Physicochemical Variation of Cyanobacterial Starch, the Insoluble α-Glucans in Cyanobacteria

Unicellular, diazotrophic species of cyanobacteria, Cyanobacterium sp. NBRC 102756, Cyanothece sp. ATCC 51142 and Cyanobacterium sp. CLg1, accumulate insoluble α-glucan inside the cells as the storage polysaccharide. The purified polysaccharides showed granular morphology, with a diameter of 0.2-0.7 µm. The three α-glucan preparations all showed A-type allomorph in X-ray diffraction analysis. Distinct thermal gelatinization temperatures were observed for these polysaccharides. The α-glucans from NBRC 102756 and ATCC 51142 strains consisted solely of branched α-glucans, or semi-amylopectin, while CLg1 contained semi-amylopectin as the primary component as well as linear or scarcely branched glucan (amylose). Separation of the debranched glucan chains by gel filtration chromatography explicitly showed the presence in the semi-amylopectin molecule of long chains corresponding to B2 chains, which connect clusters in amylopectin of plants. The relative proportions of short and long glucan chains in the branched polysaccharides differed depending on the species, and the variation was intimately correlated with the physical properties of the α-glucans. The results suggested that semi-amylopectin of the three cyanobacteria exhibit essentially similar organization with a tandem cluster structure. The polysaccharides of these strains are therefore referred to as 'cyanobacterial starch', distinct from glycogen.