In addition to chelation, the induction of mineral formation can lead to metal depletion in the direct surroundings of a bacterial cell. Biomineralization is the process in which microorganisms aid the growth of crystals by providing either a crystallization initiator or anions for mineralization. Both of these approaches to biomin-eralization depend on the presence of cells, and the latter depends on the presence of actively growing, living cells. One well-studied example of biomineral formation is provided by the intracellular magnetite-containing bodies formed by magnetot-actic bacteria (Bauerlein 2003). An extracellular biomineralization process appears to be more conducive to metal resistance. The formation of hydrozincite in zinc-rich mine effluents has been described (De Giudici et al. 2007). There, the precipitates formed with bacterial inocula were the dominant species of zinc in the AMD waters. This biomineralization would also allow growth under high nickel stress. Indeed, the formation of a new biomineral with S. acidiscabies E13 was demonstrated (Haferburg et al. 2008). The natural mineral struvite is composed of Mg (NH4)(PO4) 6H2O; nickel struvite, as we termed the biomineral observed in our cultures, is its nickel analog: Ni(NH4)(PO4) 6H2O. The crystals form on top of colonies growing on solid media, but also in liquid cultures of the strain. The medium contains not ammonia but nitrate as the nitrogen source, which does not allow mineralization without active metabolism of the streptomycete. The mineral formed is extremely pure, with only minimal magnesium impurities, which is especially interesting because the natural mineral struvite could also have formed, using magnesium from the medium. However, this was not observed, which suggests a biologically controlled process. Inoculation of the media with dead biomass did not lead to nickel/struvite formation, showing that it indeed is an active, biologically controlled process that warrants the term "biomineralization." With nickel bound to a mineral phase, the cells clearly have an adaptive advantage in terms of released nickel stress.

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