Binding of cobalt and zinc by organic acids and culture filtrates of Aspergillus niger grown in the absence or presence of insoluble cobalt or zinc phosphate

The ability of commercially-available citric, oxalic and gluconic acids to bind Co²⁺ and Zn²⁺ was investigated and compared with culture filtrates from Aspergillus niger, a fungus capable of citric, gluconic and oxalic acid production, grown in the presence and absence of cobalt or zinc phosphate. This work demonstrated that citric and oxalic acid and the A. niger culture filtrates can bind Co²⁺ and Zn²⁺ and in some cases, the culture filtrates were more efficient than commercial organic acids. Gluconic acid did not bind Co²⁺ or Zn²⁺ under the conditions used in this study. The presence of insoluble metal phosphates in the growth medium was found to markedly influence the production of organic acids and, while large concentrations of gluconic acid were produced in the presence of Co₃(PO₄)_2′ the culture filtrate was unable to bind Zn²⁺. The production of oxalic acid by A. niger when grown in the presence of Zn₃(PO₄)₂ led to the precipitation of insoluble zinc oxalate, a phenomenon with implications for metal tolerance and toxicity. The significance of these findings in relation to the environmental mobility of metals and phosphate, and the role of fungi in such transformations are discussed.