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, it is less common in oxygenated surface waters. Iron is a key micronutrient in primary productivity, 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.
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. 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.
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.
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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. Iron sulfide may have provided the energy and surfaces for the first organisms. At this time, before the onset of oxygenic photosynthesis, primary production may have been dominated by photo-ferrotrophs, which would obtain energy from sunlight, and use the electrons from Fe2+ to fix carbon.
During the Great Oxidation Event, 2.3-2.5 billion years ago, dissolved iron was oxidized by oxygen produced by cyanobacteria to form iron oxides. The iron oxides were denser than water and fell to the ocean floor forming banded iron formations (BIF). Over time, rising oxygen levels removed increasing amounts of iron from the ocean. BIFs have been a key source of iron ore in modern times.
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. 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. Aeolian dust is a critical part of the iron cycle by transporting iron particulates from the Earth's land via the atmosphere to the ocean.
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. The atmospheric transport of iron-rich dust can impact the ocean concentrations.
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. Iron limitation has been known to limit the efficiency of the biological carbon pump. The largest supply of iron to the oceans is from rivers, where it is suspended as sediment particles. Coastal waters receive inputs of iron from rivers and anoxic sediments. Other major sources of iron to the ocean include glacial particulates, atmospheric dust transport, and hydrothermal vents. Iron supply is an important factor affecting growth of phytoplankton, the base of marine food web. Offshore regions rely on atmospheric dust deposition and upwelling. Other major sources of iron to the ocean include glacial particulates, hydrothermal vents, and volcanic ash. In offshore regions, bacteria also compete with phytoplankton for uptake of iron. In HNLC regions, iron limits the productivity of phytoplankton.
Most commonly, iron was available as an inorganic source to phytoplankton; however, organic forms of iron can also be used by specific diatoms which use a process of surface reductase mechanism. Uptake of iron by phytoplankton leads to lowest iron concentrations in surface seawater. Remineralization occurs when the sinking phytoplankton are degraded by zooplankton and bacteria. Upwelling recycles iron and causes higher deep water iron concentrations. On average there is 0.07±0.04 nmol Fe kg−1 at the surface (<200 m) and 0.76±0.25 nmol Fe kg−1 at depth (>500 m). Therefore, upwelling zones contain more iron than other areas of the surface oceans. Soluble iron in ferrous form is bioavailable for utilization which commonly comes from aeolian resources.
Iron primarily is present in particulate phases as ferric iron, and the dissolved iron fraction is removed out of the water column by coagulation. For this reason, the dissolved iron pool turns over rapidly, in around 100 years.
Interactions with other elemental cycles
The iron cycle interacts significantly with the sulfur, nitrogen, and phosphorus cycles. Soluble Fe(II) can act as the electron donor, reducing oxidized organic and inorganic electron receptors, including O2 and NO3, and become oxidized to Fe(III). The oxidized form of iron can then be the electron acceptor for reduced sulfur, H2, and organic carbon compounds. This returns the iron to the reduced Fe(II) state, completing the cycle.
The transition of iron between Fe(II) and Fe(III) in aquatic systems interacts with the freshwater phosphorus cycle. With oxygen in the water, Fe(II) gets oxidized to Fe(III), either abiotically or by microbes via lithotrophic oxidation. Fe(III) can form iron hydroxides, which bind tightly to phosphorus, removing it from the bioavailable phosphorus pool, limiting primary productivity. In anoxic conditions, Fe(III) can be reduced, used by microbes to be the final electron acceptor from either organic carbon or H2. This releases the phosphorus back into the water for biological use.
The iron and sulfur cycle can interact at several points. Purple sulfur bacteria and green sulfur bacteria can use Fe(II) as an electron donor during anoxic photosynthesis. Sulfate reducing bacteria in anoxic environments can reduce sulfate to sulfide, which then binds to Fe(II) to create iron sulfide, a solid mineral that precipitates out of water and removes the iron and sulfur. The iron, phosphate, and sulfur cycles can all interact with each other. Sulfide can reduce Fe(III) from iron that is already bound to phosphate when there are no more metal ions available, which releases the phosphate and creates iron sulfide.
Iron plays an important role in the nitrogen cycle, aside from its role as part of the enzymes involved in nitrogen fixation. In anoxic conditions, Fe(II) can donate an electron that is accepted by NO3− which is oxidized to several different forms of nitrogen compounds, NO2−, N2O, N2, and NH4+, while Fe(II) is reduced to Fe(III).
Human impact on the iron cycle in the ocean is due to dust concentrations increasing at the beginning of the industrial era. Today, there is approximately double the amount of soluble iron in oceans than pre-industrial times from anthropogenic pollutants and soluble iron combustion sources. Changes in human land-use activities and climate have augmented dust fluxes which increases the amount of aeolian dust to open regions of the ocean. Other anthropogenic sources of iron are due to combustion. Highest combustion rates of iron occurs in East Asia, which contributes to 20-100% of ocean depositions around the globe.
Humans have altered the cycle for Nitrogen from fossil fuel combustion and large-scale agriculture. Due to increased Iron and Nitrogen raises marine nitrogen fixation in the subtropical North and South Pacific Ocean. In the subtropics, tropics and HNLC regions, increased inputs of iron may lead to increased CO2 uptake, impacting the global carbon cycle.
- 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.
- 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. S2CID 16985005.
- Raiswell R, Canfield DE (2012). "The iron biogeochemical cycle past and present" (PDF). Geochemical Perspectives. 1 (1): 1–232. Bibcode:2012GChP....1....1R. doi:10.7185/geochempersp.1.1.
- 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. PMID 17711233.
- Völker C, Tagliabue A (July 2015). "Modeling organic iron-binding ligands in a three-dimensional biogeochemical ocean model" (PDF). Marine Chemistry. 173: 67–77. Bibcode:2015MarCh.173...67V. doi:10.1016/j.marchem.2014.11.008.
- 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.
- 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.
- 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" (PDF). Global Biogeochemical Cycles. 25 (4): n/a. Bibcode:2011GBioC..25.4001O. doi:10.1029/2009GB003761.
- 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.
- 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.
- Tagliabue A, Bowie AR, Boyd PW, Buck KN, Johnson KS, Saito MA (March 2017). "The integral role of iron in ocean biogeochemistry" (PDF). Nature. 543 (7643): 51–59. Bibcode:2017Natur.543...51T. doi:10.1038/nature21058. PMID 28252066. S2CID 2897283.
- 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. S2CID 4325562.
- 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. S2CID 24058676.
- 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.
- Kappler, Andreas; Straub, Kristina L. (2005-01-01). "Geomicrobiological Cycling of Iron". Reviews in Mineralogy and Geochemistry. 59 (1): 85–108. doi:10.2138/rmg.2005.59.5. ISSN 1529-6466.
- 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.
- Schröder, Christian; Köhler, Inga; Muller, Francois L. L.; Chumakov, Aleksandr I.; Kupenko, Ilya; Rüffer, Rudolf; Kappler, Andreas (2016). "The biogeochemical iron cycle and astrobiology". Hyperfine Interactions. 237: 85. Bibcode:2016HyInt.237...85S. doi:10.1007/s10751-016-1289-2.
- 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.
- "The Great Oxygenation Event – when Earth took its first breath – Scientific Scribbles". Retrieved 2020-04-10.
- Thompson, Katharine J.; Kenward, Paul A.; Bauer, Kohen W.; Warchola, Tyler; Gauger, Tina; Martinez, Raul; Simister, Rachel L.; Michiels, Céline C.; Llirós, Marc; Reinhard, Christopher T.; Kappler, Andreas (2019-11-01). "Photoferrotrophy, deposition of banded iron formations, and methane production in Archean oceans". Science Advances. 5 (11): eaav2869. Bibcode:2019SciA....5.2869T. doi:10.1126/sciadv.aav2869. ISSN 2375-2548. PMC 6881150. PMID 31807693.
- Tortell, Philippe D.; Maldonado, Maria T.; Granger, Julie; Price, Neil M. (1999-05-01). "Marine bacteria and biogeochemical cycling of iron in the oceans". FEMS Microbiology Ecology. 29 (1): 1–11. doi:10.1111/j.1574-6941.1999.tb00593.x. ISSN 0168-6496.
- Johnson CM, Beard BL (August 2005). "Geochemistry. Biogeochemical cycling of iron isotopes". Science. 309 (5737): 1025–7. doi:10.1126/science.1112552. PMID 16099969. S2CID 94734488.
- Fan, Song-Miao; Moxim, Walter J.; Levy, Hiram (2006). "Aeolian input of bioavailable iron to the ocean". Geophysical Research Letters. 33 (7): L07602. Bibcode:2006GeoRL..33.7602F. doi:10.1029/2005GL024852. ISSN 0094-8276.
- Achterberg EP, Moore CM, Henson SA, Steigenberger S, Stohl A, Eckhardt S, et al. (2013). "Natural iron fertilization by the Eyjafjallajökull volcanic eruption" (PDF). Geophysical Research Letters. 40 (5): 921–926. Bibcode:2013GeoRL..40..921A. doi:10.1002/grl.50221. S2CID 55216781.
- Poulton SW (2002). "The low-temperature geochemical cycle of iron: From continental fluxes to marine sediment deposition" (PDF). American Journal of Science. 302 (9): 774–805. Bibcode:2002AmJS..302..774P. doi:10.2475/ajs.302.9.774.
- 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. Bibcode:2010BGeo....7..827D. doi:10.5194/bg-7-827-2010.
- 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.
- Leeuwen, H. P. (Herman) van, Riemsdijk, W. H. van, Hiemstra, T. J. (Tjisse), Krebs, C. J., Hiemstra, T. J. (Tjisse), & Krebs, C. J. (2008). The biogeochemical cycle of Iron: The role of Natural Organic Matter.
- Luo, Chao; Mahowald, N.; Bond, T.; Chuang, P. Y.; Artaxo, P.; Siefert, R.; Chen, Y.; Schauer, J. (2008). "Combustion iron distribution and deposition". Global Biogeochemical Cycles. 22 (1): n/a. Bibcode:2008GBioC..22.1012L. doi:10.1029/2007GB002964.
- Ratnarajah, Lavenia; Nicol, Steve; Bowie, Andrew R. (2018). "Pelagic Iron Recycling in the Southern Ocean: Exploring the Contribution of Marine Animals". Frontiers in Marine Science. 5. doi:10.3389/fmars.2018.00109. S2CID 4376458. Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
- Croot, Peter L.; Heller, Maija I. (2012). "The Importance of Kinetics and Redox in the Biogeochemical Cycling of Iron in the Surface Ocean". Frontiers in Microbiology. 3: 219. doi:10.3389/fmicb.2012.00219. PMC 3377941. PMID 22723797. Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
- Roden, Eric; Sobolev, Dmitri; Glazer, Brian; Luther, George (2004-09-01). "Potential for Microscale Bacterial Fe Redox Cycling at the Aerobic-Anaerobic Interface". Geomicrobiology Journal. 21 (6): 379–391. doi:10.1080/01490450490485872. S2CID 14296044.
- Burgin, Amy J.; Yang, Wendy H.; Hamilton, Stephen K.; Silver, Whendee L. (2011). "Beyond carbon and nitrogen: how the microbial energy economy couples elemental cycles in diverse ecosystems". Frontiers in Ecology and the Environment. 9 (1): 44–52. doi:10.1890/090227. hdl:1808/21008. ISSN 1540-9309.
- Haaijer, Suzanne; Crienen, Gijs; Jetten, Mike; Op den Camp, Huub (2012-02-03). "Anoxic Iron Cycling Bacteria from an Iron Sulfide- and Nitrate-Rich Freshwater Environment". Frontiers in Microbiology. 3: 26. doi:10.3389/fmicb.2012.00026. PMC 3271277. PMID 22347219.
- Haaijer, Suzanne C. M.; Lamers, Leon P. M.; Smolders, Alfons J. P.; Jetten, Mike S. M.; Camp, Huub J. M. Op den (2007-08-14). "Iron Sulfide and Pyrite as Potential Electron Donors for Microbial Nitrate Reduction in Freshwater Wetlands". Geomicrobiology Journal. 24 (5): 391–401. doi:10.1080/01490450701436489. ISSN 0149-0451. S2CID 97227345.
- Krishnamurthy, Aparna; Moore, J. Keith; Mahowald, Natalie; Luo, Chao; Doney, Scott C.; Lindsay, Keith; Zender, Charles S. (2009). "Impacts of increasing anthropogenic soluble iron and nitrogen deposition on ocean biogeochemistry". Global Biogeochemical Cycles. 23 (3): n/a. Bibcode:2009GBioC..23.3016K. doi:10.1029/2008GB003440. hdl:1912/3418. ISSN 1944-9224. S2CID 2839652.
- Pérez-Guzmán L, Bogner KR, Lower BH (2010). "Earth's Ferrous Wheel". Nature Education Knowledge. 3 (10): 32.