ABSTRACT
Iron, zinc, copper, molybdenum and some other transition metals are essential nutrients (Fraustro da Silva and Williams 2001). They are constitutive elements of around half of the proteins of a typical cell (Andreini et al. 2008), where they may act as structural elements, such as zinc in the zinc fi nger domain, but mostly as key elements in the active site of enzymes involved in almost every physiological process, from oxidative respiration to photosynthesis. These elements are growth-limiting nutrients for autotrophic organisms, since they form very stable complexes, resulting in low solubility and making their uptake diffi cult (Ruel and Bouis 1998, Fung et al. 2000, Grotz and Guerinot 2006). Moreover, living beings cannot accumulate high amounts of essential transition metals, since these elements can catalyze the production of free radicals in Fenton-style reactions or compete with other metals for the active site of metalloenzymes (Goldstein et al. 1993, Ranquet et al. 2007, Macomber and Imlay 2009). This is also the basis of the toxic effect of non-biogenic metals such as cadmium, lead or mercury.
