Halobacteria

Halobacteria are a class of the domain Archaea within the phylum Euryarchaeota. They are extreme halophiles that require very high salt concentrations for growth. They inhabit hypersaline environments such as saturated brines, salt lakes, and solar salterns, including the Dead Sea and Great Salt Lake. Morphologically they are often rod- or coccoid-shaped and may appear red or purple due to carotenoids such as bacterioruberin.

Physiology and metabolism: Halobacteria typically tolerate 3–5 M NaCl and some grow near saturation. Most are aerobic or microaerophilic and chemoorganotrophic, obtaining energy from organic substrates. A notable feature is bacteriorhodopsin, a light-driven proton pump that enables ATP generation under illumination, providing an alternative energy source without standard photosynthesis. They also use halorhodopsin to regulate chloride balance. Osmotic balance is achieved by high intracellular potassium and compatible solutes; their proteins are adapted to high-salt conditions.

Cell structure and genetics: Archaea-specific membranes contain ether-linked lipids with isoprenoid chains, and many Halobacteria have a protective S-layer; some lack peptidoglycan. They reproduce asexually by binary fission and often harbor plasmids; genetic systems differ from bacteria and eukaryotes.

Taxonomy and genera: Halobacteria include several genera such as Halobacterium, Haloferax, Halococcus, and Haloarcula, grouped in the order Halobacteriales within the Halobacteriaceae.

Ecology and applications: They color salt flats and serve as models for life in extreme salinity. Bacteriorhodopsin and related pigments have biotechnological uses, including research tools and sensors. Halobacteria are not known to cause disease in humans.