Abstract
The main, extracellular polysaccharide elaborated by three strains of Azotobacter vinelandii was a partly acetylated polyuronide, consisting of D-mannuronic and L-guluronic acid. The two monomers were distributed along the polymer chain in the typical, block-wise fashion previously demonstrated as characteristic for alginate from brown algae. The amount of polyuronide produced on a D-glucose-mineral salt medium was greatly increased by the addition of acetate. The ratio of the uronic acids in the polyuronide produced was strongly dependent upon the calcium concentration in the growth medium; low levels of calcium favoured the production of polyuronides rich in D-mannuronic acid, whereas the polysaccharides richest in guluronic acid seemed to be obtained at intermediate concentrations of calcium. Addition of calcium ions during the growth period demonstrated a change in the ratio of uronic acids in the direction of more L-guluronic acid. The same effect was also observed for cell-free supernatants, thus indicating the presence of an enzyme capable of epimerizing D-mannuronic to L-guluronic acid in the polymer chain.
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