LYCOS RETRIEVER 
Nickel: Iron
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Nickel plays numerous roles in the biology of microorganisms and plants, though they were not recognized until the 1970s. In fact urease (an enzyme which assists in the hydrolysis of urea) contains nickel. The NiFe-hydrogenases contain nickel in addition to iron-sulfur clusters. Such [NiFe]-hydrogenases characteristically oxidise H2. A nickel-tetrapyrrole coenzyme, F430, is present in the methyl coenzyme M reductase which powers methanogenic archaea. One of the carbon monoxide dehydrogenase enzymes consists of an Fe-Ni-S cluster.[7] Other nickel-containing enzymes include a class of superoxide dismutase[8] and a glyoxalase.[9]
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Nickel occurs naturally in surface waters from the weathering of minerals and rocks. Once nickel is in surface and ground water systems, physical and chemical interactions (complexation, precipitation/dissolution, adsorption/desorption, and oxidation/reduction) will determine its fate. Under normal conditions, nickel is overwhelmingly associated with abundant iron and manganese particles which precipitate and absorb free nickel ions. Much of the nickel in the environment is therefore found in soils and sediments. Organic material in polluted environments will keep nickel soluble.
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Nickel is deposited in the depth of the Earth. Its concentration in ultrabasic mantle layers is 0.2%. Most of the nickel on Earth is postulated to be concentrated in the Earth's core. Average Earth abundance is approximately 3%. The crustal abundance is 5.8x10-3%, mostly in deep basaltic layer. Nickel is associated with Iron and Magnesium, intercrystallised in their ores because of the same valence (II) and ionic radii.
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