Alkaliphiles are a class of extremophilic

microbes A microorganism, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells. The possible existence of unseen microbial life was suspected from antiquity, with an early attestation in ...

capable of survival in

alkaline In chemistry, an alkali (; from the Arabic word , ) is a basic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a soluble base has a pH greater than 7.0. The ...

( pH roughly 8.5–11) environments, growing optimally around a pH of 10. These bacteria can be further categorized as obligate alkaliphiles (those that require high pH to survive), facultative alkaliphiles (those able to survive in high pH, but also grow under normal conditions) and haloalkaliphiles (those that require high salt content to survive).Horikoshi, Koki. "Alkaliphiles: Some applications of their products for biotechnology." Microbiology and Molecular Biology Reviews 63.4 (1999): 735-50. Print.

Background information

Microbial growth in alkaline conditions presents several complications to normal biochemical activity and reproduction, as high pH is detrimental to normal cellular processes. For example, alkalinity can lead to denaturation of DNA, instability 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 protects the interior of a cell from the outside environment (the extr ...

and inactivation of cytosolic

enzymes An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as pro ...

, as well as other unfavorable physiological changes.Higashibata, Akira, Taketomo Fujiwara, and Yoshihiro Fukumori. "Studies on the Respiratory System in Alkaliphilic Bacillus; a Proposed New Respiratory System." Extremophiles 2 (1998): 83–92. Print. Thus, to adequately circumvent these obstacles, alkaliphiles must either possess specific cellular machinery that works best in the alkaline range, or they must have methods of acidifying the cytosol in relation to the extracellular environment. To determine which of the above possibilities an alkaliphile uses, experimentation has demonstrated that alkaliphilic enzymes possess relatively normal pH optimums. The determination that these enzymes function most efficiently near physiologically neutral pH ranges (about 7.5–8.5) was one of the primary steps in elucidating how alkaliphiles survive intensely basic environments. Since the cytosolic pH must remain nearly neutral, alkaliphiles must have one or more mechanisms of acidifying the

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

when in the presence of a highly alkaline environment.

Mechanisms of cytosolic acidification

Alkaliphiles maintain cytosolic acidification through both passive and active means. In passive acidification, it has been proposed that cell walls contain acidic

polymers A polymer () is a substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeating subunits derived from one or more species of monomers. Due to their broad spectrum of properties, b ...

composed of residues such as galacturonic acid, gluconic acid,

glutamic acid Glutamic acid (symbol Glu or E; known as glutamate in its anionic form) is an α- amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a non-essential nutrient for humans, meaning that the human body can ...

aspartic acid Aspartic acid (symbol Asp or D; the ionic form is known as aspartate), is an α-amino acid that is used in the biosynthesis of proteins. The L-isomer of aspartic acid is one of the 22 proteinogenic amino acids, i.e., the building blocks of protei ...

, and

phosphoric acid Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, ...

. Together, these residues form an acidic matrix that helps protect the

from alkaline conditions by preventing the entry of

hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It ...

ions, and allowing for the uptake of

sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...

and hydronium ions. In addition, the

peptidoglycan Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like layer (sacculus) that surrounds the bacterial cytoplasmic membrane. The sugar component consists of alternating ...

in alkaliphilic ''B. subtilis'' has been observed to contain higher levels of

hexosamines Hexosamines are amino sugars created by adding an amine group to a hexose. Examples include: * Fructosamine (based upon fructose) * Galactosamine (based upon galactose) * Glucosamine (based upon glucose) * Mannosamine (based upon mannose) Ext ...

and

amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the Proteinogenic amino acid, 22 α-amino acids incorporated into p ...

as compared to its neutrophilic counterpart. When alkaliphiles lose these acidic residues in the form of induced mutations, it has been shown that their ability to grow in alkaline conditions is severely hindered. However, it is generally agreed upon that passive methods of cytosolic acidification are not sufficient to maintain an internal pH 2-2.3 levels below that of external pH; there must also be active forms of acidification. The most characterized method of active acidification is in the form of Na⁺/H⁺ antiporters. In this model, H⁺ ions are first extruded through the

electron transport chain An electron transport chain (ETC) is a series of protein complexes and other molecules which transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples th ...

in respiring cells and to some extent through an

ATPase ATPases (, Adenosine 5'-TriPhosphatase, adenylpyrophosphatase, ATP monophosphatase, triphosphatase, ATP hydrolase, adenosine triphosphatase) are a class of enzymes that catalyze the decomposition of ATP into ADP and a free phosphate ion or ...

in fermentative cells. This

proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...

extrusion establishes a proton gradient that drives electrogenic antiporters—which drive intracellular Na⁺ out of the cell in exchange for a greater number of H⁺ ions, leading to the net accumulation of internal protons. This proton accumulation leads to a lowering of cytosolic pH. The expelled Na⁺ can be used for solute symport, which are necessary for cellular processes. It has been noted that Na⁺/H⁺ antiport is required for alkaliphilic growth, whereas either K⁺/H⁺ antiporters or Na⁺/H⁺ antiporters can be utilized by neutrophilic bacteria. If Na⁺/H⁺ antiporters are disabled through

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, ...

or another means, the bacteria are rendered neutrophilic. The sodium required for this antiport system is the reason some alkaliphiles can only grow in saline environments.

Differences in alkaliphilic ATP production

In addition to the method of proton extrusion discussed above, it is believed that the general method of cellular respiration is different in obligate alkaliphiles as compared to neutrophiles. Generally, ATP production operates by establishing a proton gradient (greater H+ concentration outside the membrane) and a transmembrane electrical potential (with a positive charge outside the membrane). However, since alkaliphiles have a reversed pH gradient, it would seem that ATP production—which is based on a strong

proton-motive force Chemiosmosis is the movement of ions across a semipermeable membrane bound structure, down their electrochemical gradient. An important example is the formation of adenosine triphosphate (ATP) by the movement of hydrogen ions (H+) across a membra ...

– would be severely reduced. However, the opposite is true. It has been proposed that while the pH gradient has been reversed, the transmembrane electrical potential is greatly increased. This increase in charge causes the production of greater amounts of ATP by each translocated proton when driven through an ATPase.Hirabayashi, Toshikazu, Toshitaka Goto, Hajime Morimoto, Kazuaki Yoshimune, Hidetoshi Matsuyama, and Isao Yumoto. "Relationship between Rates of Respiratory Proton Extrusion and ATP Synthesis in Obligately Alkaliphilic Bacillus Clarkii DSM 8720T." J Bioenerg Biomembr 44 (2012): 265-72. Print. Research in this area is ongoing.

Applications and future research

Alkaliphiles promise several interesting uses for

biotechnology Biotechnology is a multidisciplinary field that involves the integration of natural sciences and Engineering Science, engineering sciences in order to achieve the application of organisms and parts thereof for products and services. Specialists ...

and future research. Alkaliphilic methods of regulating pH and producing ATP are of interest in the scientific community. However, perhaps the greatest area of interest from alkaliphiles lies in their

: alkaline proteases; starch-degrading enzymes; cellulases; lipases; xylanases; pectinases; chitinases and their

metabolites In biochemistry, a metabolite is an intermediate or end product of metabolism. The term is usually used for small molecules. Metabolites have various functions, including fuel, structure, signaling, stimulatory and inhibitory effects on enzymes, c ...

, including: 2-phenylamine;

carotenoids Carotenoids () are yellow, orange, and red organic compound, organic pigments that are produced by plants and algae, as well as several bacteria, archaea, and Fungus, fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips ...

; siderophores; cholic acid derivatives and organic acids. It is hoped that further research into alkaliphilic enzymes will allow scientists to harvest alkaliphiles' enzymes for use in basic conditions. Research aimed at discovering alkaliphile-produced antibiotics showed some success, yet has been held at bay by the fact that some products produced at high pH are unstable and unusable at a physiological pH range.

Examples

Examples of alkaliphiles include '' Halorhodospira halochloris'', '' Natronomonas pharaonis'', and '' Thiohalospira alkaliphila''.

References