Haloquadratum walsbyi is a species of archaea which was discovered in a brine pool in the Sinai peninsula of Egypt. It is noted for both its flat, square-shaped cells, and its unusual ability to survive in aqueous environments with high concentrations of sodium chloride and magnesium chloride. The species' genus name Haloquadratum literally translates from Greek and Latin as "salt square". Haloquadratum walsbyi is a phototrophic halophilic archaea. It was the only recognized species of the genus Haloquadratum until 1999 when Haloarcula quadrata was reported as recovered from a brinepool. Haloquadratum walsbyi is very unusual in that it forms almost-perfectly square, flat-shaped cells. The genus was observed for the first time in 1980 by a British microbiologist, Prof. Anthony E. Walsby, from samples taken from the Sabkha Gavish, a brine lake in southern Sinai, in Egypt, and has been described formally in 2007 by Burnset al. They are usually abundant in most salty environments. Attempts to cultivate them were unsuccessful until 2004 and resulted in the identification of haloarcula quadrata, another species of square archaea of the genus haloarcula, distinct from H. walsbyi, less abundant and genetically quite different. The cells are large 2 to 5 microns but very thin approximately 150 nanometers and generally contain granules of polyhydroxyalkanoates. They also hold a large number of refractivevacuolesfilled with gas that ensure buoyancy in an aqueous environment, and may help to position the cells to maximize the reception of light. They can gather in sheets up to 40 μm wide, but the connections between the cells are fragile and can easily be broken. These organisms can be found in any stretch of very salty water. During the evaporation of seawater, calcium carbonate and calcium sulfateprecipitate first, leading to a brine rich in sodium chloride NaCl. If evaporation continues, NaCl precipitates in the form of halite, leaving a brine rich in magnesium chloride. H. walsbyi prospers during the final phase of the precipitation of halite, and can constitute 80% of the of this medium. The genome of H. walsbyi has been completely sequenced, allowing access to a better understanding of the phylogenetic and taxonomic classification of this organism and its role in the ecosystem. A genomic comparison of Spanish and Australian isolates strongly suggests a rapid global dispersion, as they are remarkably similar and have maintained the order of genes. Its growth in the laboratory was obtained in a medium with very high chloride concentrations, making this organism among the most haloresistant known. Its optimum growthtemperature is 40 °C.
