Ectomycorrhizal fungi are able to tolerate toxic heavy-metal concentrations in host plants, although being also negatively affected by excessive metal. Isolates of Pisolithus tinctorius and Scleroderma bovinus were grown for 20 days at 28°C in a liquid medium with increasing levels of Zn, Cu and Cd sulfates in 125 mL conical flasks containing 50 mL of MMN medium, adjusted to pH 4.8. Fungi growth was inhibited by heavy metal increasing concentration, which was different for fungi and metals. The 50% effective concentrations inhibiting growth were 2.71 x 10-3 mol L-1 of Zn, 1.18 x 10-3 mol L-1 of Cu and 12.2 x 10-6 mol L-1 of Cd to the P. tinctorius; and of 2.15 of Zn, 0.12 x 10-3 mol L-1 of Cu and 7.2 x 10-6 mol L-1 of Cd to the S. bovinus. The inhibiting heavy-metal effect on growth was in the following sequence: Cd > Cu > Zn. The fungus S. bovinus demonstrated Zn tolerance similar to P. tinctorius, being less tolerant to Cu and Cd. The growth of P. tinctorius was favored by addition of a low amount of Cu. Production of extracellular pigments by the two isolates was enhanced by the addition of all the heavy metals studied. The fungus P. tinctorius produced more pigments per gram of mycelia, suggesting that there was a positive relationship between pigment production capacity and heavy-metal tolerance.
heavy metal; tolerance; toxicity; pigments