Haloarchaea (halophilic archaea, halophilic archaebacteria, halobacteria) are a class of the Euryarchaeota,See the NCBI webpage on Halobacteria.
Data extracted from the  found in water saturated or nearly saturated with salt.
Halobacteria are now recognized as archaea, rather than bacteria and are one of the largest groups.
The name 'halobacteria' was assigned to this group of organisms before the existence of the domain Archaea was realized, and while valid according to taxonomic rules, should be updated.
Halophilic archaea are generally 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, with most species requiring more than 2.0M NaCl for growth and survival.
They are a distinct evolutionary branch of the Archaea distinguished by the possession of ether-linked lipids and the absence of murein in their cell walls.
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 solubilize phosphorus.
Phosphorus-solubilizing halophilic archaea may well play a role in P (phosphorus) nutrition to vegetation growing in hypersaline soils.
Haloarchaea may also have applications as inoculants for crops growing in hypersaline regions.
Taxonomy
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 213 species in 50 genera.
Classification of Gupta et al.
Halobacteriales
Halobacteriaceae (Type genera: Halobacterium)
Haladaptatus, Halalkalicoccus, Haloarchaeobius, Halarchaeum, Halobacterium, Halocalculus, Halorubellus, Halorussus, "Halosiccatus", Halovenus, Natronoarchaeum, Natronomonas, Salarchaeum.
Haloarculaceae (Type genera: Haloarcula)
Halapricum, Haloarcula, Halomicroarcula, Halomicrobium, Halorientalis, Halorhabdus, Halosimplex.
Halococcaceae (Type genera: Halococcus)
Halococcus.
Haloferacales
Haloferacaceae (Type genera: Haloferax)
Halabellus, Haloferax, Halogeometricum, (Halogranum), Halopelagius, Haloplanus, Haloquadratum, Halosarcina.
Halorubraceae (Type genera: Halorubrum)
Halobaculum, (Halogranum), Halohasta, Halolamina, Halonotius, Halopenitus, Halorubrum, Salinigranum.
Natrialbales
Natrialbaceae (Type genera: Natrialba)
Halobiforma, Halopiger, Halostagnicola, Haloterrigena, Halovarius, Halovivax, Natrialba, Natribaculum, Natronobacterium, Natronococcus, Natronolimnobius, Natronorubrum, Salinarchaeum.
Molecular Signatures
Detailed phylogenetic and comparative analyses of genome sequences from members of the class Haloarchaea has led to division of this class into three orders, Halobacteriales, Haloferacales and Natrialbales, which can be reliably distinguished from each other as well as all other archaea/bacteria through molecular signatures known as conserved signature indels.
These studies have also identified 68 conserved signature proteins (CSPs) whose homologs are only found in the members of these three order and 13 conserved signature indels (CSIs) in different proteins that are uniquely present in the members of the class Haloarchaea.
These CSIs are present in the following proteins: DNA topoisomerase VI, nucleotide sugar dehydrogenase, ribosomal protein L10e, RecJ-like exonuclease, ribosomal protein S15, adenylosuccinate synthase, phosphopyruvate hydratase, RNA-associated protein, threonine synthase, aspartate aminotransferase, precorrin-8x methylmutase, protoporphyrin IX magnesium chelatase and geranylgeranylglyceryl phosphate synthase-like protein.
Living environment
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).
Some of them live in underground rock salt deposits, including one from middle-late Eocene (38-41 million years ago).
Some even older ones from more than 250 million years ago have been reported.
Adaptations to environment
Haloarchaea can grow at an aw close to 0.75, yet a water activity (aw) lower than 0.90 is inhibitory to most microbes.
The number of solutes causes osmotic stress on microbes, which can cause cell lysis, unfolding of proteins and inactivation of enzymes when there is a large enough imbalance.
Haloarchaea combat this by retaining compatible solutes such as potassium chloride (KCl) in their intracellular space to allow them to balance osmotic pressure.
Retaining these salts is referred to as the “salt-in” method where the cell accumulates a high internal concentration of potassium.
Because of the elevated potassium levels, haloarchaea have specialized proteins that have a highly negative surface charge to tolerate high potassium concentrations.
Haloarchaea have adapted to use glycerol as a carbon and energy source in catabolic processes, which is often present in high salt environments due to Dunaliella species that produce glycerol in large quantities.
Phototrophy
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.
Early evolution of retinal proteins has been proposed as the purple Earth hypothesis.
Cellular shapes
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.
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.
The presence of high salt concentrations in water lowers its freezing point, in theory allowing for halophiles to exist in saltwater on Mars.
See also
Life on Mars
Purple Earth hypothesis
References
Further reading
Journals
Books
Databases
External links
An educational website on haloarchaea
HaloArchaea.com
Mike Dyall-Smith's Homepage
