The availability of water is the most important prerequisite for life of any living cell, and exposure of cells to hypersaline conditions always threatens the cells with a drastic loss of water. To re-establish the essential turgor pressure, cells increase the water activity of their cytoplasm by accumulation of compatible solutes, either by synthesis or by uptake. The ability to respond to increasing osmolality is well conserved in all three lines of descent and, here, we compare the osmoadaptive strategies of Bacteria and Archaea. The temporal sequence of events after an osmotic upshock will be discussed, with a focus on the most rapid response, notably the mechanisms of transport activation at the protein level, and different signals for osmolality will be compared. The spectrum of compatible solutes used by different organisms is rather diverse and a comparison of 'bacterial' and 'archaeal' compatible solutes will be given.

The salt adaptation of the methanogenic archaeon Methanosarcina mazei Gö1 was studied at the physiological and molecular levels. The freshwater organism M. mazei Gö1 was able to adapt to salt concentrations up to 1 M, and the addition of the compatible solute glycine betaine to the growth medium facilitated adaptation to higher salt concentrations. Transport studies with cell suspensions revealed a salt-induced glycine betaine uptake activity in M. mazei Gö1, and inhibitor studies argue for a primary transport device. Analysis of the genome of M. mazei Gö1 identified a homolog of known primary glycine betaine transporters. This gene cluster was designated Ota (osmoprotectant transporter A). Its sequence and gene organization are very similar to those of the glycine betaine transporter OpuA of Bacillus subtilis. Northern blot analysis of otaC revealed a salt-dependent transcription of this gene. Ota is the first identified salt-induced transporter for compatible solutes in Archaea.

Growth of Halobacillus halophilus is strictly chloridedependent but the physiological basis for the chloride dependence remains to be elucidated. To address the function of Cl^in H. halophilus, a physiological study was performed. It was found that uptake of the compatible solute glycine betaine under isoosmotic conditions was stimulated by increasing salt concentrations. Uptake of glycine betaine required both, Na + and Cl^. Cl^could be substituted by nitrate and bromide, but not by sulfate. Glycine betaine transport was optimal at around 0.7 M Cl^. Cells responded to an osmotic upshock by accumulating glycine betaine, but only in the presence of chloride. These studies revealed the first chloride-dependent glycine betaine transporter in a prokaryote. ß

Chloride is an abundant anion on earth but studies analyzing a possible function of chloride in prokaryotes are scarce. To address the question, we have tested 44 different Gram-negative and Gram-positive bacteria for a chloride dependence or chloride stimulation of growth. None required chloride for growth at their optimal growth (salt) conditions. However, in hyperosmotic media containing high concentrations of Na þ , 11 bacteria (Aeromonas hydrophila, Bacillus megaterium, Bacillus subtilis, Corynebacterium glutamicum, Escherichia coli, Paracoccus denitrificans, Proteus mirabilis, Proteus vulgaris, Staphylococcus aureus, Thermus thermophilus, and Vibrio fischeri) had a strict chloride dependence for growth or were significantly stimulated by chloride. These data show that chloride is essential for growth at high salt (Na þ ) concentrations in various species of the domain Bacteria.

A quantitative analysis of the Cl− dependence of growth of Halobacillus halophilus was performed. Optimal growth rates were obtained at Cl− concentrations of between 0.5 and 2.0 M, and the final yield was also strictly dependent on the Cl− concentration. Br− but not I−, SO4 2−, NO2 −, SO2 −, OCN−, SCN−, BO2 −, or BrO3 − could substitute for Cl−. To analyze the function of chloride, chloride concentration was determined. At low external Cl− (Cle −) concentrations, the growth rate was low and Cl− was excluded from the cytoplasm; increasing the Cle −concentration led to an increase in the growth rate and an energy-dependent uptake of Cl−, thus decreasing the Cle −/internal Cli − gradient from ≥10 at 0.1 M Cle − to a nearly constant value of 2 at Cle − concentrations which allowed optimal growth. Two membrane proteins with apparent molecular masses of 31 and 16 kDa which were identified to be specific for Cl−-grown cultures are possible candidates for a chloride uptake system.