the elevated potassium levels, haloarchaea have specialized proteins that have a highly negative surface charge to tolerate high potassium concentrations. Haloarchaea have adapted to use glycerol as a carbon and energy source in catabolic processes, which is often present in high salt environments due to Dunaliella species that produce glycerol in large quantities. Phototrophy Bacteriorhodopsin is used to absorb light, which provides energy to transport protons (H+) across the cellular membrane. The concentration gradient generated from this process can then be used to synthesize ATP. Many haloarchaea also possess related pigments, including halorhodopsin, which pumps chloride ions in the cell in response to photons, creating a voltage gradient and assisting in the production of energy from light. The process is unrelated to other forms of photosynthesis involving electron transport, however, and haloarchaea are incapable of fixing carbon from carbon dioxide. Early evolution of retinal proteins has been proposed as the purple Earth hypothesis. Cellular shapes Haloarchaea are often considered pleomorphic, or able to take on a range of shapes—even within a single 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". It 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. As exophiles 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. The presence of high salt concentrations in water lowers its freezing point, in theory allowing for halophiles to exist in saltwater on Mars. Recently, haloarchaea was sent 36 km (about 22 miles) up into Earths atmosphere, within a balloon. The two types that were sent up were able to survive the freezing temperatures and high radiation levels. This only further extends the theory that halophiles could exist on Mars. Medical use Certain types of haloarchaea have been found to produce carotenoids, which normally has to be synthesized using chemicals. With haloarchaea naturally producing it, there is now a