Iron-oxidizing bacteria

Iron-oxidizing bacteria are chemotrophic bacteria that derive the energy they need to live and multiply by oxidizing dissolved ferrous 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 oxidation.[1]

Iron-oxidizing bacteria in surface water

Iron is a very important element required by living organisms to carry out numerous metabolic reactions such as the formation of proteins involved in biochemical reactions, like iron–sulfur proteins, 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. Iron is one of the trace elements in marine environments.[2] 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. Iron is the most common limiting element that has a key role in structuring phytoplankton communities and determining its abundance; it's particularly important in the high-nutrient, low-chlorophyll regions, where the presence of micronutrients is mandatory for the total primary production,[3] and iron is considered one of those limiting factors.

A layman's description

When de-oxygenated water reaches a source of oxygen, these commonly called iron bacteria convert dissolved iron into an insoluble reddish-brown gelatinous slime that discolors stream beds or can stain plumbing fixtures, and clothing or utensils washed with the water carrying it.[4] 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 that groundwater has historically emerged to be exposed to atmospheric oxygen.[5] 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.[6] 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 percent) in comparison to manganese (0.1 percent) in average soils.[7] The sulfurous smell of rot or decay sometimes associated with iron-oxidizing bacteria results from enzymatic conversion of soil sulfates to volatile hydrogen sulfide as an alternative source of oxygen in anaerobic water.[8]

Habitat and iron-oxidizing bacterial groups

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, the pH variation plays an important role on driving the oxidation reaction of Fe2+/Fe3+,[2][9] at neutrophilic pH (hydrothermal vents, deep ocean basalts, groundwater iron seeps) the oxidation of iron by microorganisms is highly competitive with the rapid abiotic reaction (occurs in <1 min),[10] 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.[11][12] However, under acidic conditions only biological processes are responsible for the oxidation of ferrous,[13] where 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[14][2]

Despite being phylogenetically diverse, the microbial ferrous iron oxidation metabolic strategy (found in Archaea and Bacteria) is present in 7 phyla, being highly pronounced into the Proteobacteria phyla (Alpha, Beta, Gamma and Zetaproteobacteria classes),[15][14] and among the Archae domain in the Euryarchaeota and Chrenarcaeota phyla, also in Actinobacteria, Firmicutes, Chlorobi and Nitrosospirae phyla[14]

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 found 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 past of the earth.[16]

Ferrous iron oxidation and the early life

Unlike most lithotrophic 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) compared to other chemolithotrophic metabolisms,[14] 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).[2][17] The aerobic iron-oxidizing bacterial metabolism was known to have a remarkable contribution to the formation of the largest iron deposit (banded iron formation (BIF)) due to the advent of oxygen in the atmosphere 2.7Ga ago (by the cyanobacteria).[9]

However, with the discovery of Fe(II) oxidation carried out within anoxic conditions in the late 1990s [18] by using the light as energy source or chemolithotrophically, using a different terminal electron acceptor (mostly NO3−),[13] 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 [2] pointing the microbial anoxic phototrophic and anaerobic chemolithotrophic 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[13]

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 oxidation metabolism, the photoferrotrophic bacteria use Fe2+ as electron donor and the energy from the light to assimilate CO2 into biomass through the Calvin Benson-Bassam cycle (or rTCA cycle) in 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):[2][13]

HCO−
3 + 4Fe(II) + 10H
2O → [CH
2O] + 4Fe(OH)
3 + 7H+ (∆G°>0)

Nevertheless, some bacteria do not use the photoautotrophic Fe(II) oxidation metabolism for growth purposes [15] instead it's suggested that these groups are sensitive to Fe(II) therefore they oxidize Fe(II) into more insoluble Fe(III) oxide to reduce its toxicity, enabling them to grow in the presence of Fe(II),[15] 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 access organic carbon sources (acetate, succinate) on which the use depend on Fe(II) oxidation [19] Nonetheless many Iron-oxidizer bacteria, can use other compounds as electron donors in addition to Fe (II), or even perform dissimilatory Fe(III) reduction as the Geobacter metallireducens [15]

The dependence of photoferrotrophics on light as a crucial resource,[20][13][9] can take the bacteria to a cumbersome situation, where due to their requirement for anoxic lighted regions (near the surface)[13] 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 perform the photoferrotrophic Fe(II) oxidation deeper in the sediment/water column, with a low light availability.[13]

Nitrate-dependent Fe(II) oxidation

Light penetration can limit the Fe(II) oxidation in the water column [20] 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 in within the water column at the OMZ.[21][9] Microbes that perform this metabolism are successful in neutrophilic or alcaline environments, 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 [15][22]

2Fe2+ + NO−
3 + 5H
2O → 2Fe(OH)
3 + NO−
2 + 4H+ (∆G°=-103.5kJ/mol)

The microbial oxidation of ferrous iron couple to denitrification (with nitrite, or dinitrogen gas being the final product) [2] can be autotrophic using inorganic carbon or organic cosubstrates (acetate, butyrate, pyruvate, ethanol) performing heterotrophic growth in the absence of inorganic carbon,[15][22] it's suggested that the heterotrophic nitrate-dependent ferrous iron oxidation using organic carbon might be the most favorable process.[18] This metabolism might be very important on carrying a important step in the bioeochemical cycle within the OMZ.[23]

Ferrous iron oxidizers in the marine environment

In the marine environment, the most well-known class of iron oxidizing-bacteria is zetaproteobacteria.[24] They are the major players in marine ecosystems, being generally microaerophilic they are adapted to live in transition zones where the oxic and anoxic waters mix.[11] 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 [14] at a 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.[25] Around the vent orifices can be present heavily encrusted large mats with a gelatinous texture created by iron-oxidizing bacteria 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.[26] 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 centimeters near the orifices, the latter produces square meters mats 1m thick.[14]

The implication of climate change on iron-oxidizing bacteria

In open oceans systems that are full of dissolved iron, iron-oxidizing bacterial metabolism is ubiquitous and influences the iron cycle. Nowadays this biochemical cycle is undergoing 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: acidification, shifting of ocean currents and ocean water and groundwater hypoxia trend. [10]

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.[27][28]

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.[29]

Influence on water infrastructure

Iron-oxidizing bacteria can pose an issue for the management of water-supply wells, as they can produce insoluble ferric oxide, which appears as brown gelatinous slime that will stain plumbing fixtures, and clothing or utensils washed with the water carrying it.

Possible indicators
A burn in Scotland with iron-oxidizing bacteria.

The dramatic effects of iron bacteria are seen in surface waters as brown slimy masses on stream bottoms and lakeshores or as an oily sheen upon the water. More serious problems occur when bacteria build up in well systems. Iron bacteria in wells do not cause health problems, but they can reduce well yields by clogging screens and pipes.

Control

Treatment techniques that may be successful in removing or reducing iron bacteria include physical removal, pasteurization, and chemical treatment. Treatment of heavily infected wells may be difficult, expensive, and only partially successful. Recent application of ultrasonic devices that destroy and prevent the formation of biofilm in wells has been proven to prevent iron bacteria infection and the associated clogging very successful.

Physical removal is typically done as a first step. Small diameter pipes are sometimes cleaned with a wire brush, while larger lines can be scrubbed and flushed clean with a sewer jetter. The pumping equipment in the well must also be removed and cleaned.

Iron filters have been used to treat iron bacteria. Iron filters are similar in appearance and size to conventional water softeners but contain beds of media that have mild oxidizing power. As the iron-bearing water is passed through the bed, any soluble ferrous iron is converted to the insoluble ferric state and then filtered from the water. Any previously precipitated iron is removed by simple mechanical filtration. Several different filter media may be used in these iron filters, including manganese greensand, Birm, MTM, multi-media, sand, and other synthetic materials. In most cases, the higher oxides of manganese produce the desired oxidizing action. Iron filters do have limitations. Since the oxidizing action is relatively mild, it will not work well when organic matter, either combined with the iron or completely separate, is present in the water and iron bacteria will not be killed. Extremely high iron concentrations may require inconvenient frequent backwashing and/or regeneration. Finally, iron filter media requires high flow rates for proper backwashing and such water flows are not always available.

Wildfires may release iron-containing compounds from the soil into small wildland streams and cause a rapid but usually temporary proliferation of iron-oxidizing bacteria complete with orange coloration, the gelatinous mats, and sulphurous odors. Higher quality personal filters typically used in backpacking/trekking can successfully remove bacteria, odor, and restore water clarity. [30]

See also

References
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