Influence of Fungal Exudates On Rhizosphere Mobility of Zn and Cd in Smelter-Contaminated Soil.

High concentrations of metals in the environment are of concern, in particular their bioavailability in anthropogenically-altered settings. In the rhizosphere, plants and fungi exude organic compounds that alter the activity of metals by changing the soil’s physiochemical parameters that affect metal uptake and/or sequestration. We compared the extractability of Cd and Zn from four smelter-contaminated soils using fungal exudates, a 0.01 M mixture of low-molecular weight organic acids (rhizosphere-based method - RBM), and standard leaching techniques (0.05 M ethylenediaminetetraacetic acid - EDTA, and 0.05 M diethylenetriaminepentaacetic acid - DTPA).  Exudates were produced from the ectomycorrhizal fungus Paxillus involutus (Pi) cultured in vitro on modified Melin-Norkins Media (MMN).  For Cd, the order the extraction was EDTA (11-19% of total) ~ DTPA (10-12%) > fungal exudates (4-11%) > RBM (2-4%) ~ MMN (1-2%). For Zn, the order of extraction was DTPA (12-16%) ~ EDTA (11-14%) > fungal exudates (5-7%) > RBM (3-5%) ~ MMN (2%). On average, Pi exudates extracted a higher percentage of the total Cd and Zn than did the RBM solutions. Leaching experiments using fungal exudates may be a more accurate means for determining the impact of mycorrhizal fungi on treatment of contaminated soil.