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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, 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.

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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, and it 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. 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.

Iron is an essential micro-nutrient for almost every life form. It is a key component of hemoglobin in blood, it helps mediate nitrogen fixation in legumes and rhizobacteria, 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 major limiting nutrient in most regions of the world, both terrestrial and aquatic. Thus, the iron cycle is intrinsically linked to the cycling of other biologically-important elements.

Contents

• 1Ancient Earth
  
• 2Terrestrial
• 3Ancient earth
• 4See also
• 5References
• 6Further reading

Ancient earth[edit]

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. 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.

Oceanic[edit]

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. Other major sources of iron to the ocean include glacial particulates, atmospheric dust transport, hydrothermal vents, and volcanic ash. Iron supply is an important factor affecting growth of phytoplankton, the base of marine food web. In HNLC regions, iron limits the productivity of phytoplankton. ^[1]

Uptake of iron by phytoplankton leads to lowest iron concentrations in surface seawater. Remineralization of sinking phytoplankton by zooplankton and bacteria. recycles iron and causes higher deep water iron concentrations. Therefore, upwelling zones contain more iron than other areas of the surface ocean. Soluble iron in ferrous form is bioavailable for utilization which commonly comes from aeolian resources.  

Interactions with other elemental cycles [edit]

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 receptor for reduced sulfur, H₂, and organic carbon compounds. This returns the iron to the oxidized Fe(II) state, completing the cycle.

Terrestrial[edit]

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 e _ustimates ranging between 1000 and 2000 Tg/year. 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.

Anthropogenic Influences [edit]

Human impact on the iron cycle is due to increasing anthropogenic atmospheric alterations first majorly noticed 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.^[1]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.^[2] 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.

External links

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