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

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 >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 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.

Hyperdiverse archaea near life limits at the polyextreme geothermal Dallol area

“…A wide archaeal (and to a lesser extent, bacterial) diversity seems consistent with suggestions that NaCl-dominated brines are not as extreme as previously thought (36) but also with recent observations that the mixing of meteoric and geothermal fluids leads to hyperdiverse communities (37). Nonetheless, life at high salt requires extensive molecular 25 adaptations (9,10,16,36), which might seem at odds with multiple independent adaptations to hyperhalophily across archaea. Among those adaptations, the intracellular accumulation of K + (salt-in strategy) has been crucial.…”

Analysis of microbial communities in natural halite springs reveals a domain‐dependent relationship of species diversity to osmotic stress

“…Temperature and pH are linear gradients in which both extremes represent stressful conditions, and the minimum and maximum limits of life are well contained within the entire range. Salinity, in contrast, imposes a stress on life only at the upper range and even at NaCl saturation water activity remains well above the limit for life (Lee et al, 2018). Therefore, the nature of its effect on species diversity is fundamentally different.…”

Prokaryotic and viral community structure in the singular chaotropic salt lake Salar de Uyuni

“…ribosomes) per cell required to detect a bacterial cell using CARD-FISH is around a few dozens, depending on the sample conditions (Hoshino et al, 2008), which is very low even for environmental bacteria (Amann and Fuchs, 2008). The low detectability levels in SdU samples might therefore indicate that whereas some halophiles thrive at salt saturation (Lee et al, 2018), stress constrains the metabolic activity of SdU microbial community.…”