Haloarchaea are microrganisms member of the halophile community, in that they require high salt concentrations to grow. They are a distinct evolutionary branch of the Archaea, and are generally considered extremophiles, although not all members of this group can be considered as such.
Haloarchaea require salt concentrations in excess of 2 M (or about 10%) to grow, and optimal growth usually occurs at much higher concentrations, typically 20–25%. However, Haloarchaea can grow up to saturation (about 37% salts).
Haloarchaea are found mainly in hypersaline lakes and solar salterns. Their high densities in the water often lead to pink or red colourations of the water (the cells possessing high levels of carotenoid pigments, presumably for UV protection).
Haloarchaea are often considered pleomorphic, or able to take on a range of shapes—even within the one species. This makes identification by microscopic means difficult, and it is now more common to use gene sequencing techniques for identification instead.
One of the more unusually shaped Haloarchaea is the "Square Haloarchaeon of Walsby." Was classified in 2004 using a very low nutrition solution to allow growth along with a high salt concentration, square in shape and extremely thin (like a postage stamp). This shape is probably only permitted by the high osmolarity of the water, permitting cell shapes that would be difficult, if not impossible, under other conditions.
Haloarchaea have been proposed as a kind of life that could live on Mars; since the martian atmosphere has a pressure below the triple point of water, freshwater species would have no habitat on the martian surface.