Iron cycle
The iron cycle (Fe) is the
Iron exists in a range of
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.
Ancient earth
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Biogeochemical cycles |
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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.[16] Iron sulfide may have provided the energy and surfaces for the first organisms.[17] 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.[18]
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).[19] 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.[20][21]
Terrestrial ecosystems
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.
Oceanic ecosystem
The ocean is a critical component of the Earth's
Most commonly, iron was available as an inorganic source to phytoplankton; however, organic forms of iron can also be used by specific
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.[21]
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.[32]
The transition of iron between Fe(II) and Fe(III) in aquatic systems interacts with the freshwater
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.[34] 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.[35]
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).[33]
Anthropogenic influences
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.[29] 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.[28] 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.[29]
Humans have altered the cycle for Nitrogen from fossil fuel combustion and large-scale agriculture.
See also
References
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Further reading
- Pérez-Guzmán L, Bogner KR, Lower BH (2010). "Earth's Ferrous Wheel". Nature Education Knowledge. 3 (10): 32.