''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 Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists ...

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 Mining is the Resource extraction, extraction of valuable geological materials and minerals from the surface of the Earth. Mining is required to obtain most materials that cannot be grown through agriculture, agricultural processes, or feasib ...

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 Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the e ...

Genus ''Acidithiobacillus''

''Acidithiobacillus'' are chemolithoautotrophs that can occur as acidophilic,

mesophilic A mesophile is an organism that grows best in moderate temperature, neither too hot nor too cold, with an optimum growth range from . The optimum growth temperature for these organisms is 37 °C (about 99 °F). The term is mainly applied ...

, or mesothermophilic. ''Acidithiobacillus caldus'' can also grow mixotrophically. Currently, the genus comprises ten species which are capable of obtaining energy by oxidizing sulfur compounds, with certain species also utilizing both ferrous and ferric iron. Some species have also evolved to use hydrogen and nitrogen from the environment. They assimilate carbon from carbon dioxide using the transaldolase variant of the Calvin-Benson-Bassham cycle. The genus comprises motile, rod-shaped cells that can be isolated from low pH environments including low pH microenvironments on otherwise neutral mineral grains.

Phylogeny

The order

Acidithiobacillales The ''Acidithiobacillales'' are an order of bacteria within the class '' Acidithiobacillia'' and comprises the genera ''Acidithiobacillus'' and ''Thermithiobacillus''. Originally, both were included in the genus '' Thiobacillus'', but they are no ...

(i.e. ''Thermithiobacillus'') were formerly members of the ''Gammaproteobacteria'', with considerable debate regarding their position and that they could also fall within the ''Betaproteobacteria'', but the situation was resolved by whole-genome alignment studies and both genera have been reclassified to the new class ''Acidithiobacillia''. Some members of this genus were classified as '' Thiobacillus'' spp., before they were reclassified in 2000. * ''Acidithiobacillus ferrooxidans'' (basonym ''Thiobacillus ferrooxidans'') can be isolated from iron-sulfur minerals such as pyrite deposits, oxidising

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

and

sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...

as energy sources to support autotrophic growth and producing ferric iron and

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

. * ''Acidithiobacillus thiooxidans'' (basonym ''Thiobacillus thiooxidans'', ''Thiobacillus concretivorus'') oxidises

and produces

; first isolated from the soil, it has also been observed causing biogenic sulfide corrosion of concrete sewer pipes by altering hydrogen sulfide in sewage gas into sulfuric acid.

Bioleaching

Species within ''Acidothiobacillus'' are used in the biohydrometallurgy industry in methods called bioleaching and biomining, whereby metals are extracted from their ores through bacterial

oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

. Biomining uses radioactive waste as an ore with the bacteria to obtain gold, platinum, polonium, radon, radium, uranium, neptunium, americium, nickel, manganese, bromine, mercury, and their isotopes. ''Acidithiobacillus ferrooxidans'' has emerged as an economically significant bacterium in the field of biohydrometallurgy, in the leaching of sulfide ores since its discovery in 1950 by Colmer, Temple and Hinkle. The discovery of ''A. ferrooxidans'' led to the development of biohydrometallurgy, which deals with all aspects of microbial mediated extraction of metals from minerals or solid wastes and acid mine drainage. ''A. ferrooxidans'' has been proven as a potent leaching organism, for dissolution of metals from low-grade sulfide ores. Recently, the attention has been focused upon the treatment of mineral concentrates, as well as complex sulfide ores using batch or continuous-flow reactors. ''Acidithiobacillus ferrooxidans'' is commonly found in acid mine drainage and mine tailings. The oxidation of ferrous iron and reduced sulfur oxyanions, metal sulfides and elementary sulfur results in the production of ferric sulfate in sulfuric acid, this in turn causes the solubilization of metals and other compounds. As a result, ''A. ferrooxidans'' may be of interest for bioremediation processes. ''Acidithiobacillus'' is also commonly abundant upon inner surfaces of sewers in areas exhibiting corrosion; genetic sequencing identifies ''Acidothiobacillus thiooxidans'' as the usual species present, although it is occasionally absent from such locations.

Morphology

''Acidithiobacillus'' spp. occur as single cells or occasionally in pairs or chains, depending on growth conditions. Highly motile species have been described, as well as nonmotile ones. Motile strains have a single flagellum with the exception of ''A. albertensis'', which has a tuft of polar flagella and a glycocalyx. Nitrogen fixation also is an important ecological function carried out by some species in this genus, as is growth using molecular hydrogen as a source of energy - neither property is found in every species. Ferric iron can be used by some species as a terminal electron acceptor.

Evolution

''Acidithiobacillus'' spp. are known to inhabit diverse environments such as hot springs, acid mine drainage ( abandoned mine drainage) or mine tailings, acidic soils, and sulfidic caves. Terrestrial hot springs are currently an important research focus as they can provide known limiting conditions for the genus, but host microbial communities in which ''Acidithiobacillus'' are sometimes present. Optimum pH conditions for these bacteria vary among species, but some have been observed at the genus level in pH conditions as high as 8.94 and temperatures as high as 97.6 °C. All species of ''Acidithiobacillus'' can grow under pH and temperature conditions between 0.5 and 6.0, and 5 °C to 52 °C. They are highly tolerant of heavy metals and can flourish in environments where high concentrations of these metals are present. To obtain energy, they have evolved to couple sulfur oxidation to molecular oxygen but can also use other resources around them as electron donors or acceptors. They have adapted to living in these environments through

, but the basis by which they can survive in low pH environments likely evolved through vertical gene transfer. It is probable that the foundational genes of acid resistance in ''Acidithiobacillus'' were first inherited from a neutrophile, possibly thermophilic, and throughout their evolutionary history further acid resistance genes were obtained from neighboring

acidophile Acidophiles or acidophilic organisms are those that thrive under highly acidic conditions (usually at pH 5.0 or below). These organisms can be found in different branches of the Tree of life (biology), tree of life, including Archaea, Bacteria,Bec ...

s. While the trait of sulfur oxidation is ubiquitous among the genus, iron oxidation is specific to ''A. ferrooxidans, A. ferridurans, A. ferriphilus, A. ferrivorans, and A. ferrianus.'' The transition to modern day ''Acidithiobacillus'' spp. has occurred over hundred of millions of years involving events of gene gain and gene loss. Some evidence points to the most recent common ancestor of ''Acidithiobacillus'' appearing around the same time as ''A. caldus'', 800 million years ago. ''Acidithiobacillus'' is a significantly diverse genus, species have adapted to survive in differing environments under varying limitations such as acidity, temperature, and nutrient availability. For example ''A. caldus,'' which is the only known thermoacidophile of the genus, is adept to survive in extreme temperatures up to 52 °C, while ''A. ferrooxidans'' can survive under extremely acidic conditions with pH <1. Metabolic traits of the '' Acidithiobacillia'' class include the presence of enzymes which aid in the use of hydrogen sulfide, elemental sulfur, thiosulfate, and tetrathionate in sulfur metabolism. Species capable of iron oxidation also possess genes that are coded for nitrogen fixation and hydrogen utilization. The diversity in genomic composition allows these same species to inhabit both aerobic and anaerobic environments.

References

External links

''Acidithiobacillus ferrooxidans ATCC 23270'' Genome Page

''Thiobacillus'' sp.Type strain of ''Acidithiobacillus ferrooxidans'' at Bac''Dive'' - the Bacterial Diversity Metadatabase
{{Taxonbar, from=Q142671 Acidithiobacillia Bacteria genera