Life and survival in a magnesium chloride brine: The biology of the Dead Sea

The Dead Sea is a hypersaline terminal desert lake. Its water contains about 340 g/1 total dissolved salts. Divalent cations dominate in the brine, which presently contains about 1.89 M magnesium and 0.44 M calcium, in addition to about 1.6 M sodium and 0.2 M potassium. The main anions are chloride and bromide (99% and 1% of the anion sum, respectively). The pH of the brine is about 6.0, and its water activity was estimated at about 0.66. The lake is saturated with respect to sodium chloride. The negative water balance in recent years caused a mass precipitation of halite, with a concomitant increase in the relative concentrations of divalent cations. In spite of the fact that molar concentrations of divalent cations are strongly inhibitory to most halophilic and halotolerant microorganisms, the Dead Sea is inhabited by a variety of microorganisms. These include halophilic Archaea (Halorubrum sodomense. Halobaculum gomorrense, Haloferax volcanii. and others), well adapted to growth at high magnesium concentrations, unicellular green algae (Dunaliella parvd). and a few species of halophilic Bacteria. Dunaliella. being the sole primary producer in the lake, does not grow in undiluted Dead Sea water. However, when the upper water layers become diluted by more than 10% as a result of winter rain floods, mass blooms may develop, followed by mass development of red halophilic Archaea. which thrive on the organic material produced by the algae. During the often prolonged periods between the bloom events, during which the salinity of the brines is high and halite precipitates, a small community of Archaea remained present in a state of little activity, but ready to resume growth as soon as a suitable source of organic material becomes available.