Iron cycle

The iron cycle (Fe) is the biogeochemical cycle of iron through the atmosphere, hydrosphere, biosphere and lithosphere. While Fe is highly abundant in the Earth's crust,^[10] it is less common in oxygenated surface waters. Iron is a key micronutrient in primary productivity,^[11] and a limiting nutrient in the Southern ocean, eastern equatorial Pacific, and the subarctic Pacific referred to as High-Nutrient, Low-Chlorophyll (HNLC) regions of the ocean.^[12]

Iron exists in a range of oxidation states from -2 to +7; however, on Earth it is predominantly in its +2 or +3 redox state and is a primary redox-active metal on Earth.^[13] The cycling of iron between its +2 and +3 oxidation states is referred to as the iron cycle. This process can be entirely abiotic or facilitated by microorganisms, especially iron-oxidizing bacteria. The abiotic processes include the rusting of iron-bearing metals, where Fe²⁺ is abiotically oxidized to Fe³⁺ in the presence of oxygen, and the reduction of Fe³⁺ to Fe²⁺ by iron-sulfide minerals. The biological cycling of Fe²⁺ is done by iron oxidizing and reducing microbes.^[14][15]

Iron is an essential micronutrient for almost every life form. It is a key component of hemoglobin, important to nitrogen fixation as part of the Nitrogenase enzyme family, and as part of the iron-sulfur core of ferredoxin 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.