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 identifying square shape which makes it unique. In accordance with its name, Haloquadratum 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 is very unusual because of its unique cellular structure that resembles an almost-perfectly flat-shaped figure.

The genus was first observed in 1980 by a British microbiologist, Prof. Anthony E. Walsby, from samples taken from the Sabkha Gavish, a brine lake in southern Sinai, Egypt. This discovery is 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, another species of square archaea of the genus Haloarcula, distinct from H. walsbyi, less abundant and genetically quite different.