Effects of metals on the germination and growth of fungal isolates from New Caledonian ultramafic soils

Soils of New Caledonian mining areas have relatively high metal contents. Two ultramafic soils rich in extractable Mn, Fe, Ni and Co and two agricultural soils with much lower concentrations of metals were compared through different experiments. Microbial population numbers and their tolerance to Mn2+ and Ni2+ were estimated. The effect of five metals (Ni2+, Co2+, Fe2+, Mn2+ and Mg2+) upon spore germination and mycelial growth was investigated for fungi isolated from the two types of soils. Nickel appeared to be the most toxic metal, but the other metals also had inhibitory effects at concentrations similar to those which occur in ultramafic soils. Spore germination was more sensitive to metals than mycelial growth. The toxic effects of the five metals were not cumulative when mixed together into the medium; antagonism between ions, partly neutralizing their effects may account for this. Fungal isolates from ultramafic soils appeared to be more tolerant to metals than those from agricultural soils. Some ultramafic soil isolates appeared to actually require the presence of metals, with their growth being stimulated by relatively high concentrations of these elements. Two fungal isolates of genera which are often common in agricultural soils (i.e. Curvularia and Fusarium), and which are not present in New Caledonian ultramafic soils were also tested. Curvularia was more sensitive to the five metals than all the other isolates and Fusarium was particularly inhibited by Ni2+. However, a proportion of the spores of each isolate was able to germinate and to grow moderately well in the presence of relatively high metal concentrations. Their absence from ultramafic soils is thought to result from their elimination by competitors that are more tolerant to metals.