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Until recently, nickel was not considered to be an element of biological importance. In the last few years however, nutritional studies have shown that many eucaryotic and procaryotic organisms are dependent on nickel (Table I). Some of these require the transition metal in relatively large amounts, for example plant cells growing on urea as sole nitrogen source, most'Knallgas' bacteria growing on H2 plus O2, the acetogenic bacteria and the methanogenic bacteria. The dependence of these groups of organisms on nickel has