Molecular biology of cyanobacterial salt acclimation

Abstract

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.

Thylakoid Terminal Oxidases Are Essential for the CyanobacteriumSynechocystissp. PCC 6803 to Survive Rapidly Changing Light Intensities

“…These data suggest that the presence of COX and at least one plastoquinolreduced terminal oxidase is of physiological importance in cyanobacteria. The majority of the strains (15 out of 19) that encode COX and more than one plastoquinolreduced terminal oxidase are either halotolerant, as indicated by the presence of one or more of the nhaP genes, encoding a cytoplasmic localized Na + /H + transporter (Hagemann, 2011), and/or nitrogen fixers, identified by the presence of nifH, encoding the iron-containing component of nitrogenase (Ben-Porath and Zehr, 1994). In each case, it is possible that additional terminal oxidases are required to provide sufficient ATP for Na + export under conditions where photosynthesis is reduced (Hagemann, 2011) and to remove oxygen that may inhibit nitrogenase activity (Berman-Frank et al, 2001;Staal et al, 2003).…”

“…Energization of both the thylakoid and cytoplasmic membrane electron transfer chains is important under conditions of salt stress in Synechocystis (Jeanjean et al, 1990(Jeanjean et al, , 1993Peschek et al, 1994) and Synechococcus sp. PCC 6311 (Fry et al, 1986), possibly to provide a proton gradient for Na + /H + transporters localized in the cytoplasmic membrane under conditions when photosynthetic activity is low (Hagemann, 2011). The role and membrane localization of PTOX, which in cyanobacteria is predominantly found in marine strains, have not been investigated but may provide an additional electron sink under iron-deprived conditions, when cyt b 6 f and PSI are limited (Bailey et al, 2008).…”

The SigB σ Factor Regulates Multiple Salt Acclimation Responses of the Cyanobacterium Synechocystis sp. PCC 6803

“…Research of salt acclimation is important because large-scale cultivation of cyanobacteria, for example for bioenergy production, would best occur in brackish water or seawater, as fresh water supplies of the Earth are limited. Salt acclimation processes in cyanobacteria are already fairly well known (for recent review, see Hagemann, 2011) but the sensing and transmission of salt signals are not yet well understood.…”

“…The third phase is characterized by an exchange of Na + to K + that allows the reactivation of photosynthesis, and the beginning of synthesis of compatible solutes (Reed et al, 1985). In Synechocystis, as in many other moderately halotolerant species, the main compatible solute is glucosylglycerol (Hagemann, 2011). Salt addition activates an inactive form of glucosylglycerol-phosphate synthase, and also enhances the transcription of the ggpS gene encoding glucosylglycerol-phosphate synthase (Marin et al, 2002;Stirnberg et al, 2007;Hagemann, 2011;Novak et al, 2011).…”

“…Synechocystis is a unicellular, nontoxic, nonnitrogenfixing freshwater cyanobacterium that is moderately halotolerant. Salt acclimation in cyanobacteria occurs in five phases (Hagemann, 2011). A salt shock causes rapid shrinking of the cells due to loss of water (Blumwald et al, 1983) via aquaporins (Shapiguzov et al, 2005).…”