3.1 pH Homeostasis
The intracellular pH homeostasis is a vital adaptation shared by all alkaliphiles.
Since Garland indicated that alkaliphiles maintain lower cytoplasmic pH than its
external environment [44], the difference between the intracellular and extracellular
pH values of alkaliphiles has been studied (Table 1). The results of these studies
confirmed the established notion that alkaliphiles have an impressive capacity of
maintaining low cytoplasmic pH while thriving in high pH environment. This pH
homeostasis can uphold a difference of more than 2 pH units between the cytoplasm
and extracellular environment [46–48, 50, 51]. Alkaliphiles use a variety of adaptive
strategies to achieve this remarkable pH homeostasis (Fig. 2). Some of the most
important adaptive mechanisms that play significant role in pH homeostasis such as
acquiring H
+ from extracellular environment, reducing H
+ leakage from the cytoplasm, production of organic acid, and deterring the diffusion of OH
À from the
extracellular environment are discussed below.
Table 1 The intracellular and extracellular pH values of some alkaliphiles
Organism
Extracellular pH Intracellular pH References
Exiguobacterium aurantiacum 9.4
8.4
McLaggan et al. [45]
Clostridium paradoxum
9.8
8.5
Cook et al. [46]
B. pseudofirmus OF4
10.5
8.3
Guffanti and Hicks [47]
B. pseudofirmus OF4
10.8
8.3
Sturr et al. [48]
B. pseudofirmus OF4
11.2
8.9
Sturr et al. [48]
B. pseudofirmus OF4
11.4
9.6
Sturr et al. [48]
B. halodurans C-125
9
7.9
Aono et al. [49]
B. halodurans C-125
9.5
8.1
Aono et al. [49]
B. halodurans C-125
10
8.2
Aono et al. [49]
B. halodurans C-125
10.5
8.4
Aono et al. [49]
B. alcalophilus
9
7.6
Guffanti and Hicks [47]
B. alcalophilus
10
8.6
Guffanti and Hicks [47]
B. alcalophilus
11
9.2
Guffanti and Hicks [47]
Acquire H
+
pH Homeostasis
Generate H
+
Reduce H
+
leakage
Deter OH
_
ingress
Fig. 2 The strategies
alkaliphiles use to maintain
low intracellular pH
Challenges and Adaptations of Life in Alkaline Habitats
93