Iron cycle

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.

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.

[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.

Chemical and biological weathering break down iron-bearing minerals, releasing the nutrient into the atmosphere.

[23] 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.

[21] Other major sources of iron to the ocean include glacial particulates, atmospheric dust transport, and hydrothermal vents.

[26] Iron supply is an important factor affecting growth of phytoplankton, the base of marine food web.

[21] Other major sources of iron to the ocean include glacial particulates, hydrothermal vents, and volcanic ash.

The oxidized form of iron can then be the electron acceptor for reduced sulfur, H2, and organic carbon compounds.

In anoxic conditions, Fe(III) can be reduced, used by microbes to be the final electron acceptor from either organic carbon or H2.

[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.

[33] Human impact on the iron cycle in the ocean is due to dust concentrations increasing at the beginning of the industrial era.

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.

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 activity , [ 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 ]
Role of marine animals in the cycling of iron in the Southern Ocean [ 30 ]
Biogeochemical cycling of dissolved iron in the surface ocean [ 31 ]
L S , strong iron binding ligand; L W , weak iron binding ligand; FeL S , iron complexed by strong iron binding ligand ; FeL w , iron complexed by weak iron binding ligand; Fe(II), all sum of all Fe(II) species; Fe′, the sum of all inorganic Fe(III) species; Fe col , colloidal iron species; Fe part , iron in the particulate phase; hv, photon flux; O 2 , dissolved oxygen; and H 2 O 2 , dissolved hydrogen peroxide.