## 4 SRB Activities in Alkaline Environment
### Response of SRB to Alkaline Environment
#### 4.1.1 pH Homeostasis  
High pH environments have been found to cause in the depletion of proton in bacteria cytoplasm. One cell of _E. coli_ can contain 7.2 mole proton at pH 8 and decrease to less than 1 mole proton when environmental pH increase to 9 [76]. At high pH environment, the driving force of proton flow (pmf) is lowered because bacteria need to sustain the pH in the cytoplasm, which consequently has an energetically negative effect in pH gradient [77]. This results in dramatic decrease in ATP synthesis in bacteria metabolism. In contraction to acid stress, bacteria under alkaline stress respond increase capture and retention of protons for ATP synthesis which can be carried out by ATP synthase and antiporter cation/proton. Besides, SRB were found to regulate cell membrane and increase amino acid synthesis to maintain pH homeostasis [73].  
ATP synthase in bacteria has two domain including: F\({}_{1}\) contains three catalytic \(\alpha\) and \(\beta\) subunit pairs and single \(\gamma\), \(\delta\), and \(\varepsilon\) subunits; F\({}_{0}\) contains a single \(a\)-subunit, two  
Figure 2: Theoretical model of response of _D. vulgaris_ to alkaline environment, the \(\uparrow\) indicates the increase, \(\downarrow\) indicates the decrease in the corresponding of gene expression [73].  
\(b\)-subunits and multiple \(c\)-subunits which present different stoichiometric ratios per complex in the synthase [77]. It is common in bacteria that ATP synthases are coupled to an electrochemical gradient of protons (pmf) [78; 79]. F\({}_{1}\)F\({}_{0}\)-ATPase are also energized by an electrochemical gradient of cation which is generated by cation pumping protein complexes in the membrane. The most common cations found were Na\({}^{+}\), Li\({}^{+}\) and K\({}^{+}\). The capacity of neutralophilic as well as alkaliphilic bacteria for maintaining a cytoplasmic pH below the external pH, depends heavily upon electrogenic Na\({}^{+}\)(Li\({}^{+}\))(K\({}^{+}\))/H\({}^{+}\) antiporters. [PERSON]. [73] reported that when _D. vulgaris_ was exposed to environment with pH of 10, gene expression related to ATP synthase including _DVU10774-0780_ (\(F_{1}\varepsilon\), \(F_{1}\beta\), \(F_{1}\gamma\), \(F_{1}\alpha\), \(F_{1}\delta\), \(F_{0}B\) and \(F_{0}B\), respectively) and antiporter Na\({}^{+}\)/H\({}^{+}\) NhaC (_nhaC_) including _DVU3108_ were upregulated. This indicated that _D. vulgaris_ used mainly Na\({}^{+}\)/H\({}^{+}\) antiporter for regulating the intracellular pH homeostasis. Hence, both the electrochemical gradients of Na\({}^{+}\), or sodium motive force (smf) and pmf are essential for energizing ATP synthesis.  
Electrogenicity of the antiporter can be readily explained by assuming a certain stoichiometry between the Na\({}^{+}\) and protons and kinetics of the exchange, the K\({}_{\text{D}}\) of substrate binding, positive and negative effectors of the activity, structure-function relationships and the reaction mechanism [30; 77; 80]. An electrogenic Na\({}^{+}\)/H\({}^{+}\) antiporter when exposed to extreme environments will be driven by the \(\Delta\)pH initially formed by the primary proton pumps: Na\({}^{+}\) will be extruded and H\({}^{+}\) will be returned to the cell. The antiporters take up external H\({}^{+}\) in exchange for Na\({}^{+}\) from the cytoplasm, with a ratio of proton per sodium ion more than 1. Therefore, the \(\Delta\Psi\) (negative inside relative to the outside) drives the inward H\({}^{+}\) movement and makes it possible to acidify the cytoplasm relative to the bulk medium [30].