Metal Speciation

The total metal concentration in a system has been the most commonly employed indicator of metal inhibition of microorganisms in the environment. Clearly, other parameters are important, considering the large disparities between reported minimum inhibitory concentrations (MICs) among studies. A better indicator of metal toxicity in the environment is the concentration of the most bioavailable form, or species, of the metal.96 The most bioavailable species are those that will presumably associate most readily with enzymes and other active sites to initiate biological responses. Under most conditions, the most bioavailable form of a metal is considered to be the free, ionic, solution-phase species.97,98 Despite the free cation usually being the most abundant species, many other species are present, such as metal phosphates, carbonates, hydroxides, and sulfides, depending on the physiochemical conditions (e.g., pH, redox potential, and ionic strength) of the medium. Metals can also form hydroxo-complexes or complexes with various other organic ligands.99 It is also possible for most of the metal to speciate into insoluble precipitates, such as metal phosphates, which is likely to occur in microbiological studies employing phosphate buffers.85,89,100

Different metal species vary in their biological reactivity.98,99 For example, the free ionic form of a metal may act by substituting a cofactor for a vital enzyme. Hydroxylated metal ions have been suggested to bind to the cell surface and alter the net charge of the cell to reduce its viability.101 Because different species may have different effects on biological processes, some species may be more toxic than others. There is a paucity of information in the literature regarding the relative toxicity of different metal species.

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