Methanogens are not the only unique euryarchaeotes. A related group illustrates the limits of tolerance to another environmental condition—salinity, or saltiness. The Haloarchaea are halophilic (salt-loving) photoheterotrophs that use the protein bacteriorhodopsin to absorb energy from sunlight (see Fig. 26.6). They live in waters that are salty enough to precipitate table salt (NaCl), maintaining osmotic balance by accumulating ions (especially K+) and organic solutes in their cytoplasm. Just as extreme acidophiles require acid conditions, these extreme halophiles require high salt conditions and cannot live in dilute water.