The Order Halobacteriales
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The Order Halobacteriales AHARON OREN
Introduction Halophilic Archaea of the order Halobacteriales are found in hypersaline environments in which salt concentrations exceed 150–200 g/liter. They inhabit salt lakes such as the Great Salt Lake, Utah, the Dead Sea, and other hypersaline water bodies such as the crystallizer ponds of solar salterns in which sea water is evaporated for the production of salt. Additional habitats in which these halophilic Archaea often develop include salted fish and hides preserved by treatment with salt. They also occur in certain fermented food products in which molar concentrations of NaCl are added as part of the manufacturing process, such as Thai fish sauce. Microbial deterioration of salted hides and salted fish by members of the Halobacteriales is clearly visible by the pink-red color of the developing colonies (Clayton and Gibbs, 1927; Harrison and Kennedy, 1922; Lochhead, 1934; Shewan, 1971; Vreeland et al., 1998a). Many of the early studies on the red halophilic Archaea were initiated in an attempt to understand the cause of the damage to fish and hides treated with salt for their preservation. The account by Klebahn, 1919 on the bacteria causing the red discoloration of salted cod probably presents the first accurate description of halobacteria, at the time designated as “Bacillus halobius ruber.” That study was followed by the isolation of “Pseudomonas salinaria” from the red discoloration of cured codfish (Harrison and Kennedy, 1922). This culture is now lost, but the organism is probably very similar to later isolates known as Halobacterium salinarum. Another classic early study on the red halophilic bacteria that grow on salted fish is that of Petter, 1931. The phenomenon of bacterial degradation of salted hides was documented by Lochhead, 1934. During the 1930s also the first studies appeared of the microbiology of solar salterns and salt lakes, describing the importance of red halophilic bacteria in these ecosystems (Baas-Becking, 1931; Hof, 1935). Many of the early studies on the Halobacteriales were summarized in Larsen’s classic essay on “the halobacteria’s confusion to
biology” (Larsen, 1973). Tindall, 1992 presented an in-depth discussion on the properties, nomenclature, and taxonomic affiliations of some of the early isolates. The mode of adaptation of the Halobacteriales to life at salt concentrations at or near NaCl saturation has been the subject of in-depth studies. In contrast to most other halophilic or halotolerant microorganisms that keep intracellular ionic concentrations low, cells of the halophilic Archaea of the order Halobacteriales contain molar concentrations of ions, especially K+ and Cl−, within the cells (Christian and Waltho, 1962; Matheson et al., 1976; Oren, 1999a; Pérez-Fillol and Rodriguez-Valera, 1986). The maintenance of high intracellular salt concentrations requires unique adaptations of the enzymatic machinery to be able to function in the presence of high salt, adaptations that make the cells of m
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