compatibility of osmotica in cyanobacteria

. The solutes accumulated by cyanobacteria in response to hyper‐osmotic stress include Na+, K+, sucrose, trehalose, glucosyl‐glycerol, glyeine betaine and glutamate betaine. The compatibility of several of these solutes with glutamine synthetase activity has been examined using cell‐free extracts from a range of freshwater, marine and halotolerant cyanobacteria. All of the solutes tested were compatible with (i.e. non‐inhibitory to) enzymic activity at physiological concentrations and the results demonstrate a rank order of compatibility which correlates with the concentrations at which the organic solutes occur in cyanobacteria, i.e. glycine betaine > polyol‐derivatives > disaccharides and with the upper salinity limit for growth. The protection against inhibition by NaCl (halo‐protection) afforded by these solutes to enzymic activity was also examined. Only glycine betaine was found to exert a significant halo‐protective effect and this may be explained by differences in the mechanism of compatible solute function between small charged molecules and sugars/polyols.