Reactions between microorganisms and metals may be considered from two interrelated aspects, namely, the effect of the metal or ion on the microorganism and, secondly, the effect of the microorganism on the metal or ion. Certain abundant metals such as iron, magnesium, and manganese and a rarer metal such as zinc may be essential to the growth of many biological species, probably including most species of bacteria. Metals may also be toxic to microorganisms, in fact, all elements can be toxic at sufficiently high concentrations. Organisms also have the capability to develop tolerance to initially toxic concentrations of heavy metal ions.