Nutritional influences on the solubilization of metal phosphate by ericoid mycorrhizal fungi

Abstract

Four ericoid mycobionts (two isolates of Hymenoscyphus ericae, and two dark, sterile ericoid mycobionts isolated from metal-contaminated mine sites) were grown on solid agar plates supplemented with zinc phosphate (0.25%) containing different forms of nitrogen (nitrate, ammonium or alanine) and different concentrations of carbon (glucose) and phosphorus (K₂HPO₄). The influence of nutrient variation on solubilizing ability of the fungi was assessed by measuring the zones of solubilization appearing beneath the growing colonies. All four mycobionts were capable of zinc phosphate solubilization in the presence of all three nitrogen sources and in media containing no nitrogen. No solubilization was observed at 0 mM glucose-C but was observed with increasing glucose concentration from 300 to 600 mM C. Increasing phosphorus concentration (0–5 mM P) had no effect on the solubilizing ability of the isolates. All but one of the mycobionts were capable of solubilizing calcium phosphate (CaHPO₄), while no solubilization was observed in media containing aluminium phosphate (AlPO₄), iron phosphate (FePO₄.4H₂O) or copper phosphate (Cu₃O₈P₂.2H₂O) under conditions which were found to be optimal for zinc phosphate solubilization. Under conditions of glucose at 300 mM C and alanine as the N source in the zinc phosphate-amended agar medium, one of the mycobionts produced new crystals, which were morphologically distinct from the original zinc phosphate crystals. It is concluded that medium composition influences the metal-phosphate solubilizing ability of ericoid mycobionts. The results are discussed in relation to the possible mechanisms involved in solubilization and the potential benefits of metal-phosphate solubilization to ericoid mycobionts and their host plants.