Thermodynamics of halobacterial environments.

The ranges of the major ion composition of near neutral hypersaline lakes have been described in terms of the charge concentration and the mole fraction of monovalent cations. An ion interactive method was used to establish the thermodynamic activity of the chemical species in these model brines, which were then used in culture media for Halobacterium halobium, H. salinarium, and H. volcanii. The bacteria grew rapidly at water activities of 0.78, 0.79, and 0.925, respectively. The growth areas for these organisms were not easily defined in terms of the Na+ or Cl- activity. The morphology of H. halobium and H. salinarium changes from rod shaped to spherical when the Mg2+ activity drops below 0.15 mol . kg-1. Niches described by the chemical parameters reflect the marine origin of the environments of H. halobium and H. salinarium, and the more heterogeneous environments of H. volcanii.