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. Aqu eous solutions in the millimolar range of Fe3+ but not of Fe2+ precipi tated the exopolysaccharides from Bradyrhizobium (Chamaecytisus) strai n BGA-1 and, to a lesser extent, Bradyrhizobium japonicum USDA 110. Th e precipitation was pH dependent,with a maximum around pH 3. The preci pitate was redissolved by changing the pH and by Fe3+ reduction or che lation. Deacetylation of B. japonicum polysaccharide increased its pre cipitation by Fe3+. At pH near neutrality, the polysaccharide from Bra dyrhizobium (Chamaecytisus) strain BGA-1 stabilized Fe3+ solutions, de spite the insolubility of Fe(OH)(3). Aluminum precipitated Bradyrhizob ium (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. Pr ecipitation was inhibited by increases in the ionic strength over 10 m M. Bradyrhizobium (Chamaecytisus) polysaccharide was also precipitated by Th4+, Sn2+, Mn2+, and Co2+. The presence of Fe3+ increased the exo polysaccharide 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 se veral pH values. The results suggest that the precipitation is based o n the interaction between carboxylate groups from different polysaccha ride chains and the partially hydrolyzed aquoions of Fe3+, Al3+, Th4+, and Sn2