A range of elements (see box opposite), some of them toxic to animals and plants, can be utilised for providing energy to the cell (Ehrlich, 1996). Some are toxic at high concentrations whilst being considered essential at lower concentrations (Nies, 1999). Defining which elements are toxic is not straight forward as it depends on their availability to the organism by being bound to soil minerals especially humic acids, soil pH and which form (oxidation state) is present. Soil function may be impaired by heavy metals either through directly affecting growth or enzyme activity (Kandler et al., 1996).

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