All living organisms require several essential trace metal elements. During biological evolution of prokaryotes and later on also eukaryotes several metals became incorporated as essential factors in many biochemical functions more or less in accordance with the abundance of these metals on the planet. As a result the biological importance of first row transition metals can be ranked roughly in the order Fe, Zn, Cu, Mn, Co, Ni. The second row metals Ag and Cd or third row metals like Hg and Pb appear to have no biological function, except possibly for Cd. Iron (Fe) being the fourth most abundant element in the crust of the planet has also played a role to temper the evolution of biogenic oxygen (O-2) in the atmosphere and oceans. Yet eventually O-2 has taken over the biosphere where now both atmosphere and ocean are strongly oxidizing. Inside every living cell the primordial reducing conditions have remained however. Therefore enzyme systems based on metal couples Fe-Mn and Cu-Zn are required to protect the cell interior from damage by reactive oxygen species. The key role of metals in these and many other enzymes as well as in protein folding is one of the major vectors in biological diversity at both the molecular and the species level. Abundance and biological role of metals in the oceans are being discovered only since 1976 and many questions remain. Until now many metals appear to be tightly linked with the large scale biological cycling. Recently the significance of very fine colloids as well as dissolved organic metal-complexes has been shown. Plankton ecosystems now appear to be governed by co-limitation of several essential metals, where the biological fractionation of their stable isotopes is expected to give rise to significant shifts in isotopic ratio for any given metal element. Co-limitation of plankton growth is consistent also with observed interactions between metals. Firstly substitution of for example Co or Cd for Zn is known for some but not all phytoplankton taxa. Secondly the cellular uptake of one metal, e.g. Cd, may respond to a complex matrix of other metals like Zn, Mn and possibly Fe. By mining and industrial use of metals, land use change and irrigation, mankind has greatly modified the abundances and mutual ratio's of metals in the biosphere. Biota can to some extent resist the ensuing external stress through cellular homeostasis. However at highly elevated levels or excessive metal ratio's several biological species cannot longer exist and major shifts of ecosystems and their diversity do occur. Likely such changes have already taken place for centuries and as such have largely gone unnoticed. The past decade was the onset of the iron age in oceanography. Nowadays Fe is known to be a severely limiting element in some 40 % of the world oceans. This limitation is sometimes relieved by aeolian supply of continental dust, but most Fe supply is actually from below emanating of reducing sediments. With adequate Fe there is a systematic response by the class of very large oceanic diatoms, their massive blooms then giving rise to uptake of CO2 and export of both opal (SiO2) and organic matter into the deep-sea. Hence the supply of Fe to ocean waters is one of the major controls of plankton ecosystems and ocean element cycling (C, Si, N, P) over time scales ranging from weeks to the 100,000 year periodicity of glaciations. Understanding the cycling and biological function of metals in the oceans is a prerequisite for understanding the role of the oceans in global change of past, present and future.
