Anion exclusion in microbial and soil polysaccharides

Microorganisms in soil, especially those associated with plant roots, are s urrounded by envelopes of polysaccharides. These originate from both microb es and roots and are a characteristic feature of the rhizosphere. We have s hown these materials to selectively restrict the diffusion of anions by the measurement of diffusion potentials. Using xanthan gum as a model microbia l polysaccharide, increasing polymer concentration or polysaccharide layer thickness or the removal of acetyl and pyruvyl groups have been shown to in crease the degree of anion exclusion. The anion-exclusive behaviour of xant han has been validated independently by direct measurements of diffused ion concentrations. Data is presented showing this phenomenon to operate in KC l, KNO3 and KH2PO4 Systems. In all cases, the anion exclusion appears to be partial, restricting the diffusion of anions in the presence of a layer of 3% xanthan by 50-80%. By measurement of diffusion potentials, scleroglucan and polysaccharides produced by two soil bacteria, Azotobacter chroococcum and Enterobacter cloacae, were also shown to behave anion-exclusively. Ca- polygalacturonate, which has been used as a model root surface polymer, sho wed little ion-exclusive behaviour compared to polymers extracted from bulk soil and the rhizosphere and root surface of pea, which all showed high le vels of anion exclusion. By chemical characterisation of all polymers under study, it was possible to link the presence of uronic acids within the gel to anion-exclusive behaviour. The results suggest that anion exclusion is a common property of microbial and soil polysaccharides. The ability of the se materials to restrict the diffusion and thus the availability of nutrien t anions at the microorganism or root cell surface may be of significance t o the survival and growth of polysaccharide-producing organisms in soil.