The sodium cycle, in which a sodium motive force (SMF) is established across
the membrane to drive bioenergetics processes, has been historically associated with
adaptation to alkaline environments where establishing a proton motive force is
extremely energy demanding [40]. However, the existence of alkaliphiles that can
grow in alkaline conditions using protons as the coupling ion has since discredited
the idea of sodium cycling being an obligate requirement for alkaline adaptation
[41, 42]. Nonetheless, the sodium cycle plays an important role in the maintenance
of pH homeostasis in a restricted number of alkaliphiles. One of the main antiporters
involved in this cycle is the Mrp-antiporter, which was first identified in
B. halodurans [43]. In Bacillus pseudofirmus OF4 the operon for the mrp Na
+ /H
+
antiporter contains seven genes, all of which were found to be essential for the Na
+
exclusion and antiport activity [44]. Two other major components of this cycle are
the sodium/solute symporter and the voltage-gate channel associated with a sodium
dependent flagellar motor [45], which is found exclusive in alkaliphiles [46]. Studies
on non-alkaliphilic mutant strains of B. halodurans C-125 have revealed the importance of Na
+
/H
+ antiporters, as the alkaliphilic nature of the strain was restored after
cloning in a 3.7 kb stretch of DNA containing an Na
+
/H
+ antiporter gene [3, 47]. In
the Gram-negative bacterium Alkalimonas amylolytica, isolated from Lake
Chahannor in China, the monovalent sodium/proton antiporter NhaD provided
sodium (lithium)/proton antiport activity at pH values above 9 and over a broad
range of sodium concentrations, but activity was severely reduced under more
neutrophilic conditions [48]. It is also notable that the genome of the alkaliphile
O. iheyensis contains 18 genes coding for C4-dicarboxylate carriers, 7 of which are
also shared by B. halodurans [49] that are commonly associated with Na
+ and H
+
symport activity.
A recent transcriptomics study of the alkaliphile Halomonas sp. Y2 revealed that
it differentially expresses distinct Na
+
/H
+ antiporters in response to different
stresses. For instance, expression of the Mrp transporter did not vary across different
pHs, while its absence negatively impacted the organism’s resistance to Na
+
, Li
+
,
and K
+ ions. By comparison, the Ha-NhaD2 transporter, which is homologous to the
NhaD antiporter from Alkalimonas amylolytica, was upregulated with increased pH,
suggesting that it plays a more significant role in pH homeostasis than Mrp, under
alkaline stress [17]. In addition to the canonical Na
+
/H
+ antiport, some members of
this superfamily also show functional versatility. This is the case of the antiporter
Ap-NapA1-2 from the halotolerant alkaliphilic cyanobacterium Aphanothece
halophytica, which shows the capacity to replace proton uptake by potassium uptake
[50].
Due to the reversed proton gradient in alkaliphiles, where the cytoplasmic pH is
more acidic than the alkaline environment, processes that require a PMF, such as
motility, are severely compromised [51]. To circumvent this limitation, alkaliphiles
use SMF-driven motors to achieve motility [52]. In B. halodurans C-125 and
146
P. H. Lebre and D. A. Cowan
the membrane to drive bioenergetics processes, has been historically associated with
adaptation to alkaline environments where establishing a proton motive force is
extremely energy demanding [40]. However, the existence of alkaliphiles that can
grow in alkaline conditions using protons as the coupling ion has since discredited
the idea of sodium cycling being an obligate requirement for alkaline adaptation
[41, 42]. Nonetheless, the sodium cycle plays an important role in the maintenance
of pH homeostasis in a restricted number of alkaliphiles. One of the main antiporters
involved in this cycle is the Mrp-antiporter, which was first identified in
B. halodurans [43]. In Bacillus pseudofirmus OF4 the operon for the mrp Na
+ /H
+
antiporter contains seven genes, all of which were found to be essential for the Na
+
exclusion and antiport activity [44]. Two other major components of this cycle are
the sodium/solute symporter and the voltage-gate channel associated with a sodium
dependent flagellar motor [45], which is found exclusive in alkaliphiles [46]. Studies
on non-alkaliphilic mutant strains of B. halodurans C-125 have revealed the importance of Na
+
/H
+ antiporters, as the alkaliphilic nature of the strain was restored after
cloning in a 3.7 kb stretch of DNA containing an Na
+
/H
+ antiporter gene [3, 47]. In
the Gram-negative bacterium Alkalimonas amylolytica, isolated from Lake
Chahannor in China, the monovalent sodium/proton antiporter NhaD provided
sodium (lithium)/proton antiport activity at pH values above 9 and over a broad
range of sodium concentrations, but activity was severely reduced under more
neutrophilic conditions [48]. It is also notable that the genome of the alkaliphile
O. iheyensis contains 18 genes coding for C4-dicarboxylate carriers, 7 of which are
also shared by B. halodurans [49] that are commonly associated with Na
+ and H
+
symport activity.
A recent transcriptomics study of the alkaliphile Halomonas sp. Y2 revealed that
it differentially expresses distinct Na
+
/H
+ antiporters in response to different
stresses. For instance, expression of the Mrp transporter did not vary across different
pHs, while its absence negatively impacted the organism’s resistance to Na
+
, Li
+
,
and K
+ ions. By comparison, the Ha-NhaD2 transporter, which is homologous to the
NhaD antiporter from Alkalimonas amylolytica, was upregulated with increased pH,
suggesting that it plays a more significant role in pH homeostasis than Mrp, under
alkaline stress [17]. In addition to the canonical Na
+
/H
+ antiport, some members of
this superfamily also show functional versatility. This is the case of the antiporter
Ap-NapA1-2 from the halotolerant alkaliphilic cyanobacterium Aphanothece
halophytica, which shows the capacity to replace proton uptake by potassium uptake
[50].
Due to the reversed proton gradient in alkaliphiles, where the cytoplasmic pH is
more acidic than the alkaline environment, processes that require a PMF, such as
motility, are severely compromised [51]. To circumvent this limitation, alkaliphiles
use SMF-driven motors to achieve motility [52]. In B. halodurans C-125 and
146
P. H. Lebre and D. A. Cowan