A halophilic bacterium was isolated from bottom sediment from the Dead Sea. The organism possessed the properties of the halobacteria, but differed from the known species in two important respects, 1) the cells were disc shaped and often cupped when grown under optimum conditions, 2) the optimum requirements for sodium chloride was in the range 1.7--2.5 molar which is about half of that generally reported for the halobacteria. The organism was assigned to the genus Halobacterium and described as Halobacterium volcanni spec. rov. The optimum sodium chloride concentration for growth was close to that found in the Dead Sea. The tolerance for magnesium chloride was very high; the organism grew well in media containing magnesium chloride in the concentrations found in the Dead Sea. Halobacterium volcanii is therefore remarkably well fitted for life in the Dead Sea.

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SUMMARYWhen the NaCl concentration of a suspension of Halobacterium salinarium is gradually lowered by adding water, the rod-shaped organisms are converted to spheres which lyse. The organisms do not change in size when transformed from rods to spheres. Chemicals which do not possess a strong net charge in aqueous solutions do not protect the structure of the organism when they replace NaCl in iso-osmolar concentions. The rod shape of the organisms is only maintained by very high concentrations of ions which interact weakly with common proteins; lithium and ammonium ions are exceptions to this rule. Ions which display strong interactions with common proteins, and chemicals which are believed to break secondary bonds between protein molecules, effect a transformation of rods to spheres, and frequently a lysis of the spheres, when brought in contact with organisms suspended in strong NaCl solution. The changes in structure in hypotonic and isotonic solutions are not affected by metabolic inhibitors. Isolated cell-wall fragments disintegrate into smaller units when exposed to conditions under which whole organisms lyse. It is concluded that the structural transformations and lysis of H . salinarium in hypotonic solutions are not caused by the action of enzymes and that osmotic phenomena play no, or only a minor, role. The observations support the contention that the globular lipoprotein particles, which constitute the bulk of the material of the cell wall of these bacteria, are bound together mainly by electrostatic forces and secondary bonds. When the cells are exposed to hypotonic solutions, or to ions which bind strongly to proteins, or to chemicals which are believed to break secondary bonds between protein molecules, the linkages binding the lipoprotein particles together are weakened so that the wall structure disintegrates. Only in the presence of high concentrations of sodium and chloride ions, or other ions which bind loosely to proteins, is it possible for the proteinaceous particles of the cell wall to associate in an orderly array.