Solubilization of insoluble metal compounds by soil fungi: development of a screening method for solubilizing ability and metal tolerance

Two ubiquitous organic acid-producing soil fungi, Aspergillus niger and Penicillium simplicissimum, were screened for the ability to solubilize eight insoluble metal compounds: Al2O3, Al-x(PO4)(x), CaCO3, Ca-3(PO4)(2), Co-3(PO4)(2), Mn-x(PO4)(x), ZnO and Zn-3(PO4)(2). From these, three were selected as test metal compounds for the screening of species, strains and isolates for solubilization properties: ZnO, Zn-3(PO4)(2) and Co-3(PO4)(2). These compounds were used effectively to screen a further 56 strains of fungi, including soil isolates, for solubilizing activity (as revealed by the appearance of clear zones of solubilization around colonies) and metal tolerance (by comparison of colony growth rates). Ratios of the colony growth rate in the presence of a given metal compound (R(m)) to the control colony growth rate (R(c)), and the rate of extension of the clear zone of solubilization (R(s)) in relation to the growth rate of that colony (R(m)), were useful in characterizing these properties. Almost one-third of the natural isolates screened were able to solubilize at least one of ZnO, Zn-3(PO4)(2) and Co-3(PO4)(2), with five strains (out of 56) being capable of solubilizing all three. Cobalt phosphate was the most toxic of the three compounds; zinc phosphate was the least toxic, and was most resistant to solubilization. Zinc oxide was the most readily solubilized compound. In addition to screening for solubilizing abilities and metal tolerance, the method described can also be used for comparison of species and strains to obtain isolates with specific properties for possible biotechnological use and also to examine natural fungal populations. The incorporation of selected metal compounds in the test medium may also be used to assess responses to specific metals and their compounds as a preclude to physiological studies.