Diversity of adsorption affinity and catalytic activity of fungal phosphatases adsorbed on some tropical soils

Extracellular phosphatases from ectomycorrhizal fungi may contribute to plant nutrition, particularly in highly weathered tropical soils where available phosphorus is limited. However, the expression of their catalytic activity may be influenced by many factors including adsorption on organo-mineral surfaces of soils. We have investigated the pH-dependent adsorption and the resulting change in catalytic activity of trace amounts of extracellular fungal acid phosphatases in contact with various tropical soils. Different size fractions and the effect of chemical extraction of soil organo-mineral coatings were studied for acid phosphatases secreted by two Hebeloma cylindrosporum and one Suillus collinitus strains. The three enzymes differed in their average affinity for soil and the pH dependence. However the difference between enzymes was often greater than that between soils for a given enzyme. Only Suillus phosphatase had markedly different affinities for vertisol and ferralsol. In general, activity was largely conserved in the adsorbed state. Activity of Suillus phosphatases varied more than Hebeloma phosphatases between soils and soil physical and chemical fractionation. There was no relationship between the affinity of an enzyme for a surface and the resulting change in activity after adsorption. Suillus phosphatases showed more pH dependence of both affinity for soil and loss of activity after adsorption than the Hebeloma enzymes. These results highlight the variability of the interaction of fungal phosphatases with soil.