Haloquadratum

Haloquadratum (common abbreviation: Hqr.) is a genus of archaean, belonging to the family 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 archaean, discovered in 1980 by A.E. Walsby in the Gavish Sabkha, a coastal hypersaline pool (sabkha) on the Sinai Peninsula in Egypt, 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 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 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.