High and changing salt concentrations represent major abiotic factors limiting the growth of microorganisms. During their long evolution, cyanobacteria have adapted to aquatic habitats with various salt concentrations. High salt concentrations in the medium challenge the cell with reduced water availability and high contents of inorganic ions. The basic mechanism of salt acclimation involves the active extrusion of toxic inorganic ions and the accumulation of compatible solutes, including sucrose, trehalose, glucosylglycerol, and glycine betaine. The kinetics of these physiological processes has been exceptionally well studied in the model Synechocystis 6803, leading to the definition of five subsequent phases in reaching a new salt acclimation steady state. Recent '-omics' technologies using the advanced model Synechocystis 6803 have revealed a comprehensive picture of the dynamic process of salt acclimation involving the differential expression of hundreds of genes. However, the mechanisms involved in sensing specific salt stress signals are not well resolved. In the future, analysis of cyanobacterial salt acclimation will be directed toward defining the functions of the many unknown proteins upregulated in salt-stressed cells, identifying specific salt-sensing mechanisms, using salt-resistant strains of cyanobacteria for the production of bioenergy, and applying cyanobacterial stress genes to improve the salt tolerance of sensitive organisms.
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