3.4 Coping Intracellular Alkalinization
Although alkaliphiles are known to have an efficient pH homeostasis system that
keeps the cytoplasmic pH well below that of the habitat, the intracellular pH can go
above the neutral range. For instance, the intracellular pH of the facultative
alkaliphile B. psuedofirmus has been reported reaching 9.6 when the cells were
cultivated at pH 11.4 [48]. The maximum reported difference between the intracellular and extracellular pH of alkaliphiles is around 2.5 pH units [48]. As shown in
Table 1, it seems that the cytoplasmic pH rises even higher as the cultivation pH for
the organism reaches to its upper edge. Thus, although there is no available data, the
cytoplasmic pH of alkaliphiles such as those growing at pH 13.5 [16] may exceed
pH 10. In non-alkaliphiles, this high cytoplasmic pH can potentially impair cellular
activities and integrities and ultimately kill the cell. But, as witnessed from their
growth in extreme pH conditions, alkaliphiles evolved their intracellular system to
remain active and stable at elevated pH. The possible strategies that alkaliphiles
deploy to withstand cytoplasmic alkalinization may include (1) altering the expression/production profile of biomolecules, (2) evolving efficient intracellular repair
system, and (3) production of biomolecules that are operationally stable at high
pH. The production of biomolecules that are stable and functional at high pH is not
restricted to intracellular products; it is an absolute necessity to extracellular
products.
3.4.1 Altering the Expression/Production Profile of Biomolecules
Organisms endure stress by changing their gene expression profile and metabolic
programming. Studies have shown that alkalinization is accompanied by up- and
downregulation of several genes [102, 148, 169, 181]. Such changes bring the
desired tolerance to the rising pH by (1) switching to alkali-tolerant variants,
(2) increasing the level of biomolecules that mitigate the pH drift, (3) compensating
the loss due to denaturation, and (4) activating the protein repair and degradation
systems.
Switching Expression to Alkali-Tolerant Variants Some organisms have genes
which encode variants of a product. These organisms, up on cytoplasmic pH rise,
may switch to the expression of the alternative variant that encodes the protein which
is operationally stable at alkaline condition [182]. Thus, inactivation of the alkalisensitive proteins will not hamper the cellular process as the sensitive products are
replaced by resistant variants. Alkaliphiles are known to produce biomolecules (such
extracellular enzymes) that are active and stable at high pH. It is possible that these
organisms use the same adaptation strategy to make alkali-resistant intracellular
products. The adaptation mechanism for high pH operational stability of biomolecules is discussed in Sect. 3.5.
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G. Mamo
Although alkaliphiles are known to have an efficient pH homeostasis system that
keeps the cytoplasmic pH well below that of the habitat, the intracellular pH can go
above the neutral range. For instance, the intracellular pH of the facultative
alkaliphile B. psuedofirmus has been reported reaching 9.6 when the cells were
cultivated at pH 11.4 [48]. The maximum reported difference between the intracellular and extracellular pH of alkaliphiles is around 2.5 pH units [48]. As shown in
Table 1, it seems that the cytoplasmic pH rises even higher as the cultivation pH for
the organism reaches to its upper edge. Thus, although there is no available data, the
cytoplasmic pH of alkaliphiles such as those growing at pH 13.5 [16] may exceed
pH 10. In non-alkaliphiles, this high cytoplasmic pH can potentially impair cellular
activities and integrities and ultimately kill the cell. But, as witnessed from their
growth in extreme pH conditions, alkaliphiles evolved their intracellular system to
remain active and stable at elevated pH. The possible strategies that alkaliphiles
deploy to withstand cytoplasmic alkalinization may include (1) altering the expression/production profile of biomolecules, (2) evolving efficient intracellular repair
system, and (3) production of biomolecules that are operationally stable at high
pH. The production of biomolecules that are stable and functional at high pH is not
restricted to intracellular products; it is an absolute necessity to extracellular
products.
3.4.1 Altering the Expression/Production Profile of Biomolecules
Organisms endure stress by changing their gene expression profile and metabolic
programming. Studies have shown that alkalinization is accompanied by up- and
downregulation of several genes [102, 148, 169, 181]. Such changes bring the
desired tolerance to the rising pH by (1) switching to alkali-tolerant variants,
(2) increasing the level of biomolecules that mitigate the pH drift, (3) compensating
the loss due to denaturation, and (4) activating the protein repair and degradation
systems.
Switching Expression to Alkali-Tolerant Variants Some organisms have genes
which encode variants of a product. These organisms, up on cytoplasmic pH rise,
may switch to the expression of the alternative variant that encodes the protein which
is operationally stable at alkaline condition [182]. Thus, inactivation of the alkalisensitive proteins will not hamper the cellular process as the sensitive products are
replaced by resistant variants. Alkaliphiles are known to produce biomolecules (such
extracellular enzymes) that are active and stable at high pH. It is possible that these
organisms use the same adaptation strategy to make alkali-resistant intracellular
products. The adaptation mechanism for high pH operational stability of biomolecules is discussed in Sect. 3.5.
112
G. Mamo