Bioenergetics of alkalophilic bacteria

Abstract

The central problem for organisms which grow optimally, and in some cases obligately, at pH values of 10 to 11, is the maintenance of a relatively acidified cytoplasm. A key component of the pH homeostatic mechanism is an electrogenic Na+/H+ antiporter which--by virtue of kinetic properties and/or its concentration in the membrane--catalyzes net proton uptake while the organisms extrude protons during respiration. The antiporter is also capable of maintaining a constant pHin during profound elevations in pHout as long as Na+ entry is facilitated by the presence of solutes which are taken up with Na+. Secondary to the problem of acidifying the interior is the adverse effect of the large pH gradient, acid in, on the total pmf of alkalophile cells. For the purposes of solute uptake and motility, the organisms appear to largely bypass the problem of a low pmf by utilizing a sodium motive force for energization. However, ATP synthesis appears not to resolve the energetics problem by using Na+ or by incorporating the proton-translocating ATPase into intracellular organelles. The current data suggest that effective proton pumping carried out by the alkalophile respiratory chain at high pH may deliver at least some portion of the protons to the proton-utilizing catalysts, i.e., the F1F0-ATPase and the Na+/H+ antiporter, by some localized pathway.

Modulation of Na+/H+ Antiporter Activity by Extreme pH and Salt in the Halotolerant Alga Dunaliella salina

“…The conclusion that NH3, which induces a pH stress, leads to overproduction of the plasma membrane Na+/H+ antiporter in Dunaliella is consistent with previous observations in bacteria (15,19,26). It was demonstrated that disruption of the Na+/H+ antiporter gene in bacteria leads to failure of growth at alkaline pH, particularly in high NaCl media (15,19,26).…”

“…It was demonstrated that disruption of the Na+/H+ antiporter gene in bacteria leads to failure of growth at alkaline pH, particularly in high NaCl media (15,19,26). It is surprising that Li' adaptation, which was expected to induce an increase in the Na+/H+ antiporter activity, resulted in a decrease.…”

Experimental Demonstration of Localized Excess Protons at a WaterMembrane Interface

“…Perhaps, the most well-established scientific observations that showed the failure of the Mitchellian delocalized proton view are in alkalophilic bacteria, such as Bacillus pseuodofirmus [16][17][18]. These bacteria keep their internal pH about 2.3 pH units more acidic than the ambient bulk pH 10.5, while ∆ψ is about 180 mV in the direction from outside across the cellular membrane to the cytoplasm [19][20][21].…”

Acidification of intracellular pH results in neutral pH-sensitive growth of an alkalophilic Bacillus sp.

“…It has been reported that the acidification of pHin inhibits the growth of E. coli (2) and other bacteria (5). But in obligatory alkalophilic bacteria the explanation for the failure to grow at neutral pH is not the excessive acidification of the pHin but an inadequate ability of the respiratory chain to generate a dfH at neutral pH (2,9,(17)(18)(19). In the presence of Li+, the L1 H and di/i of this bacterium were higher than they were in medium containing Rb + (Table 1), whereas growth occurred only in medium containing Rb+ (24).…”

“…Of these bacteria, obligatory alkalophile can grow well at alkaline pH range but almost never at neutral p1-(. One possible explanation of the failure of these organisms to grow at neutral pH is the loss of membrane integrity within the neutral pH range, and low protonrnotive force (A f1H) (2,9,(17)(18)(19), but not excessive acidification of the pH;n, since the Na + /H + antiporter of alkalophiles was inactive at neutral pH (3,9,17). On the other hand, facultative alkalophiles grow in a wide range of pH, compared with obligatory alkalophile, but the growth rate is smaller at neutral pH than at alkaline pH (13,24).…”