Metal speciation and microbial growth - the hard ( and soft ) facts

Micro-organisms require certain metallic elements for growth and function (Hughes & Poole, 1989). These include, for example, the bulk elements potassium and magnesium, and trace elements such as manganese, iron, copper, zinc and molybdenum. Essential metals are shown in Fig. 1, the Periodic Table. The need for these essential metals is, or should be, acknowledged in the composition of growth media. However, higher concentrations of these metals, and the heavy metals such as silver, cadmium, mercury, tin and lead (Fig. l), may be toxic. In any study of the biological effects of either essential or toxic metals it is imperative that full account be taken of the speciation or chemical form(s) of the metal ion in the growth medium, in the organisms’ environment, and in experimental media such as buffers; the bioavailability, and hence reactivity, of the metal is critically dependent upon its speciation (Bernhard et al., 1986; Morrison et al., 1989). Unfortunately it appears that little attention is sometimes given to these matters, and that this has led to some invalid and misleading conclusions in the microbiological literature. In this review it is intended to survey the factors that control the speciation of metal ions in media, to summarize methods for establishing the speciation of metal ions in solution and to suggest some simple guidelines for estimating the bioavailability of metal ions in media. Some commonly-used media will be assessed in the light of these comments and attention drawn to errors that have resulted from a failure to recognize the importance of metal speciation. A recent and topical example will serve to illustrate the importance of speciation effects and the ease with which they are overlooked. It has been suggested in both the scientific and popular press that levels of atmospheric carbon dioxide could be reduced by ‘sowing’ the ocean with tons of iron filings to boost growth of marine algae (Davies, 1990; Martin et al., 1990). Little mention has been made of the problems of maintaining the iron in a form which is bioavailable. Significantly, Hutner (1972) cites much earlier work, which demonstrated that the 725