FIGURE 37.2 Schematic representation of elements involved in pH homeostasis in alkaliphilic microorganisms. R represents respiratory chain-mediated proton translocation; NhaC and NhaJ represent two separate H+/Na+ antiporters found in Bacillus spp.; S represents a Na+-dependent solute transporter; and F represents Na+-dependent flagella motility.
The exterior surfaces of the cell are also important for maintaining a pH differential. This is supported by evidence that the protoplasts of alkaliphilic Bacillus spp. are unstable in alkaline conditions. The peptidoglycan in these strains has a higher cross-linking rate at higher pH values, which may provide a shielding effect by "tightening" the cell wall. Large amounts of acidic compounds, including teichuronic acid, teichoic acid, uronic acids, and acidic amino acids, are evident in alkaliphilic cell walls compared to the cell walls of nonalkaliphilic microorganisms. The negative charge of these acidic substances may create a more neutral layer close to the outer surface of the cell.
Alkaliphiles have unique bioenergetic properties. Nonalkaliphilic respiring bacteria energize their cyto-plasmic membrane with a chemiosmotic driving force (Ap) by generating an electrochemical gradient of ions that has two parameters—acidic conditions outside (caused by the extrusion of protons and described by the term ApH) and a positive charge outside (described by the transmembrane electrical potential, A^). The Ap is used for proton-coupled symport of solutes, proton-driven mobility (flagella) and ATP synthesis. In alka-liphilic environments, the contribution of ApH to Ap becomes smaller with increasing extracellular pH. However, with increasing extracellular pH, sodium ion export is increased (see Section VII.B.1) and may contribute to an increased A^. This partially compensates for the decrease of Ap by the reduction of ApH. Interestingly, the sodium gradient is used to energize solute transport and flagella movement, but not for ATP synthesis. No sodium-dependent ATP-synthases have been identified in alkaliphiles. In addition, ATP-synthases in alkaliphilic Bacillus spp. have been shown to be exclusively proton translocating.
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