Extracellular polysaccharide synthesis by Nostoc strains as affected by N source and light intensity.

A study on the effect of two of the main factors affecting energy flux in N(2)-fixing cyanobacteria, i.e. light intensity and availability of combined nitrogen, on the synthesis of soluble exopolysaccharides was carried out with three strains of the genus Nostoc (PCC 7413, PCC 7936, and PCC 8113) presenting different capsular polysaccharidic morphologies and released polysaccharide productions. Strains acclimated to diazotrophic and non-diazotrophic conditions were cultured at high and low light intensities in aerated batch cultures. High light intensities enhanced total carbohydrate synthesis in all the strains but growth measured as pigment and protein concentration, total and soluble carbohydrate concentrations presented a strain-dependent response to nitrate availability. When adequately acclimated to the presence of nitrate all the capsulated strains tested became non-capsulated, with no extracellular polysaccharide being produced. Carbon availability can be on the basis of the observed correlation between the synthesis of capsular polysaccharides and diazotrophy. The slime-forming strain Nostoc PCC 7413 was the only one releasing polysaccharides into the surrounding medium under both, diazotrophic and non-diazotrophic conditions, with the highest values being obtained in the presence of nitrate. This strain presented the highest total carbohydrate (3.5 gl(-1)), soluble carbohydrate (1.8 gl(-1)) concentrations and viscosity values of all the tested strains. Different mechanisms of nitrogen-control of the synthesis of exocellular polysaccharides are reported for each strain, which results in the requirement of a species-specific optimisation of the cultivation conditions for the development of an efficient technology for the production of cyanobacterial exopolysaccharides.