Haloquadratum walsbyi

Haloquadratum walsbyi is a species of Archaea in the genus Haloquadratum, known for its square shape and halophilic nature.

First discovered in a brine pool in the Sinai Peninsula of Egypt, H. walsbyi is noted for 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 translates from Greek and Latin as "salt square". This archaean is also commonly referred to as "Walsby's Square Bacterium" because of its unique square shape. In accordance with its name, H. walsbyi are most abundantly observed in salty environments.

Haloquadratum walsbyi is a phototrophic halophilic archaeon. It was the only recognized species of the genus Haloquadratum until 1999 when Haloarcula quadrata was reported as recovered from a brine pool. Haloquadratum walsbyi has a unique cellular structure that resembles an almost-perfectly flat-shaped figure.

The genus was first observed in 1980 by British microbiologist Anthony E. Walsby from samples taken from the Sabkha Gavish, a brine lake in southern Sinai, Egypt. The discovery was formally described in 2007 by Burns et al. Attempts to cultivate the archaea were unsuccessful until 2004 and resulted in the identification of Haloarcula quadrata, a separate species of square archaea that is distinct from H. walsbyi.