Alkaliphile

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 composed of residues such as galacturonic acid, gluconic acid, glutamic acid, aspartic acid, and phosphoric acid. Together, these residues form an acidic matrix that helps protect the plasma membrane from alkaline conditions by preventing the entry of hydroxide ions, and allowing for the uptake of sodium and hydronium ions. In addition, the peptidoglycan in alkaliphilic B. subtilis has been observed to contain higher levels of hexosamines and amino acids 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 in respiring cells and to some extent through an ATPase in fermentative cells. This proton 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 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 – 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. Research in this area is ongoing.

Applications and future research

Alkaliphiles promise several interesting uses for biotechnology 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 enzymes: alkaline proteases; starch-degrading enzymes; cellulases; lipases; xylanases; pectinases; chitinases and their metabolites, including: 2-phenylamine; carotenoids; 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.