Metagenome of Marine NaCl-Saturated Brine

Around the world and wherever a body of seawater becomes isolated under arid conditions evaporation leads to the increase in salinity and produces hyper-saline environments. Given the proper conditions, the process continues until complete evaporation of water and deposition of mineral salts. Sodium chloride (common salt) has been extracted from seawater for most of human history through this process that can be considered one of the most ancient mineral extraction activities.

GC% distribution of genes from selected halophilic genomes, of
fosmid-end sequences and of crystallizer DNA.
Click to enlarge.

As sea water concentrates biological diversity starts a slow decrease that culminates at values around 10 times typical marine levels (about 400g of salts per litter of water). This is well above the level in which sodium chloride starts to precipitate and is also characterized by high concentration of magnesium, the next most common cation in seawater after sodium. The hygroscopic properties of the divalent Mg2+ ions dramatically decrease the water activity (Aw), a measure for the availability of free water molecules for biological processes. The Aw is 1.0 for pure water, 0.75 for a saturated NaCl solution and 0.3 for a saturated MgCl2 solution. The actual Aw of the MgCl2 enriched brines is unknown, but will decrease upon further concentration.

Currently an Aw of 0.6 is recognized as the lower limit for life. This means that although the organism thrives in an aqueous environment it suffers severe desiccation stress. Furthermore, magnesium is highly chaotropic and denatures macromolecules. Actually only under very arid and warm conditions are these kind of NaCl saturated brines found. Where ever they develop, however, a dense microbial community can be found that has an extremely small set of different microbes present. Primary productivity is probably strongly diminished but all depends on a single widespread species of green algae Dunaliella salina. This organism is found in relatively large numbers although its halotolerance is severely put to test at these high salinities and at lower salinities they bloom in much larger numbers. Also, measurements of primary productivity indicate low levels of activity. The densest populations (reaching up to 108cells/ml) belong to one single species of hyperhalophilic archeon. As if wanting to stress its widespread occurrence and cosmopolitan nature, it was recently isolated nearly simultaneously from salterns in Spain and Australia. Along with Haloquadratum significant numbers (up to 20%) of the total number of cells are found to belong to a bacterium related to the bacteroidetes (CFB group), Salinibacter ruber. That this is a life-edge situation is reflected by the extreme simplicity of the

Haloquadratum walsbyi DSM16790 genome and hypervariable regions
Click to enlarge.

biological diversity found but is even more clearly reflected in the fact that if the concentration process continues to reach up to ca 45% salts and roughly coinciding with the point in which Mg salts start to precipitate the dense communities just described disappear, and the habitat is naturally sterilized. Although probably many cells survive in the crystals the waters become transparent and devoid of life. A similar situation is found in the Dead Sea where life tolerance was exceeded by the mid-eighties of the past century and has stayed life-less as its name implies except for local more diluted sections around the edges.