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Iron-oxidizing bacteria (or iron bacteria) are chemotrophic bacteria that derive energy by oxidizing dissolved iron. They 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 the oxidation. When de-oxygenated water reaches a source of oxygen, iron bacteria convert dissolved iron into an insoluble reddish-brown gelatinous slime that discolors stream beds and can stain plumbing fixtures, clothing, or utensils washed with the water carrying it. Organic material dissolved in water is often the underlying cause of an iron-oxidizing bacteria population. Groundwater may be naturally de-oxygenated by decaying vegetation in swamps. Useful mineral deposits of bog iron ore have formed where groundwater has historically emerged and been exposed to atmospheric oxygen. Anthropogenic hazards like landfill leachate, septic drain fields, or leakage of light petroleum fuels like gasoline are other possible sources of organic materials allowing soil microbes to de-oxygenate groundwater. A similar reaction may form black deposits of manganese dioxide from dissolved manganese but is less common because of the relative abundance of iron (5.4%) in comparison to manganese (0.1%) in average soils. The sulfurous smell of rot or decay sometimes associated with iron-oxidizing bacteria results from the enzymatic conversion of soil sulfates to volatile hydrogen sulfide as an alternative source of oxygen in anaerobic water. Iron is a very important chemical element required by living organisms to carry out numerous metabolic reactions such as the formation of proteins involved in biochemical reactions. Examples of these proteins include iron–sulfur proteins, hemoglobin, and coordination complexes. Iron has a widespread distribution globally and is considered one of the most abundant elements in the Earth's crust, soil, and sediments. Iron is a trace element in marine environments. Its role as the electron donor of some chemolithotrophs is probably very ancient. Metabolism The anoxygenic phototrophic iron oxidation was the first anaerobic metabolism to be described within the iron anaerobic oxidation metabolism. The photoferrotrophic bacteria use Fe2+ as electron donor and the energy from light to assimilate CO2 into biomass through the Calvin Benson-Bassam cycle (or rTCA cycle) in a neutrophilic environment (pH 5.5-7.2), producing Fe3+ oxides as a waste product that precipitates as a mineral, according to the following stoichiometry (4 mM of Fe(II) can yield 1 mM of CH2O): HCO−3 + 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 has been suggested that these groups are sensitive to Fe(II) and therefore oxidize 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), it has been demonstrated that the oxidation of Fe(II) might be the mechanisms whereby the bacteria is enabled to access organic carbon sources (acetate, succinate) whose use depends on Fe(II) oxidation Nonetheless, many iron-oxidizing bacteria can use other compounds as electron donors in addition to Fe(II), or even perform dissimilatory Fe(III) reduction as the Geobacter metallireducens. The dependence of photoferrotrophics 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 by abiotic reactions due to the presence of molecular oxygen. To avoid this problem, they tolerate microaerophilic surface conditions or perform the photoferrotrophic Fe(II) oxidation deeper in the sediment/water column, with low light availability. Light penetration can limit the Fe(II) oxidation in the water column. However, nitrate dependent microbial Fe(II) oxidation is a light independent metabolism that has been shown 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 within the water column at the oxygen minimum zone. Microbes that perform this metabolism are successful in neutrophilic or alcaline environments, due to the high difference in between the redox potential of the couples Fe2+/Fe3+ and NO3−/NO2− (+200 mV and +770 mV, respectively) releasing a lot of free energy when compared to other iron oxidation metabolisms. 2Fe2+ + NO−3 + 5H2O → 2Fe(OH)3 + NO−2 + 4H+ (∆G°=-103.5 kJ/mol) The microbial oxidation of ferrous iron coupled to denitrification (with nitrite or dinitrogen gas being the final product) can be autotrophic using inorganic carbon or organic co-substrates (acetate, butyrate, pyruvate, ethanol) performing heterotrophic growth in the absence of inorganic carbon. It has been suggested that the heterotrophic nitrate-dependent ferrous iron oxidation using organic carbon might be the most favorable process. This metabolism might be very important for carrying out an important step in the biogeochemical cycle within the OMZ. Types Despite being phylogenetically diverse, the microbial ferrous iron oxidation metabolic strategy (found in Archaea and Bacteria) is present in 7 phyla, being highly pronounced in the phylum Pseudomonadota (formerly Proteobacteria), particularly the Alpha, Beta, Gamma, and Zetaproteobacteria classes, and among the Archaea domain in the "Euryarchaeota" and Thermoproteota phyla, as well as in Actinomycetota, Bacillota, Chlorobiota, and Nitrospirota phyla. There are very well-studied iron-oxidizing bacterial species 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 detected in a sample of water, indicating the presence iron-oxidizing bacteria. This biosignature has been a tool to understand the importance of iron metabolism in the Earth's past. Habitat Iron-oxidizing bacteria colonize the transition zone where de-oxygenated water from an anaerobic environment flows into an aerobic environment. Groundwater containing dissolved organic material may be de-oxygenated by microorganisms feeding on that dissolved organic material. In aerobic conditions, pH variation plays an important role in driving the oxidation reaction of Fe2+/Fe3+. At neutrophilic pHs (hydrothermal vents, deep ocean basalts, groundwater iron seeps) the oxidation of iron by microorganisms is highly competitive with the rapid abiotic reaction occurring in <1 min. Therefore, the microbial community has to inhabit microaerophilic regions.... Discover the Emily J Edwards popular books. Find the top 100 most popular Emily J Edwards books.