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 fro m Aspergillus niger, a fungus capable of citric, gluconic and oxalic acid p roduction, grown in the presence and absence of cobalt or zinc phosphate. T his 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 w ere more efficient than commercial organic acids. Gluconic acid did not bin d Co2+ or Zn2+ under the conditions used in this study. The presence of ins oluble metal phosphates in the growth medium was found to markedly influenc e the production of organic acids and, while large concentrations of glucon ic acid were produced in the presence of Co-3(PO4)(2), the culture filtrate was unable to bind Zn2+. The production of oxalic acid by A. niger when gr own in the presence of Zn-3(PO4)(2) led to the precipitation of insoluble z inc oxalate, a phenomenon with implications for metal tolerance and toxicit y. The significance of these findings in relation to the environmental mobi lity of metals and phosphate, and the role of fungi in such transformations are discussed.