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

NaCl-saturated brines such as saltern crystallizer 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 recognized 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 >121°C; pH >12; and high chaotropicity; e.g. ethanol at >18.9% w/v (24% v/v) and MgCl2 at >3.03 M) can prevent all ecosystem function and cellular metabolism. By contrast, NaCl-saturated environments contain biomass-dense, metabolically-diverse, highly-active, and complex microbial ecosystems which 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 8 M. The finite solubility of NaCl may have stabilized the genetic composition of halophile populations and limited the action of natural selection in driving the halophile evolution towards greater xerophilicity. Implications are also considered for the origin(s)-of-life and other aspects of astrobiology.