Desorption of zinc by extracellularly produced metabolites of Trichoderma harzianum, Trichoderma reesei and Coriolus versicolor.

AIMS: To determine the role of fungal metabolites in the desorption of metals. METHODS AND RESULTS: Desorption of Zn from charcoal by three different fungi was compared against metal desorption with reverse osmosis water, a 0.1% Tween 80 solution and a 0.1 mol l(-1) CaCl(2) solution. All three fungal filtrates desorbed three times more Zn than either 0.1% Tween 80 or 0.1 mol l(-1) CaCl(2). Metal chelator production in Trichoderma harzianum and Coriolus versicolor was constitutively expressed while chelator production in Trichoderma reesei was induced by Zn. The presence of Zn inhibited the production of metal chelators by C. versicolor. Only C. versicolor was found to produce oxalic acid (a strong metal chelator). All fungi caused a marked decrease in pH, although this was not enough to explain the increased desorption of the metals by the different fungal filtrates. CONCLUSIONS: Metal chelation via organic acids and proteins are the main mechanisms by which the fungal filtrates increase zinc desorption. SIGNIFICANCE AND IMPACT OF THE STUDY: The results of this study explain why plants inoculated with T. harzianum T22 take up more metal from soil, than noninoculated plants while metabolites produced by fungi could be used for metal leaching from contaminated soils.

Trichoderma harzianum Rifai 1295-22 mediates growth promotion of crack willow (Salix fragilis) saplings in both clean and metal-contaminated soil.

We investigated if the plant growth promoting fungus Trichoderma harzianum Rifai 1295-22 (also known as "T22") could be used to enhance the establishment and growth of crack willow (Salix fragilis) in a soil containing no organic or metal pollutants and in a metal-contaminated soil by comparing this fungus with noninoculated controls and an ectomycorrhizal formulation commercially used to enhance the establishment of tree saplings. Crack willow saplings were grown in a temperature-controlled growth room over a period of 5 weeks' in a garden center topsoil and over 12 weeks in a soil which had been used for disposal of building materials and sewage sludge containing elevated levels of heavy metals including cadmium (30 mg kg(-1)), lead (350 mg kg(-1)), manganese (210 mg kg(-1)), nickel (210 mg kg(-1)), and zinc (1,100 mg kg(-1)). After 5 weeks' growth in clean soil, saplings grown with T. harzianum T22 produced shoots and roots that were 40% longer than those of the controls and shoots that were 20% longer than those of saplings grown with ectomycorrhiza (ECM). T. harzianum T22 saplings produced more than double the dry biomass of controls and more than 50% extra biomass than the ECM-treated saplings. After 12 weeks' growth, saplings grown with T. harzianum T22 in the metal-contaminated soil produced 39% more dry weight biomass and were 16% taller than the noninoculated controls. This is the first report of tree growth stimulation by application of Trichoderma to roots, and is especially important as willow is a major source of wood fuel in the quest for renewable energy. These results also suggest willow trees inoculated with T. harzianum T22 could be used to increase the rate of revegetation and phytostabilization of metal-contaminated sites, a property of the fungus never previously demonstrated.

Effect of copper on the degradation of phenanthrene by soil micro-organisms.

AIMS: The effect of copper on the degradation by soil micro-organisms of phenanthrene, a polycyclic aromatic hydrocarbon, was investigated. METHODS AND RESULTS: Inert nylon filters were incubated in the soil for 28 days at 25 degrees C. Each filter was inoculated with a soil suspension, phenanthrene (400 ppm), copper (0, 70, 700 or 7000 ppm) and nitrogen/phosphorus sources. The filters were assessed for phenanthrene degradation, microbial respiration and colonization. Phenanthrene degradation proceeded even at toxic copper levels (700/7000 ppm), indicating the presence of phenanthrene-degrading, copper-resistant and/or -tolerant microbes. However, copper at these high levels reduced microbial activity (CO2 evolution). CONCLUSION: High levels of copper caused an incomplete mineralization of phenanthrene and possible accumulation of its metabolites. SIGNIFICANCE AND IMPACT OF THE STUDY: The presence of heavy metals in soils could seriously affect the bioremediation of PAH-polluted environments.

Methods for the Study of Verticillium chlamydosporium in the Rhizosphere.

Methods for screening isolates of the nematophagous fungus, V. chlamydosporium, for their ability to colonize the surface of plant roots are described. Significant differences in the extent of colonization were observed in sterile conditions and in soil; plant species and cultivars also differed in their ability to support a selected isolate of the fungus. Although fungal density could be estimated using a semi-selective medium, it was not possible to separate differences in vegetative growth from differences in sporulation. There was a weak positive correlation between estimates of fungal density on the roots of plants grown in sterile conditions and the extent of hyphal growth measured by direct observation.

The use of colony development for the characterization of bacterial communities in soil and on roots.

A simple agar plating method for the description of microbial communities is described. This method is based on the quantification of the numbers of bacterial colonies in 6-7 age-based classes as they appear on agar media over a period of 6-10 days. The method can be used to quantify microbial communities in different habitats (roots and soil) and can be related to the ecophysiology of the microbial communities present. Significant differences in distribution patterns were found in time and depth on the roots. In general, as roots matured, the microbial communities changed from one dominated by r-strategists to one that was more distributed towards K-strategists. The soil had the greatest percentage of organisms that could be characterized as K-strategists. The method was also used to compare microbial communities on wheat roots and in soil in both the field and in microcosms in the glasshouse. In general, the method enabled differentiation between r- and K-strategists in environmental samples, something that could not be done using an ecophysiological index (a modification of the Shannon diversity index) or total bacterial numbers alone.