Iron cycle
Biogeochemical iron cycle: Iron circulates through the atmosphere, lithosphere, and oceans. Labeled arrows show flux in Tg of iron per year.[1][2][3][4] Iron in the ocean cycles between plankton, aggregated particulates (non-bioavailable iron), and dissolved (bioavailable iron), and becomes sediments through burial.[1][5][6] Hydrothermal vents release ferrous iron to the ocean[7] in addition to oceanic iron inputs from land sources. Iron reaches the atmosphere through volcanism,[8] aeolian wind,[9] and some via combustion by humans. In the Anthropocene, iron is removed from mines in the crust and a portion re-deposited in waste repositories.[4][6]

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.

OceanicEdit

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]

TerrestrialEdit

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 earthEdit

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 alsoEdit

ReferencesEdit

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Further readingEdit