''Haloquadratum'' (common abbreviation: ''Hqr.'') is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of archaean, belonging to the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Haloferacaceae. The first species to be identified in this group, ''Haloquadratum walsbyi'', is unusual in that its cells are shaped like square, flat boxes. This halophilic

archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaeba ...

n, discovered in 1980 by A.E. Walsby in the Gavish Sabkha, a coastal hypersaline pool (

sabkha A sabkha ( ar, سبخة) is a coastal, supratidal mudflat or sandflat in which evaporite-saline minerals accumulate as the result of semiarid to arid climate. Sabkhas are gradational between land and intertidal zone within restricted coastal p ...

) on the

Sinai Peninsula The Sinai Peninsula, or simply Sinai (now usually ) (, , cop, Ⲥⲓⲛⲁ), is a peninsula in Egypt, and the only part of the country located in Asia. It is between the Mediterranean Sea to the north and the Red Sea to the south, and is a ...

Egypt Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a List of transcontinental countries, transcontinental country spanning the North Africa, northeast corner of Africa and Western Asia, southwest corner of Asia via a land bridg ...

, was not cultured until 2004. The cells typically contain polyhydroxyalkanoate (PHA) granules, as well as large numbers of refractile, gas-filled vacuoles which provide

buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the ...

in a watery environment and may help to position the cells to maximize light-harvesting. The cells may join with others to form fragile sheets up to 40 micrometres wide. ''H. walsbyi'' can be found anywhere in hypersaline waters. When sea water evaporates, high concentration and precipitation of calcium carbonate and

calcium sulfate Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related hydrates. In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant. One particular hydrate is better known as plaster of Paris ...

result, leading to a hypersaline

sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35. ...

-rich brine. Further evaporation results in the precipitation of sodium chloride or

halite Halite (), commonly known as rock salt, is a type of salt, the mineral (natural) form of sodium chloride ( Na Cl). Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, p ...

, and then to a concentrated magnesium chloride brine termed

bittern Bitterns are birds belonging to the subfamily Botaurinae of the heron family Ardeidae. Bitterns tend to be shorter-necked and more secretive than other members of the family. They were called ''hæferblæte'' in Old English; the word "bittern ...

. During the final stage of halite formation, before magnesium chloride concentration causes the brine to become

sterile Sterile or sterility may refer to: *Asepsis Asepsis is the state of being free from disease-causing micro-organisms (such as pathogenic bacteria, viruses, pathogenic fungi, and parasites). There are two categories of asepsis: medical and surgi ...

, ''H. walsbyi'' flourishes and can make up 80% of the brine's biomass.

Description and significance

''H. walsbyi'' was first discovered in 1980 by A.E. Walsby in the Gavish Sabkha, a coastal brine pool in the Sinai peninsula, Egypt, and formally described by Burns ''et al.'' in 2007. The organisms were notable because of their extremely thin (around 0.15 μm), square-shaped structure. It was not cultivated in the lab until 2004.

Genome structure

The mapping of ''H. walsbyis genome has been completed, giving a better understanding of the organism's genealogy and taxonomy, and the role it plays in the ecosystem. A genomic comparison of the Spanish and Australian isolates (strains HBSQ001 and C23^T) strongly suggests a rapid global dispersion, as they are so similar and have retained gene order (synteny).

Cell structure and metabolism

''H.walsbyi'' has a unique cell shape – that of an extremely thin square, its best-known characteristic. The cells possess an abundance of intracellular refractile bodies known as gas vacuoles – vacuoles filled with gas which provide buoyancy – maintaining upper position in the water column. Individual square cells are joined with others to form large sheets, sometimes as large as 40 µm. These sheets are extremely fragile and the connections between the cells are easily broken. Its mode of metabolism is not completely known; complete genetic information will give researchers necessary insight.

Ecology

''H. walsbyi'' was first noticed and taken from saline pools in Egypt, but it can be found in hypersaline bodies of water all over the world. Evaporation of water in these pools leaves high concentrations of salt, making for its optimal growth environment. According to Bolhuis ''et al.'', "In this sense, they are the most hyper halophilic organisms known, as further concentration of the magnesium salts (bitterns) leads to sterility of the brines." Its precise role in the ecosystem is not known for sure, but because of its unique morphology, learning more about it will surely provide some information on the evolution and morphological adaptation of archaeans.

References

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