## 4 SRB Activities in Alkaline Environment
### Response of SRB to Alkaline Environment
#### 4.1.3 Increase in Metabolic Production Change in Metabolic Pathway  
Previous literature reported the consumption of amino acid can lead to the protonation of cytoplasm to minimize the alkaline stress of _E. coli_[85], _Lactobacillus plantarum_[86]. Several enzymes associated with amino acid synthesis were induced when bacteria exposed to alkaline environment. In _E. coli_, these enzymes include: enzyme tryptophan deaminase (TnaA) which produces deaminases resulting in generating acid that is adaptive to alkaline challenge [85]. Enzyme o-acetylserine sulfhydrylase A (CysK), which generates NH\({}_{3}\) and acids was shown to be expressed at high level in alkaline conditions [76]. In _D. vulgaris_, gene expressions associated with amino acid were reported to be moderately upregulated [73].  
Similar to _E. coli_, gene expressions of _DVU10470_ and _DVU10471_ encoded tryptophan synthase subunits \(\beta\) and \(\alpha\) related to tryptophan deaminase were shown to be upregulated. Additionally, there were increases in gene expressions of biosynthesis and transport of amino acids, including: gene expression for cysteine synthase A (_DVU10663_) which is considered a key enzyme in producing the amino acid cysteine [87]; dihydrodipicolinate reductase (_DVU1609_) which catalyses the second step of lysine-a type of amino acid biosynthesis; isopropylmalate dehydratase (_DVU12982, 2983_) and isopropylmalate synthase (_DVU2981_) which participates in biosynthesis of L-leucine and pyruvate metabolism; and homoserine dehydrogenase (_DVU10890_) which catalyses the third step in the aspartate pathway and an intermediate in the biosynthesis of threonine, isoleucine, and methionine. This indicates the employment of multicomponent of amino acid metabolism under alkaline stress.  
In addition to increase in amino acid synthesis, SRB were found to change their metabolic pathways to adapt to the extreme environments. Genes in _D. vulgaris_ related in some energy production and central metabolic pathways were shown to be downregulated, including a decrease in lactate oxidation [73]. Besides, there were increases in gene expressions which related to energy generation and electron transfer reactions under alkaline stress. Genes related to molybdenum oxidoreductase include molybdenum oxidoreductase (_DVU10692_), respiratory nitrate reductase (_DVU10693_), molybdenum oxidoreductase--and molybdenum--binding subunit (_DVU10694_) was notably upregulated. These genes were similar to molybdenum oxidoreductase--(mopA, B, C, D) which were found in _Desulfovibrio desulfuricans G20_. These genes were shown to be involved in transferring electrons for sulphate reduction in cytoplasm [88]. Genes expression of formate dehydrogenase (_DVU10587, 0588, 2481, 2482_), thiosulphate reductase (_DVU10179_), cytoplasmic Coo hydrogen--nase (_DVU2286-2291_) and periplasmic (NiFe) hydrogen--(_D2526_) which are associated in transferring electrons were shown upregulated [73]. Due to the inhibition of lactate consumption and acetate production, the increase in these genes indicates the redirection of electron flow to sustain the redox of cytoplasm, thus controlling proton intake by pmf.