Acidophiles in acid mine drainage

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'Methods of pH homeostasis and energy generation in acidophiles' (with reference to Baker-Austin & Dopson, 2007 and Apel, Dugan, & Tuttle, 1980): (1) Direction of transmembrane electrochemical gradient (pH) and blocking of H+ by the cell membrane; (2) Reversed membrane potential through potassium transport, a modification towards maintaining a stable Donnan potential; (3) Secondary transporter protein; the H+ and Na+ gradient is harnessed to drive transport of nutrients and solutes; (4) Proton pump actively removes H+, balancing the energy gained from the H+ entry to the cytoplasm. (5) Vesicles containing protons avoid acidification of the cytoplasm, but still generate ATP from the electrochemical gradient (in A.ferrooxidans); (6) Uncouplers (uncharged compounds), such as organic acids, permeate the membrane and release their H+, leading to acidification of the cytoplasm; (7) To avoid this, heterotrophic acidophiles may degrade the uncouplers; (8) Alternatively, cytoplasmic enzymes or chemicals may bind or sequester the protons. Methods of ph homeostasis and energy generation in acidophiles.png
'Methods of pH homeostasis and energy generation in acidophiles' (with reference to Baker-Austin & Dopson, 2007 and Apel, Dugan, & Tuttle, 1980): (1) Direction of transmembrane electrochemical gradient (pH) and blocking of H+ by the cell membrane; (2) Reversed membrane potential through potassium transport, a modification towards maintaining a stable Donnan potential; (3) Secondary transporter protein; the H+ and Na+ gradient is harnessed to drive transport of nutrients and solutes; (4) Proton pump actively removes H+, balancing the energy gained from the H+ entry to the cytoplasm. (5) Vesicles containing protons avoid acidification of the cytoplasm, but still generate ATP from the electrochemical gradient (in A.ferrooxidans); (6) Uncouplers (uncharged compounds), such as organic acids, permeate the membrane and release their H+, leading to acidification of the cytoplasm; (7) To avoid this, heterotrophic acidophiles may degrade the uncouplers; (8) Alternatively, cytoplasmic enzymes or chemicals may bind or sequester the protons.

The outflow of acidic liquids and other pollutants from mines is often catalysed by acid-loving microorganisms; these are the acidophiles in acid mine drainage.

Contents

Acidophiles are not just present in exotic environments such as Yellowstone National Park [3] or deep-sea hydrothermal vents. [4] Genera such as Acidithiobacillus and Leptospirillum bacteria, and Thermoplasmatales archaea , are present in syntrophic relationships in the more mundane environments of concrete sewer pipes [5] [6] and implicated in the heavy-metal-containing, sulfurous waters of rivers such as the Rheidol. [7]

Such microorganisms are responsible for the phenomenon of acid mine drainage (AMD) and thus are important both economically and from a conservation perspective. [8] Control of these acidophiles and their harnessing for industrial biotechnology shows their effect need not be entirely negative. [1]

The use of acidophilic organisms in mining is a new technique for extracting trace metals through bioleaching, and offers solutions for acid mine drainage in mining spoils.

Introduction

Upon exposure to oxygen (O2) and water (H2O), metal sulfides undergo oxidation to produce metal-rich acidic effluent. If the pH is low enough to overcome the natural buffering capacity of the surrounding rocks (‘calcium carbonate equivalent’ or ‘acid neutralising capacity’), the surrounding area may become acidic, as well as contaminated with high levels of heavy metals. [9] [10] Though acidophiles have an important place in the iron and sulfur biogeochemical cycles, strongly acidic environments are overwhelmingly anthropogenic in cause, primarily created at the cessation of mining operations where sulfide minerals, such as pyrite (iron disulfide or FeS2), are present. [8]

Acid mine drainage may occur in the mine itself, the spoil heap (particularly colliery spoils from coal mining), or through some other activity that exposes metal sulfides at a high concentration, such as at major construction sites. [11] Banks et al. [7] provide a basic summary of the processes that occur:

2FeS2 Pyrite + 2H2O water + 7O2 oxygen ⇌2Fe2+ferrous iron + 4SO42− sulfate + 4H+ acid

Bacterial influences on acid mine drainage

The oxidation of metal sulfide (by oxygen) is slow without colonization by acidophiles, particularly Acidithiobacillus ferrooxidans (synonym Thiobacillus ferrooxidans). [12] These bacteria can accelerate pyritic oxidation by 106 times. [13] In that study, a proposal for the rate at which A.ferrooxidans can oxidise pyrite is the ability to use ferrous iron to generate a ferric iron catalyst  :

Fe2+ + 1⁄4O2 + H+ → Fe3+ + 1⁄2H2O

Under the above acidic conditions, ferric iron (Fe3+) is a more potent oxidant than oxygen, resulting in faster pyrite oxidation rates.

A.ferrooxidans is a chemolithoautotrophic bacteria, due to the oligotrophic nature (low dissolved organic carbon concentration) of acidic environments, and their lack of illumination for phototrophy. [8] Even when in vadose conditions, A.ferrooxidans can survive, if the rock retains moisture and the mine is aerated. In fact in this situation, with pioneer microorganisms, the limiting factor is likely to be the environmental circumneutral pH, which inhibits many acidophiles' growth. However, favourable geochemical conditions quickly develop with an acidic interface between the bacteria and the mineral surface, and pH is lowered to a level closer to acidophilic optimum. [13]

The process proceeds through A.ferrooxidans exhibiting a quorum level for the trigger of acid mine drainage (AMD). At first colonisation of metal sulfides there is no AMD, and as the bacteria grow into microcolonies, AMD remains absent, then at a certain colony size, the population begins to produce a measurable change in water chemistry, and AMD escalates. [13] This means pH is not a clear measure of a mine's liability to AMD; culturing A.ferrooxidans (or others) gives a definite indication of a future AMD issue. [13]

Other bacteria also implicated in AMD include Leptospirillum ferrooxidans , Acidithiobacillus thiooxidans and Sulfobacillus thermosulfidooxidans . [7]

Archaean acidophiles

Though Pseudomonadota (formerly proteobacteria) display impressive acid tolerance, most retain a circumneutral cytoplasm to avoid denaturation of their acid-labile cell constituents. [1] Archaea such as Ferroplasma acidiphilum , which oxidises ferrous iron, have a number of intracellular enzymes with an optimum similar to that of their external acidic environment. [14] This may explain their ability to survive pH as low as 1.3. [15] The differing cell membranes in archaea compared to the bacteria may hold part of the explanation; ether lipids that link isoprene, compared to Pseudomonadota's di-ester linkage, are central to the difference. [16] Though lacking a cell wall, F. acidiphilum cell membranes contain caldarchaetidylglycerol tetraether lipids, which effectively block almost all proton access, [14] Thermoplasma acidophilum also uses these bulky isoprenoid cores in its phospholipid bilayer. [17]

It is possible that the family Ferroplasmaceae may in fact be more important in AMD than the current paradigm, Acidithiobacillaceae. [14] From a practical viewpoint this changes little, as despite the myriad physiological differences between archaea and bacteria, treatments would remain the same; if pH is kept high, and water and oxygen are prohibited from the pyrite, the reaction will be negligible. [7]

The isolation from solfataric soils of two Picrophilus species of archaea P.oshimae and P.torridus are of note for their record low of survival at pH 0, [18] indicating that further AMD microorganisms may remain to be found which operate at an even lower pH. Though the genus Picrophilus is not known to be involved in AMD, [19] its extreme acidophily is of interest, for instance its proton-resistant liposomes, which could be present in AMD acidophiles. [20]

Interactions in the mine community

Tentatively, there may be examples of syntrophy between acidophilic species, and even cross-domain cooperation between archaea and bacteria. One mutualistic example is the rotation of iron between species; ferrous-oxidising chemolithotrophs use iron as an electron donor, then ferric-reducing heterotrophs use iron as an electron-acceptor. [8]

Another more synergistic behaviour is the faster oxidation of ferrous iron when A.ferrooxidans and Sulfobacillus thermosulfidooxidans are combined in low-CO2 culture. [21] S.thermosulfidooxidans is a more efficient iron-oxidiser, but this is usually inhibited by low-CO2 uptake. A.ferrooxidans has a higher affinity for the gas, but a lower iron oxidation speed, and so can supply S.thermosulfidooxidans for mutual benefit.

The community possesses diversity beyond the bacteria and archaea however; the approximately constant pH present during acid mine drainage make for a reasonably stable environment, with a community that spans a number of trophic levels, and includes obligately acidophilic eukaryotes such as fungi, yeasts, algae and protozoa. [8]

Physiology and biochemistry

Acidophiles display a great range of adaptations to not just tolerating, but thriving in an extreme pH environment (the definition of an acidophile being an organism that has a pH optimum below pH 3). Principal in these is the necessity of maintaining a large pH gradient, to ensure a circumneutral cytoplasm (normally, however not in Picrophilus species). The archaeans have already been discussed above, and further information on their and bacterial adaptations are in basic form in the Figure. To elaborate upon the figure, the bacteria also use membrane proton blocking to maintain a high cytoplasmic pH, which is a passive system as even non-respiring A.ferrooxidans exhibit it. [2] Acidophiles are also able to extrude protons against the pH gradient with unique transport proteins, a process more difficult for moderate- and hyper-thermophiles; a higher temperature causes cell membranes to become more permeable to protons, necessarily leading to increased H+ influx, in the absence of other membrane alterations. [20]

Proton motive force

To grow at low pH, acidophiles must maintain a pH gradient of several pH units across the cellular membrane. [1] Acidophiles harness the strong proton motive force (PMF), caused by the pH gradient across their cell membrane, for ATP production. A large amount of energy is available to the acidophile through proton movement across the membrane, but with it comes cytoplasmic acidity. [1] Instead ions such as sodium can be used as a substitute energy transducer to avoid this pH increase (ATPases are often Na+ linked, rather than H+ linked). [20]

Expelling H+ containing vesicles

Alternatively bacteria can use H+ containing vesicles to avoid cytoplasmic acidity (see Figure), but most require that any H+ taken in must be extruded after use in the electron transport chain (ETC). [1] On the subject of the ETC, an adaptation to living in the mine environment is in the use of different ETC electron acceptors to neutralophiles; sulfur, arsenic, selenium, uranium, iron, and manganese in solid form [22] rather than O2 (most commonly Fe in dissimilatory iron reduction, frequent in AMD).

Genomic adaptations

Genomic adaptations are also present, but not without complications in organisms like Thermoplasmatales archaea, which is both acidophilic and thermophilic. For instance, this Order expresses an increased concentration of purine-containing codons for heat-stability, whilst increasing pyramidine codons in long open reading frames for protection from acid-stress. [1] More generally, and presumably to reduce the chances of an acid-hydrolysis mutation, all obligate hyperacidophiles have truncated genomes when compared to neutralophile microorganisms. Picrophilus torridus, for instance, has the highest coding density of any non-parasitic aerobic microorganism living on organic substrates. [23]

Improved repair

Acidophiles also benefit from improved DNA and protein repair systems such as chaperones involved in protein refolding. [1] The P.torridus genome just mentioned contains a large numbers of genes concerned with repair proteins.

Biotechnology applications

Bioremediation is the primary biotech issue created by the AMD acidophiles. There are a number of methods for dealing with AMD, some crude (such as raising pH through liming, removing water, binding iron with organic wastes) and some less so (application of bactericides, biocontrol with other bacteria/archaea, offsite wetland creation, use of metal-immobilising bacteria, galvanic suppression). A number of other neutralising agents are available (pulverised fuel ash-based grouts, cattle manure, whey, brewer's yeast) many which solve a waste disposal problem from another industry. [7]

As supplies of some metals dwindle, other methods of extraction are being explored, including the use of acidophiles, in a process known as bioleaching. Though slower than conventional methods, the microorganisms (which can also include fungi) enable the exploitation of extremely low grade ores with minimum expense. [24] Projects include nickel extraction with A.ferrooxidans and Aspergillus sp. fungi [24] and sulfur removal from coal with Acidithiobacillus sp.. [25] The extraction can occur at the mine site, from waste water streams (or the main watercourse if the contamination has reached that far), in bioreactors, or at a power station (for instance to remove sulfur from coal before combustion to avoid sulfuric acid rain).

Future of the technique

AMD continues to be important in the River Rheidol, and in the near future further treatment will be needed in the area around Aberystwyth, which contains 38 of the 50 worst polluting metal mines in Wales. [26] [27]

In 2007, the UK government endorsed a return to coal as an energy source [28] and mining in the UK is increasing (for instance the open-cast coal pit at Ffos-y-fran, Merthyr Tydfil). Much preventative work will be required to avoid the AMD associated with the last generation of coal mines.

The fast and efficient protein and DNA repair systems show promise for human medical uses, particularly with regard to cancer and ageing. However further research is required to determine whether these systems really are qualitatively different, and how that can be applied from microorganisms to humans.

As discussed above, acidophiles can have the option to use electron acceptors other than oxygen. Johnson (1998) [8] points out that facultative anaerobism of acidophiles, previously dismissed, could have major implications for AMD control. Further research is needed to determine how far current methods to block oxygen will working, in light of the fact that the reaction may be able to continue anaerobically.

See also

Related Research Articles

Bioleaching is the extraction or liberation of metals from their ores through the use of living organisms. Bioleaching is one of several applications within biohydrometallurgy and several methods are used to treat ores or concentrates containing copper, zinc, lead, arsenic, antimony, nickel, molybdenum, gold, silver, and cobalt.

<span class="mw-page-title-main">Acid mine drainage</span> Outflow of acidic water from metal or coal mines

Acid mine drainage, acid and metalliferous drainage (AMD), or acid rock drainage (ARD) is the outflow of acidic water from metal mines and coal mines.

<i>Acidithiobacillus</i> Genus of bacteria

Acidithiobacillus is a genus of the Acidithiobacillia in the phylum "Pseudomonadota". This genus includes ten species of acidophilic microorganisms capable of sulfur and/or iron oxidation: Acidithiobacillus albertensis, Acidithiobacillus caldus, Acidithiobacillus cuprithermicus, Acidithiobacillus ferrianus, Acidithiobacillus ferridurans, Acidithiobacillus ferriphilus, Acidithiobacillus ferrivorans, Acidithiobacillus ferrooxidans, Acidithiobacillus sulfuriphilus, and Acidithiobacillus thiooxidans.A. ferooxidans is the most widely studied of the genus, but A. caldus and A. thiooxidans are also significant in research. Like all "Pseudomonadota", Acidithiobacillus spp. are Gram-negative and non-spore forming. They also play a significant role in the generation of acid mine drainage; a major global environmental challenge within the mining industry. Some species of Acidithiobacillus are utilized in bioleaching and biomining. A portion of the genes that support the survival of these bacteria in acidic environments are presumed to have been obtained by horizontal gene transfer.

<span class="mw-page-title-main">Sulfur-reducing bacteria</span> Microorganisms able to reduce elemental sulfur to hydrogen sulfide

Sulfur-reducing bacteria are microorganisms able to reduce elemental sulfur (S0) to hydrogen sulfide (H2S). These microbes use inorganic sulfur compounds as electron acceptors to sustain several activities such as respiration, conserving energy and growth, in absence of oxygen. The final product of these processes, sulfide, has a considerable influence on the chemistry of the environment and, in addition, is used as electron donor for a large variety of microbial metabolisms. Several types of bacteria and many non-methanogenic archaea can reduce sulfur. Microbial sulfur reduction was already shown in early studies, which highlighted the first proof of S0 reduction in a vibrioid bacterium from mud, with sulfur as electron acceptor and H
2 as electron donor. The first pure cultured species of sulfur-reducing bacteria, Desulfuromonas acetoxidans, was discovered in 1976 and described by Pfennig Norbert and Biebel Hanno as an anaerobic sulfur-reducing and acetate-oxidizing bacterium, not able to reduce sulfate. Only few taxa are true sulfur-reducing bacteria, using sulfur reduction as the only or main catabolic reaction. Normally, they couple this reaction with the oxidation of acetate, succinate or other organic compounds. In general, sulfate-reducing bacteria are able to use both sulfate and elemental sulfur as electron acceptors. Thanks to its abundancy and thermodynamic stability, sulfate is the most studied electron acceptor for anaerobic respiration that involves sulfur compounds. Elemental sulfur, however, is very abundant and important, especially in deep-sea hydrothermal vents, hot springs and other extreme environments, making its isolation more difficult. Some bacteria – such as Proteus, Campylobacter, Pseudomonas and Salmonella – have the ability to reduce sulfur, but can also use oxygen and other terminal electron acceptors.

Lithotrophs are a diverse group of organisms using an inorganic substrate to obtain reducing equivalents for use in biosynthesis or energy conservation via aerobic or anaerobic respiration. While lithotrophs in the broader sense include photolithotrophs like plants, chemolithotrophs are exclusively microorganisms; no known macrofauna possesses the ability to use inorganic compounds as electron sources. Macrofauna and lithotrophs can form symbiotic relationships, in which case the lithotrophs are called "prokaryotic symbionts". An example of this is chemolithotrophic bacteria in giant tube worms or plastids, which are organelles within plant cells that may have evolved from photolithotrophic cyanobacteria-like organisms. Chemolithotrophs belong to the domains Bacteria and Archaea. The term "lithotroph" was created from the Greek terms 'lithos' (rock) and 'troph' (consumer), meaning "eaters of rock". Many but not all lithoautotrophs are extremophiles.

<i>Ferroplasma</i> Genus of archaea

Ferroplasma is a genus of Archaea that belong to the family Ferroplasmaceae. Members of the Ferroplasma are typically acidophillic, pleomorphic, irregularly shaped cocci.

<i>Picrophilus</i> Genus of archaea

In taxonomy, Picrophilus is an archaean genus of the family Picrophilaceae.

Microbial metabolism is the means by which a microbe obtains the energy and nutrients it needs to live and reproduce. Microbes use many different types of metabolic strategies and species can often be differentiated from each other based on metabolic characteristics. The specific metabolic properties of a microbe are the major factors in determining that microbe's ecological niche, and often allow for that microbe to be useful in industrial processes or responsible for biogeochemical cycles.

Acidophiles or acidophilic organisms are those that thrive under highly acidic conditions. These organisms can be found in different branches of the tree of life, including Archaea, Bacteria, and Eukarya.

Biomining refers to any process that uses living organisms to extract metals from ores and other solid materials. Typically these processes involve prokaryotes, however fungi and plants may also be used. Biomining is one of several applications within biohydrometallurgy with applications in ore refinement, precious metal recovery, and bioremediation. The largest application currently being used is the treatment of mining waste containing iron, copper, zinc, and gold allowing for salvation of any discarded minerals. It may also be useful in maximizing the yields of increasingly low grade ore deposits. Biomining has been proposed as a relatively environmentally friendly alternative and/or supplementation to traditional mining. Current methods of biomining are modified leach mining processes. These aptly named bioleaching processes most commonly includes the inoculation of extracted rock with bacteria and acidic solution, with the leachate salvaged and processed for the metals of value. Biomining has many applications outside of metal recovery, most notably is bioremediation which has already been used to clean up coastlines after oil spills. There are also many promising future applications, like space biomining, fungal bioleaching and biomining with hybrid biomaterials.

<span class="mw-page-title-main">Iron Mountain Mine</span>

Iron Mountain Mine, also known as the Richmond Mine at Iron Mountain, is a mine near Redding in Northern California, US. Geologically classified as a "massive sulfide ore deposit", the site was mined for iron, silver, gold, copper, zinc, quartz, and pyrite intermittently from the 1860s until 1963. The mine is the source of extremely acidic mine drainage which also contains large amounts of zinc, copper, and cadmium. One of America's most toxic waste sites, it has been listed as a federal Superfund site since 1983.

<i>Acidithiobacillus ferrooxidans</i> Species of bacterium

Acidithiobacillus ferrooxidans is a bacterium that sustains its life cycle at extremely low pH values, and it is one of the very few organisms that gain energy from oxidating ferrous iron. It can make copper from ores water-soluble, and it can sequester both carbon and nitrogen from the atmosphere.

<i>Ferroplasma acidiphilum</i> Species of archaeon

Ferroplasma acidiphilum is an acidophilic, autotrophic, ferrous iron-oxidizing, cell wall-lacking, mesophilic member of the Ferroplasmaceae. F. acidophilum is a mesophile with a temperature optimum of approximately 35 °C, growing optimally at a pH of 1.7. F. acidophilum is generally found in acidic mine tailings, primarily those containing pyrite (FeS2). It is especially abundant in cases of severe acid mine drainage, where other organisms such as Acidithiobacillus and Leptospirillum lower the pH of the environment to the extent that F. acidophilum is allowed to flourish.

Leptospirillum ferriphilum is an iron-oxidising bacterium able to exist in environments of high acidity, high iron concentrations, and moderate to moderately high temperatures. It is one of the species responsible for the generation of acid mine drainage and the principal microbe used in industrial biohydrometallurgy processes to extract metals.

Acidithiobacillus caldus formerly belonged to the genus Thiobacillus prior to 2000, when it was reclassified along with a number of other bacterial species into one of three new genera that better categorize sulfur-oxidizing acidophiles. As a member of the Gammaproteobacteria class of Pseudomonadota, A. caldus may be identified as a Gram-negative bacterium that is frequently found in pairs. Considered to be one of the most common microbes involved in biomining, it is capable of oxidizing reduced inorganic sulfur compounds (RISCs) that form during the breakdown of sulfide minerals. The meaning of the prefix acidi- in the name Acidithiobacillus comes from the Latin word acidus, signifying that members of this genus love a sour, acidic environment. Thio is derived from the Greek word thios and describes the use of sulfur as an energy source, and bacillus describes the shape of these microorganisms, which are small rods. The species name, caldus, is derived from the Latin word for warm or hot, denoting this species' love of a warm environment.

<i>Acidithiobacillus thiooxidans</i> Species of bacterium

Acidithiobacillus thiooxidans, formerly known as Thiobacillus thiooxidans until its reclassification into the newly designated genus Acidithiobacillus of the Acidithiobacillia subclass of Pseudomonadota, is a Gram-negative, rod-shaped bacterium that uses sulfur as its primary energy source. It is mesophilic, with a temperature optimum of 28 °C. This bacterium is commonly found in soil, sewer pipes, and cave biofilms called snottites. A. thiooxidans is used in the mining technique known as bioleaching, where metals are extracted from their ores through the action of microbes.

<span class="mw-page-title-main">Acidithiobacillia</span> Class of bacteria

Acidithiobacillia is a class of the phylum Pseudomonadota ("Proteobacteria"). Its type order, the Acidithiobacillales, was formerly classified within the Gammaproteobacteria, and comprises two families of sulfur-oxidising autotrophs, the Acidithiobacillaceae and the Thermithiobacillaceae, which in turn include the genera Acidithiobacillus and Thermithiobacillus.

Metallosphaera sedula is a species of Metallosphaera that is originally isolated from a volcanic field in Italy. Metallosphaera sedula can be roughly translated into “metal mobilizing sphere” with the word “sedulus” meaning busy, describing its efficiency in mobilizing metals. M. sedula is a highly thermoacidophilic Archaean that is unusually tolerant of heavy metals.

Acidithrix ferrooxidans is a heterotrophic, acidophilic and Gram-positive bacterium from the genus Acidithrix. The type strain of this species, A. ferrooxidans Py-F3, was isolated from an acidic stream draining from a copper mine in Wales. This species grows in a variety of acidic environments such as streams, mines or geothermal sites. Mine lakes with a redoxcline support growth with ferrous iron as the electron donor. "A. ferrooxidans" grows rapidly in macroscopic streamer, producing greater cell densities than other streamer-forming microbes. Use in a bioreactors to remediate mine waste has been proposed due to cell densities and rapid oxidation of ferrous iron oxidation in acidic mine drainage. Exopolysaccharide production during metal substrate metabolism, such as iron oxidation helps to prevent cell encrustation by minerals.

<span class="mw-page-title-main">Microbial oxidation of sulfur</span>

Microbial oxidation of sulfur is the oxidation of sulfur by microorganisms to build their structural components. The oxidation of inorganic compounds is the strategy primarily used by chemolithotrophic microorganisms to obtain energy to survive, grow and reproduce. Some inorganic forms of reduced sulfur, mainly sulfide (H2S/HS−) and elemental sulfur (S0), can be oxidized by chemolithotrophic sulfur-oxidizing prokaryotes, usually coupled to the reduction of oxygen (O2) or nitrate (NO3−). Anaerobic sulfur oxidizers include photolithoautotrophs that obtain their energy from sunlight, hydrogen from sulfide, and carbon from carbon dioxide (CO2).

References

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  11. ↑ Akcil, Ata; Koldas, Soner (2006). "Acid Mine Drainage (AMD): Causes, treatment and case studies". Journal of Cleaner Production. 14 (12–13): 1139–45. doi:10.1016/j.jclepro.2004.09.006.
  12. ↑ Kelly, D. P.; Wood, A. P. (2000). "Reclassification of some species of Thiobacillus to the newly designated genera Acidithiobacillus gen. nov., Halothiobacillus gen. nov. and Thermithiobacillus gen. nov". International Journal of Systematic and Evolutionary Microbiology. 50 (2): 511–6. doi: 10.1099/00207713-50-2-511 . PMID   10758854.
  13. 1 2 3 4 Mielke, Randall E.; Pace, Danielle L.; Porter, Tim; Southam, Gordon (2003). "A critical stage in the formation of acid mine drainage: Colonization of pyrite by Acidithiobacillus ferrooxidans under pH-neutral conditions". Geobiology. 1: 81–90. doi:10.1046/j.1472-4669.2003.00005.x. S2CID   129323041.
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