Chemical and rheological properties of an extracellular polysaccharide produced by the cyanobacterium Anabaena sp ATCC 33047

The cyanobacterium (blue-green alga) Anabaena sp. ATCC 33047 produces an ex opolysaccharide (EPS) during the stationary growth phase in batch culture. Chemical analysis of EPS revealed a heteropolysaccharidic nature, with xylo se, glucose, galactose, and mannose the main neutral sugars found. The infr ared (IR) spectrum of EPS showed absorption bands of carboxylate groups. Th e average molecular mass of the polymer was 1.35 MDa, Aqueous dispersions a t EPS concentrations ranging from 0.2% to 0.6% (w/w) showed marked shear-th inning properties (power-law behavior). Linear dynamic viscoelastic propert ies showed that the elastic component was always higher than the viscous co mponent. Viscous and viscoelastic properties demonstrated the absence of co nformational changes within the concentration range studied. Stress-growth experiments revealed that 0.4% and 0.6% (w/w) EPS dispersions showed thixot ropic properties. A detailed com parison of the linear dynamic viscoelastic ity, transient flow, and decreasing shear rate flow curve properties was ma de for 0.4% (w/w) dispersions of xanthan gum (XG), Alkemir 110 (AG), and EP S. Viscoelastic spectra demonstrated that the EPS dispersion turned out to be more "fluidlike" than the AG and XG dispersions. The flow indexes indica ted that the EPS dispersion was less shear-sensitive than that of XG, showi ng essentially the same viscosity, that is, >50 s(-1). The fact that viscos ities of EPS and AG dispersions were not substantially different within the shear-rate range covered must be emphasized, in relation to EPS potential applications. The rheological behavior of EPS dispersions indicates the for mation of an intermediate structure between a random-coil polysaccharide an d a weak gel. (C) 2000 John Wiley & Sons, Inc.