Energetically Synthesizing Solutes

Energetically Synthesizing Solutes

All halophiles must maintain their cytoplasm isoosmotic with their surrounding medium. Salt tolerance requires that compatible solutes accumulate in the cytosol and organelles where these function in osmotic adjustment and osmoprotection (Rhodes and Hanson, 1993). Some compatible osmolytes are essential elemental ions, such as K+, but the majority are organic solutesBiological membranes are permeable to water, and active energy dependent inward transport of water to compensate for water lost by osmotic processes is energetically not feasible. Moreover, cells that keep a turgor need even to maintain their intracellular osmotic pressure higher than that of their environment.

There are two fundamentally different strategies used by halophilic microorganisms to balance their cytoplasm osmotically with their medium (Galinski and Trüper, 1994; Zahran, 1997). (1) Accumulation of molar concentrations of potassium and chloride. This strategy requires extensive adaptation of the intracellular enzymatic machinery to the presence of salt, as the proteins should maintain their proper conformation and activity at near–saturating salt concentrations. The proteome of such organisms is highly acidic, and most proteins denature when suspended in low salt. It is called the'high–salt–in strategy'. They maintain osmotically equivalent internal concentrations by accumulating high concentrations of potassium chloride. Potassium ions enter the cell passively via a uniporter. Sodium ions are

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