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Introduction
Iron-oxidizing bacteria are chemotrophic bacteria that derive the energy needed to live and multiply by oxidizing dissolved Ferrous ion (Fe2+) to Ferric iron (Fe3+ ). Those bacteria are known to grow and proliferate in waters containing iron concentrations as low as 0.1 mg/L. However, at least 0.3 ppm of dissolved oxygen is needed to carry out oxidation.

In aerobic conditions, the pH variation plays an important role on driving the oxidation reaction of Fe2+/Fe3+, at neutrophilic pH (hydrothermal vents, deep ocean basalts, groudwater iron seeps) the oxidation of iron by microorganisms is highly competitive with the rapid abiotic reaction (occurs in <1 min) , for that reason the microbial community has to inhabit microaerophilic regions, where the low oxygen concentration allow the cell to oxidize Fe(II) and produce energy to grow. However, under acidic conditions only biological processes are responsible for the oxidation of ferrous, whereas Ferrous iron is more soluble and stable even in the presence of oxygen, thus making Ferrous iron oxidation the major metabolic strategy in rich-iron acidic environments

Iron is a very important element highly required by living organism to carry out numberless metabolic reactions such as the formation of proteins involved in biochemical reactions, like Iron–sulfur protein s, Hemoglobin and Coordination complexes.This element has a widespread distribution in the planet and is considered one of the most abundant in the Earth's crust, soil and sediments, while in the marine environments is one of the trace elements its role in the metabolism of some chemolithotrophs is probably very ancient. As Liebig's law of the minimum says, the element present in the smallest amount (called Limiting factor) is the one that determines the growth rate of a population of organisms. The Iron is the most common limiting element that have a key role in structuring phytoplankton communities and determining its abundance, it's particularly important in the HNLC (High-nutrient, low-chlorophyll regions), where the presence of micronutrient s is mandatory for the total primary production, and Iron is considered one of those limiting factors.

Microbial Ferrous Iron Oxidation Metabolism

Anoxygenic Phototrophic Ferrous Iron Oxidation 

The anoxygenic phototrophic iron oxidation was the first anaerobic metabolism to be described within the iron anaerobic oxidizer metabolism The photoferrotrophic bacteria use Fe 2+ as electron donor, and the energy from the light to assimilate CO2 into Biomass through the Calvin Benson-Bassam cycle (or rTCA cycle) within a neutrophilic environment (pH5.5-7.2), producing Fe3+oxides as a waste product that precipitates as a mineral, according to the following stoichiometry (4mM of Fe(II) can yield 1mM of CH2O) :

HCO3-+ 4Fe(II)+ 10H2O → [CH2O] + 4Fe(OH)3 + 7H+ ∆G°> 0

Nevertheless some bacteria do not use the photoautotrophic Fe(II) oxidation metabolism for growth purposes instead it's suggested that these groups are sensitive to Fe(II) therefore the oxidation of Fe(II) into more insoluble Fe(III) oxide to reduce its toxicity, enabling them to grow in the presence of Fe(II), on the other hand based on experiments with R. capsulatus SB1003 (photoheterotrophic), was demonstrated that the oxidation of Fe(II) might be the mechanisms whereby the bacteria is enable to ccess organic carbon sources (acetate, succinate) on which the use depend on Fe(II) oxidation Nonetheless many Iron-oxidizer bacterias, can use other compounds as electron donors in addition to Fe (II), or even perfom dissimilatory Fe(III) reduction as the Geobacter metallireducens

The dependence of potoferrotrophics on light as a crucial resource, can take the bacteria to a cumbersome situation, where due to their requirement for anoxic lighted regions (near the surface) they could be faced with competition matter with the abiotical reaction because of the presence of molecular oxygen, however to evade this problem they tolerate microaerophilic surface conditions, or perfom the photoferrotrophic Fe(II) odxidation deeper in the sediment/water column, with a low light availaiability

Nitrate-dependent Fe(II) oxidation

Light penetration can limit the Fe(II) oxidation in the water column however nitrate dependent microbial Fe(II) oxidation metabolism has been showed to support microbial growth in various freshwater and marine sediments (paddy soil, stream, brackish lagoon, hydrothermal, deep-sea sediments) and later on demonstrated as a pronounced metabolism in within the water column at the OMZ. Microbes that perform this metabolism are successful in neutrophilic or alcaline environmets, due to the high difference in between the redox potencial of the couples Fe2+/Fe3+ and NO3-/NO2- (+200mV and +770mv respectively) generating a high free energy when compared to other iron oxidation metabolisms

2Fe2+ + NO3- + H2O → Fe(OH)3 + NO2- + 4H+       ∆G°= -103.5kJ/mol

The microbial oxidation of Ferrous iron couple to Denitrification (with nitrite, or dinitrogen gas being the final product) can be autotrophic using inorganic carbon or organic cosubstrates (acetate, butyrate, pyruvate, ethanol) performing heterotrophic growth in the absence of inorganic carbon, it's suggested that the heterotrophic nitrate-dependent ferrous iron oxidation using organic carbon might be the most favorable process. This metabolism might be very important on carrying a important step to the biogeochemical in the oxygen minimum zones

Groups of iron Oxidizing Microorganisms
Despite being phylogenetically diverse the microbial Ferrous iron oxidation metabolic strategy (found in prokaryots: Archaea and Bacteria) it is distributed in 7 phyla highly pronounced into the Proteobacteria phyla (Alpha, Beta, Gamma and Zetaproteobacteria classes) ,and among the Archae domain in the Euryarchaeota and Chrenarcaeota phyla, and also in Actinobacteria, Firmicutes, Chlorobi and Nitrosospirae phyla

here are very well studied species on the FeOB such as Thiobacillus ferrooxidans and Leptospirillum ferrooxidans, and some like Gallionella ferruginea and Mariprofundis ferrooxydans are able to produce a particular extracellular stalk-ribbon structure rich in iron, known as a typical Biosignature of microbial Iron-oxidation. These structures can be easily found in a sample of water, indicating the presence FeOB, this biosignature has been a tool to understand the importance of Iron metabolism in the past of the earth.

Ferrous iron oxidation and the Early Life

Unluckily most litotrophic metabolisms, the oxidation of Fe2+/ to Fe3+ yields very little energy to the a cell (∆G°=29kJ mol-1 /∆G°=-90kJ mol-1 acidic and neutrophilic environments respectively) while compared to other chemolitothrophic metabolisms, therefore the cell must oxidize large amounts of Fe2+ to fulfill its metabolic requirements, withal contributing to the mineralization process (through the excretion of twisted stalks). The aerobic IOB metabolism was known to have a remarkable contribution to the formation of the largest iron deposist (Banded iron formation (BIF)) due to the advent of oxygen in the atmosphere 2.7Ga ago (by the Cyanobacteria).

However with the discover of Fe(II) oxidation carried out within anoxic conditions in the late 1990s by using the light as energy source or chemolitothrophically, using a different terminal electron acceptor (mostly NO3-), arose the suggestion that the anoxic Fe2+ metabolism, pre-dates the anaerobic Fe2+ oxidation, whereas the age of the BIF pre-dates the oxygenic photosynthesis pointing the microbial anoxic phototrophic and anaerobic chemolitotrophic metabolism may have been present in the ancient earth, and together with the Fe(III) reducers, they had been the responsible for the BIF in the Pre-Cambrian era

Ferrous Iron Oxidation in the Marine Environment
In the marine environment the most well-known class of iron oxidizing-bacteria (FeOB) is Zetaproteobacteria. These bacteria are generally microaerophilic, they adapted themselves to live in transition zones where the oxic and anoxic waters mix .The Zetaproteobacteria are present in different Fe(II)-rich habitats, found in deep ocean sites associated with hydrothermal activity and in coastal and terrestrial habitats, been reported in the surface of shallow sediments, beach aquifer, and surface water.

Mariprofundus ferrooxydans is one of the most common and well-studied species of Zetaproteobacteria. It was first isolated from the Loihi seamount vent field, near Hawaii at depth between 1100 and 1325 meters, on the summit of this shield volcano. Vents can be found ranging from slightly above ambient (10°C) to high temperature (167°C). The vent waters are rich of CO2, Fe(II) and Mn. Around the vent orifices can be present heavily encrusted large mats with a gelatinous texture created by the FeOB as a by-product (iron-oxyhydroxide precipitation), these areas can be colonized by other bacterial communities, those can able to change the chemical composition and the flow of the local waters. There are two different types of vents at Loihi seamount: one with a focus and high temperature flow (above 50°C) and the other with a cooler (10-30°C) diffuse flow. The former creates mats of some centimetres near the orifices, the latter produces square meters mats 1m thick.

Implication of climate change on FeOB
In open oceans systems that are full of dissolved iron, the IOB is ubiquitously and influences significantly the iron cycle. Nowadays this biogechemical cycle is undergoing highly modifications, due to pollution and climate change, nonetheless the normal distribution of ferrous iron in the ocean could be affected by the global warming under the following conditions : ocean water hypoxia trend, acidification, and the shifting of ocean currents.

These are all consequences of the substantial increase of CO2 emissions into the atmosphere from anthropogenic sources, currently the concentration of carbon dioxide in the atmosphere is around 380 ppm (80 ppm more than 20 million years ago), and about a quarter of the total CO2 emission enters to the oceans (2.2 pg C year-1) and reacting with seawater it produces bicarbonate ion (HCO-3) and thus the increasing ocean acidity.Furthermore, the temperature of the ocean has increased by almost a degree (0.74 ° C) causing the melting of big quantities of glaciers contributing to the sea level rise, thus lowering of O2 solubility by inhibiting the oxygen exchange between surface waters, where the O2 is very abundant, and anoxic deep waters.

All these changes in the marine parameters (temperature, acidity and oxygenation) impact the Iron biogeochemical cycle, and could have several and critical implications on ferrous iron oxidizers microbes, hypoxic and acid conditions could improve primary productivity in the superficial and coastal waters because that would increase the availability of ferrous iron Fe(II) for microbial iron oxidation, but at the same time this scenario could also disrupt cascade effect to the sediment in deep water and cause the death of benthonic animals. Moreover is very important to consider that iron and phosphate cycles are strictly interconnected and balanced, so that a small change in the first could have substantial consequences on the second.