oxidation. Some of the NRBs are also capable of incomplete reduction of nitrate
under stress conditions such as temperature, high nitrate concentration in environment which leads to conversion of nitrate to nitrite only. The nitrite so produced is a
strong inhibitor of the Dsr enzyme of SRB and therefore limits the growth of SRB
population. Besides, nitrate reduction occurs at a high redox potential which also
limits SRB growth with increasing redox potential.
6.2.4 Methanogens
Methanogens are strictly anaerobic archaea which produce methane as a by-product
during metabolism. These organisms can be present in any part of the sediments
when the sulfate is depleted. Methanogens typically use methanol, methylamines,
methyl sulfate, formate, H 2 + CO 2 or CO and acetate as a carbon and energy source
(Schlegel and Müller 2011). They are of two types: acetoclastic methanogen and
hydrogenotrophic methanogen. Acetoclastic methanogens use acetate, while
hydrogenotrophs use hydrogen and CO 2 . These communities are present in swamps,
digestive systems of animals, marine sediments, hot springs, hydrothermal vents,
and oil fields.
Methanogens belong to the phylum Euryarchaeota in the archaea domain. They
are further classified into seven orders: Methanococcales, Methanobacteriales,
Methanosarcinales, Methanomicrobiales, Methanopyrales, Methanocellales, and
Methanomassiliicoccales (Enzmann et al. 2018). In Methanosarcinales order
acetoclastic methanogens are present, while in order Methanococcales,
Methanobacteriales, Methanomicrobiales, and Methanopyrales hydrogenotrophic
methanogens are present. Methylotrophic methanogens use methylated compounds
like methanol, methylamines, or methylated thiols. These are present in the order
Methanomassiliicoccales, Methanobacteriales, and Methanosarcinales.
6.2.5 Acid Producing and Fermenting Bacteria
Acid producing bacteria (APB) produce the acids (e.g., sulfuric acids) during their
metabolic activities. This produced acid causes corrosion in the oil field installations.
These are fermentative bacteria known to gain energy from the substrate level
phosphorylation. These bacterial population use a wide range of organic compounds
like sugars, peptides, amino acids, and organic acids. Many reports have also shown
APB to use inorganic sulfur compound, ferric ions, and nitrate as a terminal electron
acceptor. Besides, fermentative bacteria can grow at wide ranges of salinity and
temperature conditions. Mesophilic, saccharolytic fermentative Halanaerobium uses
disaccharide and monosaccharide in fermentative pathways (Ollivier and Magot
2005). Fermentative bacteria in oil fields are present in separators and filters where
extensive involves in biodegradation of oil. Thermophilic fermentative bacteria are
also present in oil fields and belong to the order Thermotogales (Prajapat et al. 2019).
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under stress conditions such as temperature, high nitrate concentration in environment which leads to conversion of nitrate to nitrite only. The nitrite so produced is a
strong inhibitor of the Dsr enzyme of SRB and therefore limits the growth of SRB
population. Besides, nitrate reduction occurs at a high redox potential which also
limits SRB growth with increasing redox potential.
6.2.4 Methanogens
Methanogens are strictly anaerobic archaea which produce methane as a by-product
during metabolism. These organisms can be present in any part of the sediments
when the sulfate is depleted. Methanogens typically use methanol, methylamines,
methyl sulfate, formate, H 2 + CO 2 or CO and acetate as a carbon and energy source
(Schlegel and Müller 2011). They are of two types: acetoclastic methanogen and
hydrogenotrophic methanogen. Acetoclastic methanogens use acetate, while
hydrogenotrophs use hydrogen and CO 2 . These communities are present in swamps,
digestive systems of animals, marine sediments, hot springs, hydrothermal vents,
and oil fields.
Methanogens belong to the phylum Euryarchaeota in the archaea domain. They
are further classified into seven orders: Methanococcales, Methanobacteriales,
Methanosarcinales, Methanomicrobiales, Methanopyrales, Methanocellales, and
Methanomassiliicoccales (Enzmann et al. 2018). In Methanosarcinales order
acetoclastic methanogens are present, while in order Methanococcales,
Methanobacteriales, Methanomicrobiales, and Methanopyrales hydrogenotrophic
methanogens are present. Methylotrophic methanogens use methylated compounds
like methanol, methylamines, or methylated thiols. These are present in the order
Methanomassiliicoccales, Methanobacteriales, and Methanosarcinales.
6.2.5 Acid Producing and Fermenting Bacteria
Acid producing bacteria (APB) produce the acids (e.g., sulfuric acids) during their
metabolic activities. This produced acid causes corrosion in the oil field installations.
These are fermentative bacteria known to gain energy from the substrate level
phosphorylation. These bacterial population use a wide range of organic compounds
like sugars, peptides, amino acids, and organic acids. Many reports have also shown
APB to use inorganic sulfur compound, ferric ions, and nitrate as a terminal electron
acceptor. Besides, fermentative bacteria can grow at wide ranges of salinity and
temperature conditions. Mesophilic, saccharolytic fermentative Halanaerobium uses
disaccharide and monosaccharide in fermentative pathways (Ollivier and Magot
2005). Fermentative bacteria in oil fields are present in separators and filters where
extensive involves in biodegradation of oil. Thermophilic fermentative bacteria are
also present in oil fields and belong to the order Thermotogales (Prajapat et al. 2019).
118
G. Prajapat et al.