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 pyromorphitesolubilizing ability. B. caledonica 4, which demonstrated the highest ability to dissolve pyromorphite, accumulated the highest watersoluble 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.

Fungal Bioweathering of Mimetite and a General Geomycological Model for Lead Apatite Mineral Biotransformations

“…1; Table 1). The bioleaching of other lead apatites, such as pyromorphite and vanadinite, by A. niger and other organic acid-producing fungi with the simultaneous bioprecipitation of lead oxalate has been described in the literature (39,44,45). This suggests the existence of a common pattern in fungal bioleaching of the main lead apatites Fig.…”

“…Organic acids can provide a source of protons and contribute to acidification, which can also result from the proton-translocating plasma membrane ATPase, nutrient uptake in exchange for H ϩ , and respiration if carbonic acid is formed (39,67). More importantly, organic acids provide a source of ligands which can form complexes with metals and enhance the efficiency and rate of mineral dissolution (39,44,45). Furthermore, organic acids can be a source of electrons which can reduce metals to lower oxidation states.…”

“…This work examined the stability of natural mimetite [Pb 5 (AsO 4 ) 3 Cl] in the presence of the ubiquitous soil fungus Aspergillus niger, which possesses well-known geoactive properties (1,2,39,40,(44)(45)(46)(47)(48)(49)(50)(51)(52)(53), including citric, gluconic, and oxalic acid production (51,54,55). Experiments clearly revealed the complete bioweathering of a pure natural mimetite by the fungus and the simultaneous bioprecipitation of pure lead oxalate [Pb(C 2 O 4 )] as a new mycogenic biomineral ( Fig.…”

“…3), which has been supported here with the geochemical simulations. Fungi, as well as other microbes (e.g., bacteria, algae, and protists), can mediate chemical weathering of rocks and minerals through several mechanisms, including acidolysis, complexolysis, redoxolysis, and metal accumulation in the biomass (1,2,39,45,50,56,(67)(68)(69)(70). The excretion of organic acids (e.g., oxalic, citric, gluconic, and lactic acids) can be strongly influenced by growth conditions, such as the presence of toxic metal minerals, pH, buffering capacity, nutrient availability, and the C, P, S, and N sources (1, 2, 39, 45, 51, 52, 71-74).…”

“…The organism used for the bioleaching tests was Aspergillus niger van Tieghem (ATCC 201373), from the culture collection of the Geomicrobiology Group (University of Dundee). A. niger was chosen because it has been shown to have significant geoactive properties in mineral bioweathering of the lead apatites pyromorphite and vanadinite (1,2,39,40,(44)(45)(46)(47)(48)(49)(50)(51)(52)(53). Moreover, this fungus is a common soil saprotroph and can produce substantial amounts of organic acids, e.g., oxalic, citric, and gluconic acids (51,54,55 15.…”