Differential tolerance to Cd and Zn of arbuscular mycorrhizal (AM) fungal spores isolated from heavy metal-polluted and unpolluted soils

Spores of arbuscular mycorrhizal fungi were isolated from two soils of field trials at INRA-Bordeaux (France) polluted by long-term application of a zinc-polluted sewage sludge (S2 soil) or treated with cadmium nitrate (Cd40 soil) and from corresponding unpolluted soils (F and Cd0 soils). These AM fungi were tested for their tolerance to Cd and Zn added as salt solutions with increasing concentrations (0 to 10 mg L⁻¹) in a simple spore germination device. According to preliminary identification the predominant species in S2 and F cultures was Glomus mosseae, whereas Cd40 and Cd0 cultures contained a mixture of at least G. mosseae and G. etunicatum. Germination of Cd40 spores was more tolerant to Cd and Zn than for Cd0 spores, with EC₅₀ values of 73 and 158 µmol L⁻¹ added Cd and Zn corresponding to approximately 10 and 13 µmol L⁻¹ remaining in solution in the device. The S2 spores from the sludge contaminated soil were more tolerant to Zn (EC₅₀ = 87 µmol L⁻¹ but not to Cd (EC₅₀ = 7.5 µmol L⁻¹ than the spores from the farmyard manure-treated F soil (EC₅₀ = 38 and 8.8 µmol L⁻¹ respectively). Thus, S2 culture exhibited a specific tolerance to Zn, which was lower than the unspecific tolerance of Cd40 culture to both Cd and Zn, despite the much higher Zn availability in S2 soil. These results indicate that AM fungi from different soils may differ in their metal susceptibility and that both metal specific and unspecific tolerance mechanisms may be selected in metal polluted soils.