Metals in the Natural World
Most metals in the Earth’s crust are located in highly inorganic environments– as compo nents of rocks or soils on land or under water. Where metals are aggregated in local high concentrations through geological processes these may be sufficient in amount and con centration to represent an ore deposit, which is really an economic rather than a scientific definition. In addition, metals are present in water bodies as dissolved cations; their concentrations can be in a very few cases substantial, as is the case with sodium ion in seawater. However, even if present in very low concentration, as for gold in seawater, the size of the oceans means that there is a substantial amount of gold (and other metals) dispersed in the aquatic environment. The other location of metals is within living organisms, where, of the transition metals, iron, zinc and copper predominate. On rare occasions the concentration of another metal may be relatively high; this is the case in some plants that tolerate and concentrate particular metal ions, such as nickel in Hybanthus floribundus native to Western Australia, which can be hyper accumulated up to ∼50 mg per gram dry weight. Levels of metal ions in animals and in particular plants vary with species and environment. However, generally metals are present in nature in only trace amounts (Table 1.1). High levels of most metal ions are toxic to living species; for example ryegrass displays a toxicity order Cu>Ni >Mn> Pb >Cd >Zn >Al> Hg> Cr> Fe with each species displaying a unique trend.
Metals were eventually recognized as having a presence in a range of biomolecules. Where metal cations appear in living things their presence is rarely if ever simply fortuitous. Rather, they play a particular role, from simply providing an ionic environment through to being at the key active site for reactions in a large enzyme. Notably, it is the lighter alkali, alkaline earth and transition elements that dominate the metals present in living organisms. Of transition metals, although iron, copper and zinc are most dominant, almost all of the first-row transition elements play some part in the functioning of organisms. Nevertheless, even heavier elements such as molybdenum and tungsten are found to have some roles.
Keys to metal ion roles are their high charge (and surface charge density) capacity to bind organic entities through strong coordinate bonds and ability in many cases to vary oxidation states.
