Metals and microorganisms: a problem of definition.

Metals are directly a n d / o r indirectly involved in all aspects of microbial growth, metabolism and differentiation. Many metals are essential, e.g. K, Na, Mg, Ca, Mn, Fe, Co, Ni, Cu, Zn, Mo, whereas others have no known essential biological function(s), e.g. AI, Ag, Cd, Sn, Au, Sr, Hg, T1, Pb. All these elements can interact with microbial cells and be accumulated as a result of physico-chemical mechanisms and transport systems of varying specificity, independent of, or directly and indirectly dependent on, metabolism [1-4]. Some of these processes are of global importance being components of major biogeochemical cycles, including microfossil formation, iron and manganese deposition, silver and uranium mineralisation, as well as resulting in transfer to other organisms via food chains [2,5-8]. Some interactions are of biotechnologicai importance being relevant to metal removal a n d / o r