Adsorption of IIb-metals by three common soil fungi—comparison and assessment of importance for metal distribution in natural soil systems

Interactions of IIb-elements, Zn, Cd and Hg, with three common soil fungi, Trichoderma harzianum, Penicillium spinulosum and Mortierella isabellina, have been studied. The accumulation of the metals by the fungi was studied as a function of pH at constant ionic strength and at concentration levels of the metals representative of natural systems. Two stages of fungal activity were considered in the experiments. The fungi generally exhibited high affinity for metal ions indicated by distribution coefficients (log Kd, in 1 kg−1) of about 3.5±1, 2.5±1 and 4±1 for Zn, Cd and Hg, respectively. The pH-dependence of the accumulation as well as the isotherms at constant pH were similar between the fungi, and the maximum capacities were at least 50 mmoles kg−1 mycelium (dw). Metal accumulation by starved mycelia was almost independent of pH, while non-starved mycelia in two cases accumulated more metals at low pH. Calculations of the distribution of metals in a model soil system of inorganic and organic constituents as well as fungal biomass indicated that the amounts of metal associated to the fungi are negligible at neutral pH. However, due to the ability of these fungi to accumulate metals independently of pH, the fraction of metals associated to fungal biomass at low pH may be significant, and, in some cases, predominant. This illustrates that the effects of fungi on metal distribution in soil should not be neglected, e.g. during a progressing acidification.