Acidophile (organisms)

Acidophile (organisms)

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Acidophilic organisms are those that thrive under highly acidic conditions (usually at pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 2.0 or below) . These organisms can be found in different branches of the tree of life
Tree of life
The concept of a tree of life, a many-branched tree illustrating the idea that all life on earth is related, has been used in science , religion, philosophy, mythology, and other areas...

, including Archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

, Bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, and Eukaryotes. A partial list of these organisms include:
Archaea
  • Sulfolobales
    Sulfolobales
    In taxonomy, the Sulfolobales are an order of the Thermoprotei.- External links :...

    , an order in the Crenarchaeota
    Crenarchaeota
    In taxonomy, the Crenarchaeota has been classified as either a phylum of the Archaea kingdom or a kingdom of its own...

     branch of Archaea
    Archaea
    The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

  • Thermoplasmatales
    Thermoplasmatales
    In taxonomy, the Thermoplasmatales are an order of the Thermoplasmata. All are acidophiles, growing optimally at pH below 2. Picrophilus is currently the most acidophilic of all known organisms growing at a minimum pH of 0.06. Many of these organisms do not contain a cell wall, although this is...

    , an order in the Euryarchaeota branch of Archaea
    Archaea
    The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

  • ARMAN
    Arman
    Arman was a French-born American artist. Born Armand Pierre Fernandez in Nice, France, Arman is a painter who moved from using the objects as paintbrushes to using them as the painting itself...

    , in the Euryarchaeota branch of Archaea
    Archaea
    The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

  • Acidianus brierleyi, A. infernus, facultatively anaerobic thermoacidophilic archaebacteria
  • Metallosphaera sedula, thermoacidophilic

Bacteria
  • Acidobacterium, a phylum of Bacteria
  • Acidithiobacillales
    Acidithiobacillales
    The Acidithiobacillales are a recently recognized order of Proteobacteria with only two genera, placed in separate families. Originally both were included in the genus Thiobacillus, but they are not related to the type species, which belongs to the beta proteobacteria....

    , an order of Proteobacteria
    Proteobacteria
    The Proteobacteria are a major group of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera....

     e.g. A.ferrooxidans, A. thiooxidans
  • Thiobacillus prosperus, T. acidophilus, T. organovorus, T. cuprinus
  • Acetobacter aceti, a bacterium that produces acetic acid
    Acetic acid
    Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...

     (vinegar) from the oxidation of ethanol.

Mechanisms of adaptation to acidic environments


Most acidophilic organisms have evolved extremely efficient mechanisms to pump protons out of the intracellular
Intracellular
Not to be confused with intercellular, meaning "between cells".In cell biology, molecular biology and related fields, the word intracellular means "inside the cell".It is used in contrast to extracellular...

 space in order to keep the cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...

 at or near neutral pH. Therefore, intracellular proteins do not need to develop acid stability through evolution. However, other acidophiles, such as Acetobacter
Acetobacter
Acetobacter is a genus of acetic acid bacteria characterized by the ability to convert ethanol to acetic acid in the presence of oxygen. There are several species within this genus, and there are other bacteria capable of forming acetic acid under various conditions; but all of the Acetobacter are...

 aceti
, 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
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 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
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 from Sulfolobus acidocaldarius), 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
Salt bridge (protein)
Salt bridges fall into the broader category of noncovalent interactions. A salt bridge is actually a combination of two noncovalent interactions: hydrogen bonding and electrostatic interactions . This is most commonly observed to contribute stability to the entropically unfavorable folded...

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.