The major elemental cations (Na⁺, K⁺, Mg²⁺, and Ca²⁺) are required for a variety of biochemical and physiological processes. K⁺ is the most abundant monovalent cation in prokaryotic cells and is important in maintenance of pH and electrochemical gradients in neutrophilic (Kroll and Booth, 1983) and acidiophilic (Matin, 1990) microorganisms. Extreme halophiles concentrate K⁺ in the cytoplasm up to 3 M to maintain turgor pressure (Galinski, 1995). Prokaryotes use Na⁺ gradients for solute transport (Wilson and Ding, 2001) and, in some species, to drive flagellar motors (Imae and Atsumi, 1989) or generate reducing power (Dimroth, 1997). Most organisms use Na⁺/H⁺ antiporters (Padan et al, 2001) to generate Na⁺ gradients, but some bacteria have primary Na⁺ translocation systems that couple Na⁺ efflux to respiration (Tokuda and Unemoto, 1982) or the decarboxylation of organic acids (Hilpert and Dimroth, 1991).

The roles of Mg²⁺ and Ca²⁺ in prokaryotes are not as clearly defined. Mg²⁺ is the most abundant divalent cation in cells and functions as a cofactor for many enzymes, stabilizes ribosomes and membranes, and signals through the PhoP/PhoQ two-component system in some pathogenic bacteria (Smith and Maguire, 1998). Ca²⁺ is found in prokaryotes in micromolar concentrations and has been implicated in a range of cellular functions including chemotaxis, sporulation, enzyme activation, and membrane stabilization (Smith, 1995). Calmodulin-like proteins have been identified in bacteria (Onek and Smith, 1992) and Ca²⁺ signaling in prokaryotes is being explored (Norris et al, 1996).

For more information:

Matin, A. Keeping a neutral cytoplasm; the bioenergetics of obligate acidophiles. FEMS Microbiol Rev. 1990;75:307-318.