Many organisms accumulate compatible solutes under environmental stress conditions. Cyanobacteria accumulate compatible solutes in response to increased external salinity, with tolerance increasing from Suc (sucrose) or trehalose to 2-O-(α-D-glucopyranosyl)-glycerol and glycinebetaine accumulating species. It is not clear how these different solutes influence salt tolerance. One possible explanation may be a differential ability of these solutes to stabilize membranes under stress conditions. We therefore performed drying experiments with liposomes in the presence of compatible solutes. Suc, trehalose and sorbitol protected liposomes from leakage of a soluble marker and from membrane fusion during drying and rehydration. 2-O-(α-D-glucopyranosyl)-glycerol was less effective and glycinebetaine showed hardly any effect. In combination with Suc, the latter two solutes showed improved protection. Lipid-phase transitions are known to contribute to solute leakage from liposomes. We determined phase transitions in dry membranes in the absence or presence of the solutes, using Fourier-transform infrared spectroscopy. The ability of the solutes to decrease the phase transition temperature corresponded closely to their ability to protect the liposomes against solute leakage. All solutes interacted with the phosphate in the lipid headgroups. The magnitude of the shift in the asymmetric P=O stretching vibration correlated closely with the lipid-phase transition temperature. This indicates that the degree of membrane protection afforded by the solutes is mainly determined by their ability to interact with the membrane lipids. However, this is not a determinant of cellular protection against salt stress, as the solutes show a reverse order when ranked with regard to protection against these stresses.

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