographical note

Publisher Copyright:
© FEMS 2018. All rights reserved.

Keywords

  • Dunaliella salina
  • Extreme halophiles
  • Habitability of Mars
  • Hypersaline brines
  • Limits-of-life on Earth
  • Solar salterns

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