Unexpected diversity of heterotrophic prokaryotes living at the highest salt concentrations

Halophilic Archaea (family Halobacteriaceae) were until recently the only heterotrophic microorganisms known to grow optimally at near-saturating salt concentrations. It now appears that Salinibacter ruber, an extremely halophilic bacterium related to the Flavobacterium-Cytophaga group, may occur in large numbers in saltern crystallizer ponds. The organism is red, and harbors a novel carotenoid pigment. There is a surprising similarity between the physiology of Salinibacter and the Halobacteriaceae. In both cases the cells contain molar concentrations of KCl, and organic compatible solutes contribute little to osmotic adaptation. The proteins of Salinibacter contain a large excess of acidic amino acids and are depleted in hydrophobic amino acids. Its cytoplasmic enzymes function at high salt. The discovery of Salinibacter and the elucidation of its properties show that similar modes of adaptation to life at high salt concentrations and low water activities may have evolved independently in phylogenetically distant groups.