Anion exclusion in microbial and soil polysaccharides

Abstract.

Microorganisms in soil, especially those associated with plant roots, are surrounded by envelopes of polysaccharides. These originate from both microbes and roots and are a characteristic feature of the rhizosphere. We have shown these materials to selectively restrict the diffusion of anions by the measurement of diffusion potentials. Using xanthan gum as a model microbial polysaccharide, increasing polymer concentration or polysaccharide layer thickness or the removal of acetyl and pyruvyl groups have been shown to increase the degree of anion exclusion. The anion-exclusive behaviour of xanthan has been validated independently by direct measurements of diffused ion concentrations. Data is presented showing this phenomenon to operate in KCl, 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, showed little ion-exclusive behaviour compared to polymers extracted from bulk soil and the rhizosphere and root surface of pea, which all showed high levels 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 these materials to restrict the diffusion and thus the availability of nutrient anions at the microorganism or root cell surface may be of significance to the survival and growth of polysaccharide-producing organisms in soil.