Zinc Phosphate and Pyromorphite Solubilization by Soil Plant-Symbiotic Fungi

Abstract

Acidolysis, complexolysis and metal accumulation were involved in solubilization of zinc phosphate and pyromorphite by a selection of soil fungi representing ericoid and ectomycorrhizal plant symbionts and an endophytic/entomopathogenic fungus, Beauveria caledonica. Zinc phosphate was much more readily solubilized than pyromorphite. According to the relationship between metal mobilization and pH, acidolysis (protonation) was found to be the major mechanism of both zinc phosphate and pyromorphite dissolution for most of the fungi examined. In general, the more metal tolerant fungal strains yielded more biomass, acidified the medium more and dissolved more of the metal mineral than less tolerant strains. However, B. caledonica 4 excreted a substantial amount of oxalic acid (∼ 0.8 mM) in the presence of pyromorphite that coincided with a dramatic increase in lead mobilization providing a clear example of complexolysis. Organic acid excretion by fungi was inter- and intraspecific and was strongly influenced by the presence of the toxic metal minerals. When grown on zinc phosphate or pyromorphite, Hymenoscyphus ericae DGC3(UZ) accumulated the lowest metal concentration, but Thelephora terrestris accumulated the highest metal concentration in the biomass. The ability to accumulate water-soluble lead species, representing mainly cytosolic and vacuolar pools, seemed to be connected with pyromorphite-solubilizing ability. B. caledonica 4, which demonstrated the highest ability to dissolve pyromorphite, accumulated the highest water-soluble fraction and total lead concentration in the mycelium. Generally, isolates with a higher zinc-tolerance accumulated significantly less total zinc from zinc phosphate (including the sum of water-soluble and NaCl-extractable zinc) than non-tolerant strains.

Keywords:

• entomopathogenic fungi
• metal accumulation
• mycorrhizal fungi
• organic acids
• oxalic acid
• pyromorphite
• solubilization
• zinc phosphate

This research was funded by the BBSRC/BIRE program (94/BRE13640). We thank Professor Andy Meharg and Dr. David Genney (University of Aberdeen, Scotland), Dr. Elena Martino (University of Torino, Italy), and Dr. Jan Colpaert and Miss Kristin Adriaensen (Limburgs University Centre, Belgium) for the provision of fungal strains.

Notes

*Maximum values of organic acid concentrations in medium detected after either 10, 15, or 20 days of fungal growth are shown in round brackets. Values are means derived from at least three replicate determinations ±SEM. “—” no organic acids detected.