Morphological and biochemical characterization of the exocellular investments of polysaccharide-producing Nostoc strains from the Pasteur Culture Collection

A total of 40 Nostoc strains, belonging to the Pasteur Culture Collection a nd originally isolated from different habitats, were photoautotrophically g rown in liquid cultures and tested for the presence of exocellular polysacc haridic investments surrounding the trichomes. However, 25 of them showed a significant presence of these structures, coupled with the release of poly saccharidic material (RPS) into the medium. A rather large number of differ ent morphological forms was observed in the cultures during growth, but at the time of harvesting the predominant morphological form was, in most case s, the vegetative trichome. With regard to the exocellular mucilaginous inv estments, three main types of morphologies were observed: (i) capsules surr ounded by an external pellicle, (ii) capsules with sharp outlines but witho ut an external pellicle, (iii) slimy investments that either loosely surrou nded the trichomes without following their shape or were organized in large globular lumps. Among the twenty-five strains that released polysaccharide s into the culture medium, three showed mean daily productivities ranging f rom 30 to 50 mg (RPS) l(-1) d(-1), values comparable with those of the most productive cyanobacterial strains so far described. The morphological char acteristics of the polysaccharidic investments produced by the Nostoc strai ns seem not to be related to their original habitats. Furthermore, the diff erences in RPS productivities observed among the strains seem not to be rel ated to the shape of the mucilaginous exocellular investments. Chemical ana lysis of purified samples of the polysaccharides demonstrated that all the polymers possess an acidic nature, due to the presence of uronic acids, and that they are characterized by the presence of a peptidic moiety and of am ino sugars.