Haloquadratum (="salt square") is a genus of the Halobacteriaceae. The first species to be identified in this group, Haloquadratum walsbyi, is highly unusual since its cells are flat, rectangular boxes in shape.
First discovered in 1980 by A.E. Walsby in the Gavish Sabkha, a coastal brine pool on the Sinai Peninsula in Egypt, this archaeon was not successfully cultured until 2004. While attempting to culture Haloquadratum walsbyi, a new species, Haloarcula quadra, was found. Haloquadratum are remarkable for their shape, motility, and relative abundance in halophilic environments.
The cells contain large numbers of refractile gas-filled vacuoles which provide buoyancy in a watery environment and may help to position the cells to maximise light-harvesting . The cells may join with others to form fragile sheets as extensive as 40 microns. They also have one to several flagella, making them highly motile.
Haloquadratum walsbyi can be found anywhere in hypersaline waters. When sea water evaporates, high concentration and precipitation of calcium carbonate and calcium sulphate result, leading to a hypersaline sodium chloride rich brine. Further evaporation results in the precipitation of sodium chloride or halite, and then to a concentrated magnesium chloride brine termed bittern. During the final stage of halite formation, before magnesium chloride concentration causes the brine to become sterile, H. walsbyi flourishes and can make up 80% of the brine's biomass.