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For microorganisms to live under extreme physical conditions requires important adaptations of the cells. In many organisms the use of Na(+) instead of protons as coupling ion in energy transduction is associated with such adaptation. This review focuses on the enzymes that are responsible for the generation and utilization of Na(+) gradients in extremophilic microorganisms. Aspects that are dealt with include: bioenergetics and ion homeostasis in extremophilic Bacteria and Archaea; the molecular mechanism of Na(+) translocation; and (dis)advantages of Na(+) as coupling ion in energy transduction.

Bacterial growth at the extremes of temperature has remained a fascinating aspect in the study of membrane function and structure. The stability of the integral membrane proteins of thermophiles make them particularly amenable to study. Respiratory enzymes of thermophiles appear to be functionally similar to the mesophilic enzymes but differ in their thermostability and unusual high turnover rates. Energy coupling at extreme temperatures seems inefficient as suggested by the high maintenance coefficients and the high permeability of the cell membrane to protons. Nevertheless, membranes maintain their structure at these extremes through changes in fatty acid acyl chain composition. Archaebacteria synthesize novel membrane-spanning lipids with unique physical characteristics. Thermophiles have adapted to life at extreme temperatures by using sodium ions rather than protons as coupling ions in solute transport. Genetic and biochemical studies of these systems now reveal fundamental principles of such adaptations. The recent development of reconstitution techniques using membrane-spanning lipids allows a rigorous biochemical characterization of membrane proteins of extreme thermophiles in their natural environment.