Nostoc (Cyanophyceae) goes nude: extracellular polysaccharides serve as a sink for reducing power under unbalanced C/N metabolism

ABSTRACT: Many species of the filamentous N 2 -fixing hetero- cyst-forming Cyanobacteria of the genus Nostoc produce large amounts of extracellular polymeric substances (EPS), but hitherto no general model has been proposed of the factors that control their synthesis. Previously, we demonstrated a strong correlation between the presence of a glycocalyx (or EPS capsule) and diazotrophic growth in the genus Nostoc. When grown with nitrate, nude morpho- types lacking a glycocalyx were obtained for all the capsulated strains tested. CO 2 availability was pro- posed as a key factor that controls the synthesis of the capsule. To test this hypothesis, Nostoc PCC 7936 was cultured diazotrophically (N 2 ) or with nitrate with different CO 2 supplies. By tuning the pH and the supply of CO 2 , capsulated or nude mor-photypes were obtained irrespective of the source of nitrogen. Exocellular polysaccharides were synthesized only when the fixed carbon exceeded the amount of nitrogen available. The glycocalyx is not needed for the optimal function- ing of nitrogenase because diazotrophic cultures grew equally well, irrespective of whether they were capsulated or nude. Capsulated cultures possessed protein to carbohydrate ratios that ranged between 1 and 1.5, whereas in nude cultures the ratio ranged between 2 and 2.5. Low protein to carbohydrate ratios were indicative for either nitrogen-limited or carbon-oversaturated cultures. The results demo- nstrate that in Nostoc EPS serve as a sink for the excess fixed carbon under unbalanced C/N metabolism.