Halobacteria (also Halobacteriacea) are a class of the Euryarchaeota, found in water saturated or nearly saturated with salt. Halobacteria are now recognized as archaea, rather than bacteria. The name 'halobacteria' was assigned to this group of organisms before the existence of the domain Archaea was realized, and remains valid according to taxonomic rules. In a non-taxonomic context, halophilic archaea are referred to as haloarchaea to distinguish them from halophilic bacteria.
These microorganisms are members of the halophile community, in that they require high salt concentrations to grow. They are a distinct evolutionary branch of the Archaea, and are generally considered extremophiles, although not all members of this group can be regarded as such.
Haloarchaea can grow aerobically or anaerobically. Parts of the membranes of haloarchaea are purplish in color, and large blooms of haloarchaea appear reddish, from the pigment bacteriorhodopsin, related to the retinal pigment rhodopsin which it uses to transform light energy into chemical energy by a process unrelated to chlorophyll-based photosynthesis.
Haloarchaea have a potential to solubilized phosphorus. P- solubilizing halophilic archaea may play a role in P nutrition to vegetation growing in hypersaline soils. Haloarchaea may have application as inoculants for crop growing in hypersaline regions.
The extremely halophilic, aerobic members of Archaea are classified within the family Halobacteriaceae, order Halobacteriales in Class III. Halobacteria of the phylum Euryarchaeota (International Committee on Systematics of Prokaryotes, Subcommittee on the taxonomy of Halobacteriaceae). As of May 2016, the family Halobacteriaceae comprises 50 genera 213 species.
Domain : Archaea
Haladaptatus paucihalophilus (Type species)
Halalkalicoccus tibetensis (Type species)
"Halanaeroarchaeum sulfurireducens" (Type species) (IJSEM, in press)
Halapricum salinum (Type species)
Halarchaeum acidiphilum (Type species)
Haloarchaeobius iranensis (Type species)
Haloarcula vallismortis (Type species)
Halobacterium [Hbt.] (Type genus)
"Halobacterium piscisalsi" (subjective junior synonym of Halobacterium salinarum)
Halobacterium salinarum (Type species)
Halobaculum gomorrense (Type species)
Halobellus clavatus (Type species)
Halobiforma haloterrestris (Type species)
Halocalculus aciditolerans (Type species)
Halococcus morrhuae (Type species)
Haloferax volcanii (Type species)
Halogeometricum borinquense (Type species)
Halogranum rubrum (Type species)
Halohasta litorea (Type species)
Halolamina pelagica (Type species)
Halomarina oriensis (Type species)
Halomicroarcula pellucida (Type species)
Halomicrobium mukohataei (Type species)
Halonotius pteroides (Type species)
"Haloparvum sedimenti" (Type species) (IJSEM, in press)
Halopelagius inordinatus (Type species)
Halopenitus persicus (Type species)
Halopiger xanaduensis (Type species)
Haloplanus natans (Type species)
Haloquadratum walsbyi (Type species)
Halorhabdus utahensis (Type species)
"Halorhabdus rudnickae" (Syst Appl Microbiol., in press)
Haloarchaea require salt concentrations in excess of 2 M (or about 10%) to grow, and optimal growth usually occurs at much higher concentrations, typically 20–25%. However, Haloarchaea can grow up to saturation (about 37% salts).
Haloarchaea are found mainly in hypersaline lakes and solar salterns. Their high densities in the water often lead to pink or red colourations of the water (the cells possessing high levels of carotenoid pigments, presumably for UV protection).
Phototrophy in haloarchaea
Bacteriorhodopsin is used to absorb light, which provides energy to transport protons (H+) across the cellular membrane. The concentration gradient generated from this process can then be used to synthesize ATP. Many haloarchaea also possess related pigments, including halorhodopsin, which pumps chloride ions in the cell in response to photons, creating a voltage gradient and assisting in the production of energy from light. The process is unrelated to other forms of photosynthesis involving electron transport however, and haloarchaea are incapable of fixing carbon from carbon dioxide.
Haloarchaea are often considered pleomorphic, or able to take on a range of shapes—even within a single species. This makes identification by microscopic means difficult, and it is now more common to use gene sequencing techniques for identification instead.
One of the more unusually shaped Haloarchaea is the "Square Haloarchaeon of Walsby." It was classified in 2004 using a very low nutrition solution to allow growth along with a high salt concentration, square in shape and extremely thin (like a postage stamp). This shape is probably only permitted by the high osmolarity of the water, permitting cell shapes that would be difficult, if not impossible, under other conditions.
Haloarchaea as Exophiles
Haloarchaea have been proposed as a kind of life that could live on Mars; since the Martian atmosphere has a pressure below the triple point of water, freshwater species would have no habitat on the Martian surface.