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 The tree of life is a fundamental archetype in many of the world's mythology, mythological, religion, religious, and philosophy, philosophical traditions. It is closely related to the concept of the sacred tree.Giovino, Mariana (2007). ''The ...

, including

Archaea Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...

Bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

,Becker, A.
Types of Bacteria Living in Acidic pH"
Retrieved 10 May 2017. and

Eukarya The eukaryotes ( ) constitute the domain of Eukaryota or Eukarya, organisms whose cells have a membrane-bound nucleus. All animals, plants, fungi, seaweeds, and many unicellular organisms are eukaryotes. They constitute a major group of l ...

Examples

A list of these organisms includes:

Archaea

Sulfolobales Sulfolobales is an order of archaeans in the class Thermoprotei. Phylogeny The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) and National Center for Biotechnology Information (NCBI) ...

, an order in the

Thermoproteota The Thermoproteota are prokaryotes that have been classified as a phylum (biology), phylum of the domain Archaea. Initially, the Thermoproteota were thought to be sulfur-dependent extremophiles but recent studies have identified characteristic T ...

branch of

Thermoplasmatales Thermoplasmatales is an order (biology), order of archaeans in the class Thermoplasmata. All are acidophiles, growing optimally at pH below 2. ''Picrophilus'' is currently the most acidophile, acidophilic of all known organisms, being capable of ...

, an order in the

Euryarchaeota Methanobacteriota is a phylum in the domain Archaea. Taxonomy The phylum ''Methanobacteriota'' was introduced to prokaryotic nomenclature in 2023. It contains following classes: *Archaeoglobi Garrity & Holt (2002) *Halobacteria Grant ''et al ...

branch of Archaea :*

ARMAN Arman (November 17, 1928 – October 22, 2005) was a French and American artist. Born Armand Fernandez in Nice, France, Arman was a painter who moved from using objects for the ink or paint traces they leave (''cachets'', ''allures d'objet'') t ...

, in the Euryarchaeota branch of Archaea :* ''

Acidianus ''Acidianus'' is a genus of archaeans in the family Sulfolobaceae. Phylogeny The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) and National Center for Biotechnology Information (NCBI) ...

brierleyi, A. infernus'', facultatively anaerobic thermoacidophilic archaebacteria :* ''

Halarchaeum acidiphilum Halobacteriaceae is a family in the order Halobacteriales and the domain Archaea. ''Halobacteriaceae'' represent a large part of halophilic Archaea, along with members in two other methanogenic families, ''Methanosarcinaceae'' and ''Methanocalcul ...

'', acidophilic member of the Halobacteriacaeae :* ''

Metallosphaera ''Metallosphaera'' is a genus of archaeans in the family Sulfolobaceae. Phylogeny The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) and National Center for Biotechnology Information ( ...

sedula'', thermoacidophilic

Bacteria

Acidobacteriota Acidobacteriota is a phylum of Gram-negative bacteria. Its members are physiologically diverse and ubiquitous, especially in soils, but are under-represented in culture. Description Members of this phylum are physiologically diverse, and can be ...

, a phylum of

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 ...

, an order of

Pseudomonadota Pseudomonadota (synonym "Proteobacteria") is a major phylum of gram-negative bacteria. Currently, they are considered the predominant phylum within the domain of bacteria. They are naturally found as pathogenic and free-living (non- parasitic) ...

e.g. ''A. ferrooxidans, A. thiooxidans'' :*''

Thiobacillus ''Thiobacillus'' is a genus of Gram-negative Betaproteobacteria. ''Thiobacillus thioparus'' is the type species of the genus, and the type strain thereof is the StarkeyT strain, isolated by Robert Starkey in the 1930s from a field at Rutgers U ...

prosperus, T. acidophilus, T. organovorus, T. cuprinus'' :*'' Acetobacter aceti'', a bacterium that produces

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compone ...

(vinegar) from the oxidation of ethanol. :*''

Alicyclobacillus ''Alicyclobacillus'' is a genus of Gram-variable, rod-shaped, spore-forming bacteria. The bacteria are able to grow in acidic conditions, while the spores are able to survive typical pasteurization procedures. Overview ''Alicyclobacilli'' are ...

'', a genus of bacteria that can contaminate fruit juices.

Eukarya

:* ''

Mucor racemosus ''Mucor racemosus'' is a rapidly growing, weedy mould belonging to the division Mucoromycota. It is one of the earliest fungi to be grown in pure culture and was first isolated in 1886. It has a worldwide distribution and colonizes many habitat ...

'' :* ''Urotricha'' :* ''Dunaliella acidophila'' :* Members of the algal class Cyanidiophyceae, including '' Cyanidioschyzon merolae''

Mechanisms of adaptation to acidic environments

Most acidophile organisms have evolved extremely efficient mechanisms to pump protons out of the

intracellular space Intracellular space is the interior space of the plasma membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and prote ...

in order to keep the

cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...

at or near neutral pH. Therefore, intracellular proteins do not need to develop acid stability through evolution. However, other acidophiles, such as ''

Acetobacter aceti ''Acetobacter aceti,'' a Gram-negative bacterium that moves using its peritrichous flagella, was discovered when Louis Pasteur proved it to be the cause of conversion of ethanol to acetic acid in 1864. Today, ''A.'' ''aceti'' is recognized as a s ...

'', have an acidified cytoplasm which forces nearly all proteins in the genome to evolve acid stability. For this reason, ''Acetobacter aceti'' has become a valuable resource for understanding the mechanisms by which proteins can attain acid stability. Studies of proteins adapted to low pH have revealed a few general mechanisms by which proteins can achieve acid stability. In most acid stable proteins (such as pepsin and the soxF

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

from ''

Sulfolobus acidocaldarius ''Sulfolobus acidocaldarius'' is a thermoacidophilic archaeon that belongs to the phylum Thermoproteota. ''S. acidocaldarius'' was the first ''Sulfolobus'' species to be described, in 1972 by Thomas D. Brock and collaborators. This species was ...

''), there is an overabundance of acidic residues which minimizes low pH destabilization induced by a buildup of positive charge. Other mechanisms include minimization of solvent accessibility of acidic residues or binding of metal cofactors. In a specialized case of acid stability, the NAPase protein from ''Nocardiopsis alba'' was shown to have relocated acid-sensitive salt bridges away from regions that play an important role in the unfolding process. In this case of kinetic acid stability, protein longevity is accomplished across a wide range of pH, both acidic and basic.

Examples

Archaea

Bacteria

Eukarya

Mechanisms of adaptation to acidic environments

See also

References

Further reading