transporting it out of the blood. Most fish do have the genes necessary for this
biochemical process; however, only those fish which are adapted to alkaline habitats
are able to express these genes throughout their life [240].
The work done so far has improved our understanding how fish can adapt to high
pH environment. However, there are still some unanswered questions such as
adaptive mechanism of gills proteins/membranes which are directly exposed to the
alkaline water. Moreover, due to the external fertilization of fish reproductive
process, the gametes are deposited directly into the extreme habitats. How these
gamete cells survive the high pH is yet to be discovered. Studies on other eukaryotic
organisms’ adaptation to the high pH environment will certainly add to the existing
knowledge and should be encouraged.
5 Conclusion
It has been over four decades since researchers started unraveling the secretes of high
pH adaptation. Over these years, very fascinating adaptive strategies of alkaliphiles
have been described in numerous publications. To thrive in high pH environments,
organisms evolved multilevel adaptations that are reflected in their unique functional
and structural makeups. Adaptations related to pH homeostasis and bioenergetics of
alkaliphilic prokaryotes have been widely and deeply studied. However, there are
still issues that are waiting for proper scientific look. One of such issues that seem
overlooked is the cytoplasmic alkalinization of extreme alkaliphiles like those
growing around pH 13 and the associated physiological adaptations. At the extreme
pH, though it is not experimentally proven, there is a possibility that the cytoplasmic
pH can drift above pH 10, and this can, at least, theoretically affect the transcription
and translation processes, DNA replication, the activities and stabilities of biomolecules including enzymes, DNA and RNA, etc. However, the fact that these unique
organisms are growing in the extreme habitats indicate the cytoplasmic system is
functional and hence must be adapted to high pH. On the other hand, although it is
unlikely, there is a possibility that these organisms manage to keep the cytoplasmic
pH below pH 10. If this happens, the intracellular and extracellular pH difference can
reach over 3.5 pH units for alkaliphiles thriving at pH 13.5, and this obviously
requires an extremely efficient pH homeostasis even by alkaliphiles standard. Thus,
these extreme alkaliphiles to thrive in their habitats should evolve either an extraordinary pH homeostasis mechanism or unique adaptation that protects their cellular
activities and biomolecules from the deleterious effect of high cytoplasmic pH
(>pH 10). Which one of these alternatives nature has chosen remains to be seen?
Probably one of the most studied high pH adaptations is the bioenergetics. It is
widely accepted that cell membranes of alkaliphiles have low proton motive force
(pmf) which makes oxidative phosphorylation-based ATP production challenging.
However, it seems that alkaliphiles solved this challenge primarily by evolving
efficient respiratory complexes and ATP synthase that aggregates in a patch by the
cardiolipin. The respiratory complexes pump H
+ faster, and the headgroup of the
120
G. Mamo
biochemical process; however, only those fish which are adapted to alkaline habitats
are able to express these genes throughout their life [240].
The work done so far has improved our understanding how fish can adapt to high
pH environment. However, there are still some unanswered questions such as
adaptive mechanism of gills proteins/membranes which are directly exposed to the
alkaline water. Moreover, due to the external fertilization of fish reproductive
process, the gametes are deposited directly into the extreme habitats. How these
gamete cells survive the high pH is yet to be discovered. Studies on other eukaryotic
organisms’ adaptation to the high pH environment will certainly add to the existing
knowledge and should be encouraged.
5 Conclusion
It has been over four decades since researchers started unraveling the secretes of high
pH adaptation. Over these years, very fascinating adaptive strategies of alkaliphiles
have been described in numerous publications. To thrive in high pH environments,
organisms evolved multilevel adaptations that are reflected in their unique functional
and structural makeups. Adaptations related to pH homeostasis and bioenergetics of
alkaliphilic prokaryotes have been widely and deeply studied. However, there are
still issues that are waiting for proper scientific look. One of such issues that seem
overlooked is the cytoplasmic alkalinization of extreme alkaliphiles like those
growing around pH 13 and the associated physiological adaptations. At the extreme
pH, though it is not experimentally proven, there is a possibility that the cytoplasmic
pH can drift above pH 10, and this can, at least, theoretically affect the transcription
and translation processes, DNA replication, the activities and stabilities of biomolecules including enzymes, DNA and RNA, etc. However, the fact that these unique
organisms are growing in the extreme habitats indicate the cytoplasmic system is
functional and hence must be adapted to high pH. On the other hand, although it is
unlikely, there is a possibility that these organisms manage to keep the cytoplasmic
pH below pH 10. If this happens, the intracellular and extracellular pH difference can
reach over 3.5 pH units for alkaliphiles thriving at pH 13.5, and this obviously
requires an extremely efficient pH homeostasis even by alkaliphiles standard. Thus,
these extreme alkaliphiles to thrive in their habitats should evolve either an extraordinary pH homeostasis mechanism or unique adaptation that protects their cellular
activities and biomolecules from the deleterious effect of high cytoplasmic pH
(>pH 10). Which one of these alternatives nature has chosen remains to be seen?
Probably one of the most studied high pH adaptations is the bioenergetics. It is
widely accepted that cell membranes of alkaliphiles have low proton motive force
(pmf) which makes oxidative phosphorylation-based ATP production challenging.
However, it seems that alkaliphiles solved this challenge primarily by evolving
efficient respiratory complexes and ATP synthase that aggregates in a patch by the
cardiolipin. The respiratory complexes pump H
+ faster, and the headgroup of the
120
G. Mamo