Lead mineral transformation by fungi

Pyromorphite (Pb5(PO4)3Cl), the most stable lead mineral under a wide range of geochemical conditions [1], can form in urban and industrially contaminated soils [2–5]. It has been suggested that the low solubility of this mineral could reduce the bioavailability of lead, and several studies have advocated pyromorphite formation as a remediation technique for lead-contaminated land [3,5,6], if necessary using addition of phosphate [6]. Many microorganisms can, however, make insoluble soil phosphate bioavailable [7–10], and the solubilisation of insoluble metal phosphates by free-living and symbiotic fungi has been reported [11–15]. If pyromorphite can be solubilised by microbial phosphate-solubilising mechanisms, the question arises of what would happen to the released lead. We have now clearly demonstrated that pyromorphite can be solubilised by organic-acid-producing fungi, for example Aspergillus niger, and that plants grown with pyromorphite as sole phosphorus source take up both phosphorus and lead. We have also discovered the production of lead oxalate dihydrate by A. niger during pyromorphite transformation, which is the first recorded biogenic formation of this mineral. These mechanisms of lead solubilisation, or its immobilisation as a novel lead oxalate, have significant implications for metal mobility and transfer to other environmental compartments and organisms. The importance of considering microbial processes when developing remediation techniques for toxic metals in soils is therefore emphasised.