Haloquadratum walsbyi is a square, non-motile, pigmented halophilic archaea that dominates in most thalassic NaCl-saturated environments, reaching population densities of over 10(7) cells per ml. Two unique features of these extremely fragile cells are the wafer-like rectangular shape, usually between 2 and 5 um, and a cell thickness of not more than 0.1 um. It requires >14% salt for growth. Cells are challenged by the sub-lethal conditions of an extremely high MgCl(2) concentration and high solar irradiance and suffer severe desiccation stress. Special mechanisms are therefore required to maintain optimal water activity within the cell and at the cell surface. Additionally the amount of dissolved oxygen decreases to near anoxia and some essential nutrients (e.g. phosphates) become unavailable due to their complexing with Mg(2+). H.walsbyi encodes the largest archaeal protein known to date, halomucin (9159 residues), which is similar to animal mucins. Animal mucins play an important role in protecting various tissues against desiccation or harsh chemical conditions. Halomucin is thought to be exported outside the cell, where it may be glycosylated, sulfated and sialated; H.walsbyi is the first archaea identified that is probably able to synthesize sialic acid. These modifications, along with the potential capacity to synthesize and poly-gamma-glutamate capsule, probably create an aqueous shield and in the case of the capsule may also help maintain the unique shape of the cell. C23 is the type strain for the species, and was isolated from an Australian solar saltern. It differs from the other sequenced H.walsybi (strain HBSQ001, HALWD, isolated in Spain) in possessing a conventional two-layered structure consisting of the cell membrane covered by a single, external protein S-layer. HALWD has an atypical triple-layered cell wall. Despite being isolated approximately 17,000 km apart 84% of the 2 genomes are 98.6% identical. The genomes have evolved by uptake and precise integration of foreign DNA, probably originating from close relatives. Change is also driven by numerous types of mobile genetic elements, most of which seem to be active. It is probably subject to rapid global dispersal, possibly by airborne salt particles or migratory birds (adapted from PMID 21701686).