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

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.

High growth rate and substrate exhaustion results in rapid cell death and lysis in the thermophilic bacterium Geobacillus thermoleovorans.

Batch cultures of the thermophilic bacterium Geobacillus thermoleovorans T80 attained extremely high-specific glucose utilization rates leading to high specific growth rates, followed by extensive cell death and lysis with the onset of substrate exhaustion. The dramatic decrease in live cell numbers, as determined by flow cytometry, was accompanied by the release of soluble protein. Once the growth phase reached the point of commitment to lysis created by the impending exhaustion of substrate, the addition of extra carbon substrate did not halt the rapid death rate and lysis, although, towards the end of the exponential growth phase, the substrate was utilized producing only a small additional biomass concentration as a result of the net effect of cell growth and death. This lytic phenomenon was observed when a range of different carbon substrates (glucose, pyruvate, acetate, n-hexadecane, nutrient broth), as well as ammonium (the nitrogen source) in the presence of excess carbon source, reached near exhaustion. The rate and extent of cell death and the ensuing lysis depend on the culture growth rate. Cultures batch grown with a lower initial substrate concentration, or at a lower temperature, or at lower dilution rates for continuous-flow cultures, exhibited a lower rate and extent of cell death and lysis. Batch re-culture of the persister cells resulted in a behavior identical to that of the original culture indicating that these cells were not genetically modified. The glucose utilization, cell growth and death rates were mathematically described based on Monod kinetics and estimated values of pertinent biokinetic constants are reported.

Utilisation of aminomethane sulfonate by Chromohalobacter marismortui VH1.

Chromohalobacter marismortui VH1 was screened for its ability to utilise organosulfonate compounds at a range of NaCl concentrations. Only aminomethane sulfonate, of seven sulfonates tested, was utilised. Length of lag phase during growth on aminomethane sulfonate, as either nitrogen and/or sulfur source, increased with increasing NaCl concentration. Cell yields increased linearly with increasing aminomethane sulfonate concentration up to 5 mM. Resting cells pregrown on aminomethane sulfonate as sole nitrogen source exhibited carbon-sulfur bond cleaving [0.123 nmol sulfate accumulated h(-1) (mg cells)(-1)] and sulfite-oxidising [0.185 nmol sulfate accumulated h(-1) (mg cells)(-1)] activities. C. marismortui VH1 is capable of sulfur-starvation deregulated metabolism of aminomethane sulfonate under high salt conditions.

Iminodiacetate and nitrilotriacetate degradation by Kluyveromyces marxianus IMB3.

The thermotolerant yeast Kluyveromyces marxianus IMB3 was capable of utilising either iminodiacetate or nitrilotriacetate as a sole source of nitrogen for growth. Cell extracts contained iminodiacetate dehydrogenase and nitrilotriacetate monooxygenase activities, suggesting the presence in the yeast of orthologues of these bacterial enzymes. The activities were not detectable in complete medium-growth cells, nor in nitrogen-starved cells, suggesting an inducible biodedgradation pathway for biodegradation of these xenobiotics, which has not been previously reported in a eukaryotic cell system. This observation emphasises the hitherto unrealised importance of yeast strains in the biodegradation of xenobiotics in the environment.