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Precipitation of Metallic Cations by the Acidic Exopolysaccharides from Bradyrhizobium japonicum and Bradyrhizobium (Chamaecytisus) Strain BGA-1

The interaction between the acidic exopolysaccharides produced by two Bradyrhizobium strains and several metal cations has been studied. Aqueous solutions in the millimolar range of Fe3+ but not of Fe2+ precipitated the exopolysaccharides from Bradyrhizobium (Chamaecytisus) strain BGA-1 and, to a lesser extent, Bradyrhizobium japonicum USDA 110. The precipitation was pH dependent, with a maximum around pH 3. The precipitate was redissolved by changing the pH and by Fe3+ reduction or chelation. Deacetylation of B. japonicum polysaccharide increased its precipitation by Fe3+. At pH near neutrality, the polysaccharide from Bradyrhizobium (Chamaecytisus) strain BGA-1 stabilized Fe3+ solutions, despite the insolubility of Fe(OH)3. Aluminum precipitated Bradyrhizobium (Chamaecytisus) polysaccharide but not the polysaccharide produced by B. japonicum. The precipitation showed a maximum at about pH 4.8, and the precipitate was redissolved after Al3+ chelation with EDTA. Precipitation was inhibited by increases in the ionic strength over 10 mM. Bradyrhizobium (Chamaecytisus) polysaccharide was also precipitated by Th4+, Sn2+, Mn2+, and Co2+. The presence of Fe3+ increased the exopolysaccharide precipitation by aluminum. No precipitation, gelation, or increase in turbidity of polysaccharide solutions occurred when K+, Na+, Ca2+, Mg2+, Cu2+, Cd2+, Pb2+, Zn2+, Hg2+, or U6+ was added at several pH values. The results suggest that the precipitation is based on the interaction between carboxylate groups from different polysaccharide chains and the partially hydrolyzed aquoions of Fe3+, Al3+, Th4+, and Sn2+.