Osmotic stress in Synechocystis sp. PCC 6803: low tolerance towards nonionic osmotic stress results from lacking activation of glucosylglycerol accumulation.

In order to compare the molecular principles of the acclimatization of bacterial cells to salt and nonionic osmotic stress, the moderately halotolerant cyanobacterium Synechocystis sp. PCC 6803 was challenged by salt (NaCl), and the osmolytes sorbitol and maltose. The physiological response towards each of the three compounds was found to be different. After salt addition, the cell volume remained unchanged, and the accumulation of the osmoprotective compound glucosylglycerol (GG) was observed after activation of the key enzyme GgpS at the biochemical and gene (ggpS) expression level. Sorbitol addition had only minor effects on the cell volume. In spite of the fact that the ggpS expression was increased, the GgpS enzyme was not activated, resulting in the absence of GG accumulation. In contrast the cells accumulated sorbitol, which served as a compatible solute and assured a certain osmotic resistance. In comparison to NaCl and sorbitol, the addition of maltose caused a strong decrease in cell volume indicating water efflux. However, no osmolyte accumulation was observed, resulting in an osmosensitive phenotype. Consequently, a successful response of Synechocystis cells to an osmotic challenge is indicative of the de novo synthesis of GG upon salt-dependent activation of the GgpS enzyme or the uptake of external solutes.