Cation exchange capacity and lead sorption in ectomycorrhizal fungi

Two ectomycorrhizal fungi, Paxillus involutus 533 and Laccaria bicolor S238, differing greatly in their mycelial characteristics, were investigated with regard to their cation exchange capacity and Pb-binding capacity in vitro after growth with either NO3- or NH4+ as N source. The CECs of 800-1200 mu mol g-l dry weight for Paxillus involutus: 533 and 2000-3000 mu mol g-l dry weight for Laccaria bicolor S238, were high compared to plant roots. The fungal mycelium also had a high Pb sorption capacity. It was higher in Laccaria bicolor S238 than in Paxillus involutus 533 and higher after pregrowth in NO3- compared to NH4+. Both the higher CEC and the higher Pb sorption capacity of Laccaria bicolor S238 compared to Paxillus involutus 533 might have been the result of the hydrophilic nature of the of Laccaria bicolor S238 mycelium. It would have absorbed the solutions better than the hydrophobic mycelium of Paxillus involutus 533. X-ray microanalysis of the cell walls revealed that the Pb content of the cell walls was higher in Paxillus involutus 533 than in Laccaria bicolor S238. Nevertheless, electron dense deposits in the cell walls of Laccaria bicolor S238 contained large amounts of Pb, P and S. Thus, while Pb was evenly distributed in the cell walls of Paxillus involutus 533, Pb was accumulated in electron dense deposits in Laccaria bicolor S238. The results are discussed in view of their significance for the mycorrhizal symbiosis.