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Reference.org
Iron cycle
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<p>The iron cycle (Fe) is the <a href="/facts/Biogeochemical_cycle/3vG5KBu4">biogeochemical cycle</a> of <a href="/facts/Iron/QK1JYy8E">iron</a> through the <a href="/facts/Atmosphere/oUpLaGRF">atmosphere</a>, <a href="/facts/Hydrosphere/RbJmcSX6">hydrosphere</a>, <a href="/facts/Biosphere/P1ltA4sb">biosphere</a> and <a href="/facts/Lithosphere/1mqaxASd">lithosphere</a>. While Fe is highly abundant in the Earth's crust, it is less common in oxygenated surface waters. Iron is a key <a href="/facts/Micronutrient/nwzKuSMQ">micronutrient</a> in <a href="/facts/Primary_productivity/14RzcF5E">primary productivity</a>, and a limiting nutrient in the Southern ocean, eastern equatorial Pacific, and the subarctic Pacific referred to as <a href="/facts/High-nutrient,_low-chlorophyll_regions/7fUdBWs1">High-Nutrient, Low-Chlorophyll (HNLC) regions</a> of the ocean. </p><p>While iron can exist in a range of <a href="/facts/Oxidation_states/W53W6s6V">oxidation states</a> from −2 to +7; however, on Earth it is predominantly in its +2 or +3 redox state. It is a primary redox-active metal in nature. The cycling of iron between its +2 and +3 oxidation states is referred to as the iron cycle. This process can be entirely <a href="/facts/Abiotic/CA3Dwtta">abiotic</a> or facilitated by <a href="/facts/Microorganisms/pysJYcE2">microorganisms</a>, especially <a href="/facts/Iron-oxidizing_bacteria/tUk32vjY">iron-oxidizing bacteria</a>. The abiotic processes include the <a href="/facts/Rust/sajUmaz8">rusting</a> of metallic which, in addition to oxidation of the metal, involves oxidation of Fe(II) in the presence of oxygen. Another type of abiotic process is the reduction of Fe3+ to Fe2+ by sulfide minerals. The biological cycling of Fe2+ is mediated by iron oxidizing and reducing microbes. </p><p>Iron is an essential micronutrient for life form. It is a key component of hemoglobin, important to nitrogen fixation as part of the <a href="/facts/Nitrogenase/gIhxgCaL">Nitrogenase</a> enzyme family, and as part of the iron-sulfur core of <a href="/facts/Ferredoxin/1qJJjQHk">ferredoxin</a> it facilitates electron transport in chloroplasts, eukaryotic mitochondria, and bacteria. Due to the high reactivity of Fe2+ with oxygen and low solubility of Fe3+, iron is a limiting nutrient in most regions of the world. </p>