Abstract
Alkaline pH homeostasis at pH ≥ 10 has been most extensively studied in extremely alkaliphilic Bacillus pseudofirmus OF4. Comparisons between B. pseudofirmus OF4 and neutrophile Bacillus subtilis show that the capacity for alkaline pH homeostasis is more robust in the alkaliphile and that it sets the upper pH limit for alkaliphile growth. Active pH homeostasis mechanisms are essential, including a Na+ cycle in which cytoplasmic accumulation of H+ relative to the outside is mediated by Na+/H+ antiporters. Additional transporters have a role in H+ capture and/or retention, e.g. a specially adapted H+-ATP synthase and an ammonium transporter of alkaliphiles. Several alkaliphile adaptations in support of pH homeostasis disadvantage their growth at pH ≤ 7.5, i.e. alkaliphiles are “hard-wired” for alkaliphily. At pH values at which their capacity for pH homeostasis no longer maintains a cytoplasmic pH < 8, extreme alkaliphiles still exhibit growth. They grow well when the cytoplasmic pH is 8.3-9.6, values that preclude most neutrophile growth. This indicates that there are adaptations of cytoplasmic processes of alkaliphiles.
