The major organic osmoregulatory solutes of 36 cyanobacteria from a wide range of environmental sources have been examined using 3C nuclear magnetic resonance spectre scopy. These strains were also examined for their salt-tolerance, and could be arranged in three salt-tolerance groups, designated freshwater, marine and hypersaline. The most salt-tolerant cyanobacteria in the hypersaline group are properly classified as moderately halophilic. Cyanobacteria from all habitats and taxonomic groups accumulated organic osmoregulatory solutes, and the chemical class of the solute correlated with the salt-tolerance and habitat of the strain. Freshwater strains accumulated simple saccharides, predominantly sucrose and trehalose ; marine strains accumulated the heteroside 0-a-D-glucopyranosyl-( 1 + 2)-glycerol, and hypersaline strains accumulated sucrose and/or trehalose together with glycine betaine or the novel solute L-glutamate betaine (N-trimethyl-L-glutamate) or they accumulated glycine betaine alone. The results suggest that the presence of certain major organic osmoregulatory solutes may be useful in the numerical taxonomy of cyanobacteria, and in the identification of some ionic characteristics of the environment of origin of each isolate.

The process of osmoregulation in a unicellular blue-green alga, Synechococcus sp., has been studied by natural-abundance carbon-13 nuclear magnetic resonance spectroscopy of intact cells and cell extracts. 2-O-alpha-D-Glucopyranosylglycerol was identified as the major organic osmoregulatory solute. This demonstrates the presence of a major osmoregulatory solute in a blue-green alga and is also an example of an osmoregulatory role for glucosylglycerol.

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Three groups of cyanobacteria are recognized on the basis of their organic osmotica and upper salinity limit for growth. In general, the least halotolerant forms accumulate disaccharides, while cyanobacteria of intermediate halotolerance synthesize the heteroside glucosylglycerol and the most halotolerant isolates accumulate betaines in response to salt stress. However, certain strains also accumulate additional organic solutes, depending upon the growth temperature, the ambient salinity and the duration of salt stress.

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