Salinibacter: an extremely halophilic bacterium with archaeal properties.

The existence of large number of a member of the Bacteroidetes in NaCl-saturated brines in saltern crystallizer ponds was first documented in 1999 based on fluorescence in situ hybridization studies. Isolation of the organism and its description as Salinibacter ruber followed soon. It is a rod-shaped, red-orange pigmented, extreme halophile that grows optimally at 20-30% salt. The genus is distributed worldwide in hypersaline environments. Today, the genus Salinibacter includes three species, and a somewhat less halophilic relative, Salisaeta longa, has also been documented. Although belonging to the Bacteria, Salinibacter shares many features with the Archaea of the family Halobacteriaceae that live in the same habitat. Both groups use KCl for osmotic adjustment of their cytoplasm, both mainly possess salt-requiring enzymes with a large excess of acidic amino acids, and both contain different retinal pigments: light-driven proton pumps, chloride pumps, and light sensors. Salinibacter produces an unusual carotenoid, salinixanthin that forms a light antenna and transfers energy to the retinal group of xanthorhodopsin, a light-driven proton pump. Other unusual features of Salinibacter and Salisaeta include the presence of novel sulfonolipids (halocapnine derivatives). Salinibacter has become an excellent model for metagenomic, biogeographic, ecological, and evolutionary studies.