The uptake and transport of lead in some ectomycorrhizal fungi in culture

Various fungi interact with heavy metals (HM) differently regarding their binding ability and their tolerance. Besides the ability of the fungus to form an extensive extramatrical mycelium, its binding and transport properties are the most important factors in ameliorating the effects of HM toxicity on the mycorrhizal symbiosis. The tolerance of seven ectomycorrhizal fungi to elevated lead concentrations was tested using agar cultures and the results were compared to those from liquid culture. Critical toxic concentrations, where the growth of the mycelium was inhibited to 50%, were lower in liquid cultures (around 0.2 mM) in comparison to agar cultures (0.5-2 mM). The less tolerant fungi in agar culture were Amanita muscaria and Laccaria laccata, the most tolerant Lactarius piperatus, Pisolithus tinctorius and two strains of Suillus bovinus, DB83 from a lead polluted area and DB84 from a non-polluted area. Tolerance indices, based on the dry weight of mycelia from liquid culture, differ from the tolerance indices obtained from solid culture. L. laccata growth was limited to a lesser extent than other fungi and the growth of S. bovinus DB83 was strongly stimulated at a Pb concentration of 30 mu M, which could indicate lower susceptibility of this strain to low lead concentrations. The uptake, transport and release of Pb were then studied with Pb-210 tracer using Conway cells with the active tracer added to the inoculated medium in the centre compartment. The uptake of Pb by P. tinctorius (1.1% of the added activity) was much lower than the uptake by L. laccata and S. bovinus DB83 (6.2 and 5.4% of the added activity respectively). The same situation was reflected in the activity of the tracer transported in the mycelia. Further, the release of transported Pb from mycelia to the outer medium differs between L. laccata and S, bovinus DB83 mycelia, reaching 45 and 10% of the transported activity, respectively, indicating stronger binding of lead to S. bovinus DB83. The results of the tracer study are discussed with respect to various experiments with ectomycorrhizal seedlings and lead.