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 comprised of residues such as galacturonic acid, gluconic acid, glutamic acid, aspartic acid, and phosphoric acid. Together, these residues form an acidic matrix which help 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 blow 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 is then used to drive 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 extruded 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.
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