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 High-Nutrient, Low-Chlorophyll (HNLC) regions of the ocean.[12] A critical component of the iron cycle is aeolian dust, which is transported from the Earth's land via the atmosphere to the ocean.

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. 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. Some examples of this include the rusting of iron-bearing metals (in this case, Fe2+ is abiotically oxidized to Fe3+) by oxygen, and the abiotic reduction of Fe3+ to Fe2+ by iron-sulfide minerals or the biological cycling of Fe2+-oxidizing microbes.[13]

Iron is an essential micro-nutrient for almost every life form, and is a primary redox-active metal on Earth.[14] 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. Thus, the iron cycle is intrinsically linked to the cycling of other biologically-important elements.

Iron cycle7
The biogeochemical iron cycle: Iron circulates through the atmosphere, lithosphere, and oceans. Labeled arrows show the flux between iron reservoirs, with flux values in Tg of iron per year where literature values are available.[1][2][3][4] Iron in the ocean cycles between plankton, aggregated particulates (non-bioavailable iron), and dissolved (free and ligand-complexed bioavailable iron), and is deposited into sediments through burial.[1][5][6] Hydrothermal vents release ferrous iron to the ocean[7] in addition to oceanic iron inputs from sediments, glaciers, icebergs, rivers, and atmospheric dust. Iron reaches the atmosphere through stochastic volcanism,[8] aeolian wind,[9] and to a lesser extent through combustion by humans. In the Anthropocene, iron is removed from the crust through mining with a portion re-deposited in slag, landfills, and other waste repositories.[4][6]


The ocean is a critical component of the Earth's climate system, and the iron cycle plays a key role in ocean primary productivity and marine ecosystem function. The largest supply of iron to the oceans is from rivers, where it is suspended as sediment.[15] Other major sources of iron to the ocean include glacial particulates, atmospheric dust transport, and hydrothermal vents.[16] Iron supply is an important factor affecting growth of phytoplankton, the base of marine food web.[17] Uptake of iron by phytoplankton leads to lowest iron concentrations in surface seawater. Remineralization of sinking phytoplankton by zooplankton and bacteria.[11] recycles iron and causes higher deep water iron concentrations. Therefore, upwelling zones contain more iron than other areas of the surface ocean.[2]


The iron cycle is an important component of the terrestrial ecosystems. The ferrous form of iron, Fe2+, is dominant in the Earth's mantle, core, or deep crust. The ferric form, Fe3+, is more stable in the presence of oxygen gas.[18] Dust is a key component in the Earth's iron cycle. Chemical and biological weathering break down iron-bearing minerals, releasing the nutrient into the atmosphere. Changes in hydrological cycle and vegetative cover impact these patterns and have a large impact on global dust production, with dust deposition estimates ranging between 1000 and 2000 Tg/year.[2] Volcanic eruptions are also a key contributor to the terrestrial iron cycle, releasing iron-rich dust into the atmosphere in either a large burst or in smaller spurts over time.[19] The atmospheric transport of iron-rich dust can impact the ocean concentrations.[2]

Ancient earth

On the early Earth, when atmospheric oxygen levels were 0.001% of those present today, dissolved Fe2+ was thought to have been a lot more abundant in the oceans, and thus more bioavailable to microbial life in that era.[20] At this time, before the onset of oxygenic photosynthesis, primary production may have been dominated by photoferrotrophs, which would obtain energy from sunlight, and use the electrons from Fe2+ to fix carbon.[21]

See also


  1. ^ a b Nickelsen L, Keller D, Oschlies A (2015-05-12). "A dynamic marine iron cycle module coupled to the University of Victoria Earth System Model: the Kiel Marine Biogeochemical Model 2 for UVic 2.9". Geoscientific Model Development. 8 (5): 1357–1381. Bibcode:2015GMD.....8.1357N. doi:10.5194/gmd-8-1357-2015.
  2. ^ a b c d Jickells TD, An ZS, Andersen KK, Baker AR, Bergametti G, Brooks N, et al. (April 2005). "Global iron connections between desert dust, ocean biogeochemistry, and climate". Science. 308 (5718): 67–71. Bibcode:2005Sci...308...67J. doi:10.1126/science.1105959. PMID 15802595.
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  4. ^ a b Wang T, Müller DB, Graedel TE (2007-07-01). "Forging the Anthropogenic Iron Cycle". Environmental Science & Technology. 41 (14): 5120–5129. Bibcode:2007EnST...41.5120W. doi:10.1021/es062761t.
  5. ^ Völker C, Tagliabue A (July 2015). "Modeling organic iron-binding ligands in a three-dimensional biogeochemical ocean model". Marine Chemistry. 173: 67–77. doi:10.1016/j.marchem.2014.11.008.
  6. ^ a b Matsui H, Mahowald NM, Moteki N, Hamilton DS, Ohata S, Yoshida A, Koike M, Scanza RA, Flanner MG (April 2018). "Anthropogenic combustion iron as a complex climate forcer". Nature Communications. 9 (1): 1593. Bibcode:2018NatCo...9.1593M. doi:10.1038/s41467-018-03997-0. PMC 5913250. PMID 29686300.
  7. ^ Emerson D (2016). "The Irony of Iron - Biogenic Iron Oxides as an Iron Source to the Ocean". Frontiers in Microbiology. 6: 1502. doi:10.3389/fmicb.2015.01502. PMC 4701967. PMID 26779157.
  8. ^ Olgun N, Duggen S, Croot PL, Delmelle P, Dietze H, Schacht U, et al. (2011). "Surface ocean iron fertilization: The role of airborne volcanic ash from subduction zone and hot spot volcanoes and related iron fluxes into the Pacific Ocean". Global Biogeochemical Cycles. 25 (4): n/a. Bibcode:2011GBioC..25.4001O. doi:10.1029/2009GB003761.
  9. ^ Gao Y, Kaufman YJ, Tanre D, Kolber D, Falkowski PG (2001-01-01). "Seasonal distributions of aeolian iron fluxes to the global ocean". Geophysical Research Letters. 28 (1): 29–32. Bibcode:2001GeoRL..28...29G. doi:10.1029/2000GL011926.
  10. ^ Taylor SR (1964). "Abundance of chemical elements in the continental crust: a new table". Geochimica et Cosmochimica Acta. 28 (8): 1273–1285. Bibcode:1964GeCoA..28.1273T. doi:10.1016/0016-7037(64)90129-2.
  11. ^ a b Tagliabue A, Bowie AR, Boyd PW, Buck KN, Johnson KS, Saito MA (March 2017). "The integral role of iron in ocean biogeochemistry". Nature. 543 (7643): 51–59. Bibcode:2017Natur.543...51T. doi:10.1038/nature21058. PMID 28252066.
  12. ^ Martin JH, Fitzwater SE (1988). "Iron deficiency limits phytoplankton growth in the north-east Pacific subarctic". Nature. 331 (6154): 341–343. Bibcode:1988Natur.331..341M. doi:10.1038/331341a0.
  13. ^ Schmidt C, Behrens S, Kappler A (2010). "Ecosystem functioning from a geomicrobiological perspective – a conceptual framework for biogeochemical iron cycling". Environmental Chemistry. 7 (5): 399. doi:10.1071/EN10040.
  14. ^ Melton ED, Swanner ED, Behrens S, Schmidt C, Kappler A (December 2014). "The interplay of microbially mediated and abiotic reactions in the biogeochemical Fe cycle". Nature Reviews. Microbiology. 12 (12): 797–808. doi:10.1038/nrmicro3347. PMID 25329406.
  15. ^ Poulton SW (2002). "The low-temperature geochemical cycle of iron: From continental fluxes to marine sediment deposition". American Journal of Science. 302 (9): 774–805. Bibcode:2002AmJS..302..774P. doi:10.2475/ajs.302.9.774.
  16. ^ Duggen S, Olgun N, Croot P, Hoffmann LJ, Dietze H, Delmelle P, Teschner C (2010). "The role of airborne volcanic ash for the surface ocean biogeochemical iron-cycle: a review". Biogeosciences. 7 (3): 827–844. doi:10.5194/bg-7-827-2010.
  17. ^ Hutchins DA, Boyd PW (2016). "Marine phytoplankton and the changing ocean iron cycle". Nature Climate Change. 6 (12): 1072–1079. Bibcode:2016NatCC...6.1072H. doi:10.1038/nclimate3147.
  18. ^ Johnson CM, Beard BL (August 2005). "Geochemistry. Biogeochemical cycling of iron isotopes". Science. 309 (5737): 1025–7. doi:10.1126/science.1112552. PMID 16099969.
  19. ^ Achterberg EP, Moore CM, Henson SA, Steigenberger S, Stohl A, Eckhardt S, et al. (2013). "Natural iron fertilization by the Eyjafjallajökull volcanic eruption". Geophysical Research Letters. 40 (5): 921–926. Bibcode:2013GeoRL..40..921A. doi:10.1002/grl.50221.
  20. ^ Canfield DE, Rosing MT, Bjerrum C (October 2006). "Early anaerobic metabolisms". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 361 (1474): 1819–34, discussion 1835–6. doi:10.1098/rstb.2006.1906. PMC 1664682. PMID 17008221.
  21. ^ Camacho A, Walter XA, Picazo A, Zopfi J (2017). "Photoferrotrophy: Remains of an Ancient Photosynthesis in Modern Environments". Frontiers in Microbiology. 8: 323. doi:10.3389/fmicb.2017.00323. PMC 5359306. PMID 28377745.

Further reading

Biogeochemical cycle

In ecology and Earth science, a biogeochemical cycle or substance turnover or cycling of substances is a pathway by which a chemical substance moves through biotic (biosphere) and abiotic (lithosphere, atmosphere, and hydrosphere) compartments of Earth. There are biogeochemical cycles for the chemical elements calcium, carbon, hydrogen, mercury, nitrogen, oxygen, phosphorus, selenium, and sulfur; molecular cycles for water and silica; macroscopic cycles such as the rock cycle; as well as human-induced cycles for synthetic compounds such as polychlorinated biphenyl (PCB). In some cycles there are reservoirs where a substance remains for a long period of time (such as an ocean or lake for water).

Hydrothermal vent

A hydrothermal vent is a fissure on the seafloor from which geothermally heated water issues. Hydrothermal vents are commonly found near volcanically active places, areas where tectonic plates are moving apart at spreading centers, ocean basins, and hotspots. Hydrothermal deposits are rocks and mineral ore deposits formed by the action of hydrothermal vents.

Hydrothermal vents exist because the earth is both geologically active and has large amounts of water on its surface and within its crust. Under the sea, hydrothermal vents may form features called black smokers or white smokers. Relative to the majority of the deep sea, the areas around submarine hydrothermal vents are biologically more productive, often hosting complex communities fueled by the chemicals dissolved in the vent fluids. Chemosynthetic bacteria and archaea form the base of the food chain, supporting diverse organisms, including giant tube worms, clams, limpets and shrimp. Active hydrothermal vents are believed to exist on Jupiter's moon Europa, and Saturn's moon Enceladus, and it is speculated that ancient hydrothermal vents once existed on Mars.

Index of meteorology articles

This is a list of meteorology topics. The terms relate to meteorology, the interdisciplinary scientific study of the atmosphere that focuses on weather processes and forecasting. (see also: List of meteorological phenomena)



Iron () is a chemical element with symbol Fe (from Latin: ferrum) and atomic number 26. It is a metal, that belongs to the first transition series and group 8 of the periodic table. It is by mass the most common element on Earth, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust.

Pure iron is very rare on the Earth's crust, basically being limited to meteorites. Iron ores are quite abundant, but extracting usable metal from them requires kilns or furnaces capable of reaching 1500 °C or higher, about 500 °C higher than what is enough to smelt copper. Humans started to dominate that process in Eurasia only about 2000 BCE, and iron began to displace copper alloys for tools and weapons, in some regions, only around 1200 BCE. That event is considered the transition from the Bronze Age to the Iron Age. Iron alloys, such as steel, inox, and special steels are now by far the most common industrial metals, because of their mechanical properties and their low cost.

Pristine and smooth pure iron surfaces are mirror-like silvery-gray. However, iron reacts readily with oxygen and water to give brown to black hydrated iron oxides, commonly known as rust. Unlike the oxides of some other metals, that form passivating layers, rust occupies more volume than the metal and thus flakes off, exposing fresh surfaces for corrosion.

The body of an adult human contains about 4 grams (0.005% body weight) of iron, mostly in hemoglobin and myoglobin. These two proteins play essential roles in vertebrate metabolism, respectively oxygen transport by blood and oxygen storage in muscles. To maintain the necessary levels, human iron metabolism requires a minimum of iron in the diet. Iron is also the metal at the active site of many important redox enzymes dealing with cellular respiration and oxidation and reduction in plants and animals.Chemically, the most common oxidation states of iron are iron(II) and iron(III). Iron shares many properties of other transition metals, including the other group 8 elements, ruthenium and osmium. Iron forms compounds in a wide range of oxidation states, −2 to +7. Iron also forms many coordination compounds; some of them, such as ferrocene, ferrioxalate, and Prussian blue, have substantial industrial, medical, or research applications.


In chemistry, iron(II) refers to the element iron in its +2 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe2+.

The adjective ferrous or the prefix ferro- is often used to specify such compounds — as in "ferrous chloride" for iron(II) chloride, FeCl2. The adjective "ferric" is used instead for iron(III) salts, containing the cation or Fe3+. The word ferrous is derived from the Latin word ferrum for iron.

Iron(II) atoms may also occur as coordination complexes, such as the polymer iron(II) oxalate dihydrate, [Fe(C2O4)(H2O)2]n or [Fe2+][C2O2−4][H2O]2n; and organometallic compounds, such as the neutral molecule ferrocene, Fe(C2H5)2 or [Fe2+][C5H−5]2.

Iron is almost always encountered in the oxidation states 0 (as in the metal), +2, or +3. Solid iron(II) salts are relatively stable in air, but in the presence of air and water they tend to oxidize to iron(III) salts that include hydroxide (HO−) or oxide (O2− anions.

Iron fertilization

Iron fertilization is the intentional introduction of iron to iron-poor areas of the ocean surface to stimulate phytoplankton production. This is intended to enhance biological productivity and/or accelerate carbon dioxide (CO2) sequestration from the atmosphere.

Iron is a trace element necessary for photosynthesis in plants. It is highly insoluble in sea water and in a variety of locations is the limiting nutrient for phytoplankton growth. Large algal blooms can be created by supplying iron to iron-deficient ocean waters. These blooms can nourish other organisms.

Multiple ocean labs, scientists and businesses have explored fertilization. Beginning in 1993, thirteen research teams completed ocean trials demonstrating that phytoplankton blooms can be stimulated by iron augmentation. Controversy remains over the effectiveness of atmospheric CO2 sequestration and ecological effects. The most recent open ocean trials of ocean iron fertilization were in 2009 (January to March) in the South Atlantic by project Lohafex, and in July 2012 in the North Pacific off the coast of British Columbia, Canada, by the Haida Salmon Restoration Corporation (HSRC).Fertilization occurs naturally when upwellings bring nutrient-rich water to the surface, as occurs when ocean currents meet an ocean bank or a sea mount. This form of fertilization produces the world's largest marine habitats. Fertilization can also occur when weather carries wind blown dust long distances over the ocean, or iron-rich minerals are carried into the ocean by glaciers, rivers and icebergs.

Iron ore

Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in colour from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the form of magnetite (Fe3O4, 72.4% Fe), hematite (Fe2O3, 69.9% Fe), goethite (FeO(OH), 62.9% Fe), limonite (FeO(OH)·n(H2O), 55% Fe) or siderite (FeCO3, 48.2% Fe).

Ores containing very high quantities of hematite or magnetite (greater than about 60% iron) are known as "natural ore" or "direct shipping ore", meaning they can be fed directly into iron-making blast furnaces. Iron ore is the raw material used to make pig iron, which is one of the main raw materials to make steel—98% of the mined iron ore is used to make steel. Indeed, it has been argued that iron ore is "more integral to the global economy than any other commodity, except perhaps oil".

Iron oxide

Iron oxides are chemical compounds composed of iron and oxygen. There are sixteen known iron oxides and oxyhydroxides, the best known of which is rust, a form of iron(III) oxide.Iron oxides and oxyhydroxides are widespread in nature and play an important role in many geological and biological processes. They are used as iron ores, pigments, catalysts, and in thermite, and occur in hemoglobin. Iron oxides are inexpensive and durable pigments in paints, coatings and colored concretes. Colors commonly available are in the "earthy" end of the yellow/orange/red/brown/black range. When used as a food coloring, it has E number E172.


The Nitrosomonadales are an order of the class Betaproteobacteria in the phylum "Proteobacteria". Like all members of their class, they are Gram-negative.

The order is divided into six families:Nitrosomonadaceae (type family) comprises the genera Nitrosomonas (type genus), Nitrosolobus and Nitrosospira. Methylophilaceae comprises the genera Methylophilus (type genus), Methylobacillus and Methylovorus. Spirillaceae comprises the genus Spirillum (type genus) Thiobacillaceae comprises the genera Thiobacillus (type genus), Annwoodia, Sulfuritortus. Gallionellaceae comprises the genera Gallionella (type genus), Ferriphaselus, Sulfuriferula, Sulfurirhabdus and Sulfuricella. Sterolibacteraceae comprises the genera Sterolibacterium, Sulfurisoma, Denitratisoma, Sulfuritalea, Georgfuchsia, Sulfurisoma and Methyloversatilis.Members of the genus Nitrosomonas oxidize ammonium ions into nitrite, - a process called nitrification - and are important in the nitrogen cycle. Other autotrophic genera such as Thiobacillus and Annwoodia oxidize reduced inorganic sulfur ions such as thiosulfate and sulfide into sulfate and have key roles in the sulfur cycle. Methylotrophs such as Methylophilus oxidize compounds such as methanol into carbon dioxide and are key to the carbon cycle. Gallionella and Ferriphaselus oxidise ferric iron (Fe3+) ions into ferric hydroxide (Fe(OH)3) during autotrophic growth, and thus have roles in the carbon cycle and the iron cycle. As such, the Nitrosomonadales are critical to biogeochemical cycling of the elements and many species have key roles in principal biochemical processes.


An oxide is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– atom. Metal oxides thus typically contain an anion of oxygen in the oxidation state of −2. Most of the Earth's crust consists of solid oxides, the result of elements being oxidized by the oxygen in air or in water. Hydrocarbon combustion affords the two principal carbon oxides: carbon monoxide and carbon dioxide. Even materials considered pure elements often develop an oxide coating. For example, aluminium foil develops a thin skin of Al2O3 (called a passivation layer) that protects the foil from further corrosion. Individual elements can often form multiple oxides, each containing different amounts of the element and oxygen. In some cases these are distinguished by specifying the number of atoms as in carbon monoxide and carbon dioxide, and in other cases by specifying the element's oxidation number, as in iron(II) oxide and iron(III) oxide. Certain elements can form many different oxides, such as those of nitrogen. other examples are siicon, iron, titanium, and aluminium oxides.

Sponge iron reaction

The sponge iron reaction (SIR) is a chemical process based on redox cycling of an iron-based contact mass, the first cycle is a conversion step between iron metal (Fe) and wuestite (FeO), the second cycle is a conversion step between wuestite (FeO) and magnetite (Fe3O4). In application, the SIT is used in the reformer sponge iron cycle (RESC) in combination with a steam reforming unit.

Steam reforming

Steam reforming or steam methane reforming is a chemical synthesis for producing syngas (hydrogen and carbon monoxide) from hydrocarbons such as natural gas. This is achieved in a reformer which reacts steam at high temperature and pressure with methane in the presence of a nickel catalyst. The steam methane reformer is widely used in industry to make hydrogen.


Syngas, or synthesis gas, is a fuel gas mixture consisting primarily of hydrogen, carbon monoxide, and very often some carbon dioxide. The name comes from its use as intermediates in creating synthetic natural gas (SNG) and for producing ammonia or methanol. Syngas is usually a product of gasification and the main application is electricity generation. Syngas is combustible and can be used as a fuel of internal combustion engines. Historically, syngas has been used as a replacement for gasoline, when gasoline supply has been limited; for example, wood gas was used to power cars in Europe during WWII (in Germany alone half a million cars were built or rebuilt to run on wood gas). Syngas, however, has less than half the energy density of natural gas.Syngas can be produced from many sources, including natural gas, coal, biomass, or virtually any hydrocarbon feedstock, by reaction with steam (steam reforming), carbon dioxide (dry reforming) or oxygen (partial oxidation). Syngas is a crucial intermediate resource for production of hydrogen, ammonia, methanol, and synthetic hydrocarbon fuels. Syngas is also used as an intermediate in producing synthetic petroleum for use as a fuel or lubricant via the Fischer–Tropsch process and previously the Mobil methanol to gasoline process.

Production methods include steam reforming of natural gas or liquid hydrocarbons to produce hydrogen, the gasification of coal, biomass, and in some types of waste-to-energy gasification facilities.

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