The extracellular sheath was isolated from cultures of a unicellular diazotrophic cyanobacterium, Glwothece ATCC 27 152. Chemical analysis of lyophilized cell-free preparations indicated that the sheath was composed of an acidic heteropolysaccharide containing the neutral sugars rhamnose, 2-O-methylxylose, xylose, mannose, galactose and glucose, and the uronic acids mannuronic, glucuronic and galacturonic acid. Protein, pyruvic acid and 0-acetyl and sulphate groups were also detected. Quantitative differences in the carbohydrate and protein content of the sheath were observed between material collected from cultures grown with and without a source of combined nitrogen. The extent to which the external layers bound cadmium ions was affected by these differences.

Cells of the unicellular cyanobacterium Gloeothece sp. PCC 6909 are surrounded by an inner (enclosing 1-2 cells) and an outer (enclosing cell groups) sheath. Using conventional Epon-embedding in combination with ruthenium-red staining, the inner and outer sheaths appeared similar and displayed multiple bands of electron-dense subunits. However, embedding in Nanoplast resin to avoid shrinkage led to the detection of two distinct zones (inner and outer zone) each with several distinct layers. The zone delimited by the electron-dense thick inner sheath layer, and the zone enclosed by the thin electron-dense outer sheath layer, are composed of a homogeneous material of little electron-contrast. Whereas the outer zone appears to be of even contrast, the inner zone is characterized by a distinct electron-transparent layer. Element distribution analysis revealed that the electron-transparent layer contained relatively large amounts of sulfur, carbon, and oxygen but only little nitrogen. Inner and outer sheath fractions were isolated by differential mechanical cell breakage and centrifugation. The outer sheath fraction was less hydrated than the inner one. The two fractions differed little in their contents of uronic acids, carbohydrate and protein, although the outer sheath fraction contained less sulfate. A soluble polysaccharide with a chemical composition similar to that of inner and outer sheath fractions was also obtained from the culture supernatant.

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