Assessment of the potential of Nostoc strains from the Pasteur Culture Collection for the production of polysaccharides of applied interest

The exocellular polysaccharides released during the photoautotrophic growth by 25 Nostoc strains belonging to the Pasteur Culture Collection (PCC) were investigated with regard to their chemical and rheological properties in order to assess their potential for the production of polysaccharides of industrial interest. All the released polysaccharides (RPSs) were complex anionic heteropolymers, composed of a number of monosaccharides ranging from six to nine, always including glucose and fucose, and most frequently galactose. Ribose, rarely reported to be present in cyanobacterial RPSs, was found in seven polymers. The RPSs were shown to possess an anionic charge due to the presence of uronic acids and in most cases also of pyruvil and sulphate groups. Moreover, a large number of polymers showed the presence of significant amounts of constituents such as acetyl groups, peptidic moieties and deoxysugars, that may contribute to the hydrophobicity of the macromolecules. The viscosity dependence on the shear rate of aqueous solutions of the RPSs was, in three cases, comparable with that of xanthan gum, and one RPS showed a very satisfactory stability of the viscosity over a wide range of pH, temperature and salinity values. The most promising PCC Nostoc strains for the production of RPSs for specific applications were thus singled out, based on the characteristics of their polymers.