extracellular pH values and the sole dependence of alkaliphiles on Na
+
/H
+
antiporters unlike neutralophiles which also involve K
+
/H
+ antiporters [21, 52].
Several studies have shown the vital importance of Na
+
/H
+ antiporters in adapting
high pH environments [73, 74]. This is clearly shown in the growth profile of
neutralophiles with and without Na
+
/H
+ antiporters. The growth of neutralophiles
that lack functioning Na
+
/H
+ antiporters is limited around neutrality, pH 6.3–7.7
[75, 76]. In these organisms, the rise in the environmental pH to alkaline range is
accompanied by rapid alkalinization due to inefficient intracellular pH homeostasis,
which hampers growth. But neutralophiles equipped with functional Na
+
/H
+
antiporters can maintain their cytoplasmic pH around 7.5 and grow in environments
with pH values of up to 8.5. Often, when the external pH value exceeds 8.5,
neutralophiles start to grow slowly, and when the pH is over 9, their growth becomes
severely impaired [21, 22]. At higher alkalinity, the neutralophiles fail to maintain
their cytoplasmic pH below 8. Moreover, the physiology of these organisms is not
adapted to function at high pH, and this results in a dramatic drop in growth rate as
the external pH values increase. However, alkaliphiles which are endowed with high
level of Na
+ /H
+ antiporters can maintain their cytoplasmic pH at 7.5 even when
growing in an environment of pH 9.5. Some of these alkaliphiles can grow at much
higher pH, and in those conditions, the intracellular pH is expected to become way
above the pH values at which neutralophiles can survive and/or grow.
Monovalent cation/proton antiporters can be products of a single gene or heterooligomers assembled from multiple gene products. The hetero-oligomer monovalent
cation antiporters are known as Mrp [77]. Mrps are widely distributed among
bacteria and archaea [77, 78] and involved in several physiological processes. In
archaea, Mrps are used in the conversion of energy involved in metabolism and
hydrogen production, while in bacteria, it is involved in nitrogen fixation, bile salt
tolerance, arsenic oxidation, and pathogenesis [78]. In alkaliphiles it is believed that
it plays a dominant role in pH homeostasis and sodium tolerance [21, 22, 60, 77, 79,
80]. The Mrp operon has six or seven genes which encode hydrophobic proteins
required for optimal activity [79]. Structural analysis predicted that these antiporters
have large surface which can facilitate the capturing of proton and funneling it into
the antiporter [21, 22, 80].
The Na
+
/H
+ antiporter systems exchange cytoplasmic Na
+ for extracellular H
+
.
However, the Na
+ leaving the cytoplasm must be replenished so that the antiporterdependent pH homeostasis system works effectively; this is especially important
when the extracellular Na
+ concentration is low [21, 22, 81–85]. Alkaliphiles use
Na
+ solute symporters and Na
+
-coupled motility channels known as MotPS for
reentry of Na
+ to the cytoplasm [21, 78, 86, 87, 88]. Moreover, Na
+ uptake by
alkaliphiles is accomplished through voltage-gated Na
+ channels known as
NaChBac and NaVBP [86, 89–93]. The major Na
+ and H
+ entry and exit pathways
are shown in Fig. 3.
Challenges and Adaptations of Life in Alkaline Habitats
95
+
/H
+
antiporters unlike neutralophiles which also involve K
+
/H
+ antiporters [21, 52].
Several studies have shown the vital importance of Na
+
/H
+ antiporters in adapting
high pH environments [73, 74]. This is clearly shown in the growth profile of
neutralophiles with and without Na
+
/H
+ antiporters. The growth of neutralophiles
that lack functioning Na
+
/H
+ antiporters is limited around neutrality, pH 6.3–7.7
[75, 76]. In these organisms, the rise in the environmental pH to alkaline range is
accompanied by rapid alkalinization due to inefficient intracellular pH homeostasis,
which hampers growth. But neutralophiles equipped with functional Na
+
/H
+
antiporters can maintain their cytoplasmic pH around 7.5 and grow in environments
with pH values of up to 8.5. Often, when the external pH value exceeds 8.5,
neutralophiles start to grow slowly, and when the pH is over 9, their growth becomes
severely impaired [21, 22]. At higher alkalinity, the neutralophiles fail to maintain
their cytoplasmic pH below 8. Moreover, the physiology of these organisms is not
adapted to function at high pH, and this results in a dramatic drop in growth rate as
the external pH values increase. However, alkaliphiles which are endowed with high
level of Na
+ /H
+ antiporters can maintain their cytoplasmic pH at 7.5 even when
growing in an environment of pH 9.5. Some of these alkaliphiles can grow at much
higher pH, and in those conditions, the intracellular pH is expected to become way
above the pH values at which neutralophiles can survive and/or grow.
Monovalent cation/proton antiporters can be products of a single gene or heterooligomers assembled from multiple gene products. The hetero-oligomer monovalent
cation antiporters are known as Mrp [77]. Mrps are widely distributed among
bacteria and archaea [77, 78] and involved in several physiological processes. In
archaea, Mrps are used in the conversion of energy involved in metabolism and
hydrogen production, while in bacteria, it is involved in nitrogen fixation, bile salt
tolerance, arsenic oxidation, and pathogenesis [78]. In alkaliphiles it is believed that
it plays a dominant role in pH homeostasis and sodium tolerance [21, 22, 60, 77, 79,
80]. The Mrp operon has six or seven genes which encode hydrophobic proteins
required for optimal activity [79]. Structural analysis predicted that these antiporters
have large surface which can facilitate the capturing of proton and funneling it into
the antiporter [21, 22, 80].
The Na
+
/H
+ antiporter systems exchange cytoplasmic Na
+ for extracellular H
+
.
However, the Na
+ leaving the cytoplasm must be replenished so that the antiporterdependent pH homeostasis system works effectively; this is especially important
when the extracellular Na
+ concentration is low [21, 22, 81–85]. Alkaliphiles use
Na
+ solute symporters and Na
+
-coupled motility channels known as MotPS for
reentry of Na
+ to the cytoplasm [21, 78, 86, 87, 88]. Moreover, Na
+ uptake by
alkaliphiles is accomplished through voltage-gated Na
+ channels known as
NaChBac and NaVBP [86, 89–93]. The major Na
+ and H
+ entry and exit pathways
are shown in Fig. 3.
Challenges and Adaptations of Life in Alkaline Habitats
95