Haloquadratum walsbyi DSM 16790

A square, non-motile, pigmented halophilic archaea that dominates in most thalassic NaCl-saturated environments, reaching population densities of over 107 cells per ml, this strain was isolated from a Spanish solar saltern. The two unique features of these cells are the wafer like rectangular shape,usually between 2 and 5 micro m and a cell thickness of not more than 0.1 micro m. In their specific habitat these squares are challenged by the sub-lethal conditions of an extremely high MgCl2 concentration and high solar irradiance. This means that although the organism thrives in an aqueous environment it suffers severe desiccation stress. Special mechanisms are therefore required to maintain optimal water activity within the cell and at the cell surface. Concomitant with the extremely high salinity, the amount of dissolved oxygen decreases to near anoxia and some essential nutrients (e.g. phosphates) become unavailable due to complexation with Mg2+. It 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 is 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. (HAMAP: HALWD)