thermophilic, halotolerant, and halophilic in nature. They are the causes for corrosion of pumping facilities and storage tanks, lower quality of oil products, e.g., sulfur
content and a number of limiting factors.
6.2.2 Iron Reducing and Oxidizing Bacteria
Iron reducing bacteria (IRB) reduce the ferric iron (Fe
3+ ) into ferrous (Fe
2+ ) iron
with the oxidation of organic substrates. This reduction of ferric can be coupled with
the fermentation process. Besides, the ferric iron acts as a terminal electron acceptor,
and with other terminal electrons can be used by the IRB (e.g., sulfate). Moreover, as
iron is insoluble in nature; therefore, during the iron reduction process, membrane
bound ferric reductase enzyme is present in the IRB (Lovley 1993). Additionally, the
assimilation of iron after reduction is not possible in IRB. Also, the phylogeny of
IRB is closely related to SRB phylogeny as some IRB have the capabilities to reduce
sulfur. However, in the presence of H 2 , IRB outcompete SRB and methanogens
(Fredrickson and Gorby 1996). Examples of IRB are Geobacter, Shewanella,
Desulfuromonas, and Pelobacter.
Iron oxidizing bacteria (IOB), on the other hand, oxidize the ferrous iron (Fe
2+ )
into ferric iron. These types of bacteria are generally microaerophilic in nature. They
are also found to be autotrophic, heterotrophic, and mixotrophic in nature (Bridge
and Johnson 1998). Examples of some IOB species are Sulfobacillus
thermosulfidooxidans, Sulfobacillus acidophilus, Acidimicrobium, Gallionella
ferruginea, Leptothrix, Sideroxydans, Mariprofundus ferrooxydans, and
Ferritrophicum radicicola. One hypothesis also concludes that IOB indirectly
involve microbial influenced corrosion by deoxygenation and lead to SRB growth.
Thus, these communities are actively involved in corrosion (Emerson et al. 2010).
6.2.3 Nitrate Reducing Bacteria
Nitrate reducing bacteria (NRB) use nitrate as a terminal electron acceptor and
contribute in the nitrogen cycle. Generally, NRB are heterotrophic in nature and
can be facultative anaerobes in nature, sometimes. Also, few of the NRB species can
grow autotrophically with the oxidation of sulfur and iron containing compounds,
and use carbon dioxide or bicarbonate as the carbon source (Matějů et al. 1992).
These different types of their metabolism switch according to the surrounding
environmental conditions. NRB respire nitrate in the limiting oxygen condition
through the membrane bound nitrate reductase enzyme. Fewer NRB species can
also oxidize the sulfide into sulfate or sulfur elements, and are known as sulfide
oxidizing nitrate reducing bacteria (so-NRB). In the oil fields, NRB activity is
induced when the nitrate is injected as a strategy for controlling microbial souring.
During injection of nitrate, NRB outcompete the sulfate reducing bacteria for similar
energy source (oil components). Besides, the so-NRB activity also increases during
this process and decreases sulfide concentration in the production fluids by sulfide
6 Application of Biotechnology in Oil and Gas Industries
117
content and a number of limiting factors.
6.2.2 Iron Reducing and Oxidizing Bacteria
Iron reducing bacteria (IRB) reduce the ferric iron (Fe
3+ ) into ferrous (Fe
2+ ) iron
with the oxidation of organic substrates. This reduction of ferric can be coupled with
the fermentation process. Besides, the ferric iron acts as a terminal electron acceptor,
and with other terminal electrons can be used by the IRB (e.g., sulfate). Moreover, as
iron is insoluble in nature; therefore, during the iron reduction process, membrane
bound ferric reductase enzyme is present in the IRB (Lovley 1993). Additionally, the
assimilation of iron after reduction is not possible in IRB. Also, the phylogeny of
IRB is closely related to SRB phylogeny as some IRB have the capabilities to reduce
sulfur. However, in the presence of H 2 , IRB outcompete SRB and methanogens
(Fredrickson and Gorby 1996). Examples of IRB are Geobacter, Shewanella,
Desulfuromonas, and Pelobacter.
Iron oxidizing bacteria (IOB), on the other hand, oxidize the ferrous iron (Fe
2+ )
into ferric iron. These types of bacteria are generally microaerophilic in nature. They
are also found to be autotrophic, heterotrophic, and mixotrophic in nature (Bridge
and Johnson 1998). Examples of some IOB species are Sulfobacillus
thermosulfidooxidans, Sulfobacillus acidophilus, Acidimicrobium, Gallionella
ferruginea, Leptothrix, Sideroxydans, Mariprofundus ferrooxydans, and
Ferritrophicum radicicola. One hypothesis also concludes that IOB indirectly
involve microbial influenced corrosion by deoxygenation and lead to SRB growth.
Thus, these communities are actively involved in corrosion (Emerson et al. 2010).
6.2.3 Nitrate Reducing Bacteria
Nitrate reducing bacteria (NRB) use nitrate as a terminal electron acceptor and
contribute in the nitrogen cycle. Generally, NRB are heterotrophic in nature and
can be facultative anaerobes in nature, sometimes. Also, few of the NRB species can
grow autotrophically with the oxidation of sulfur and iron containing compounds,
and use carbon dioxide or bicarbonate as the carbon source (Matějů et al. 1992).
These different types of their metabolism switch according to the surrounding
environmental conditions. NRB respire nitrate in the limiting oxygen condition
through the membrane bound nitrate reductase enzyme. Fewer NRB species can
also oxidize the sulfide into sulfate or sulfur elements, and are known as sulfide
oxidizing nitrate reducing bacteria (so-NRB). In the oil fields, NRB activity is
induced when the nitrate is injected as a strategy for controlling microbial souring.
During injection of nitrate, NRB outcompete the sulfate reducing bacteria for similar
energy source (oil components). Besides, the so-NRB activity also increases during
this process and decreases sulfide concentration in the production fluids by sulfide
6 Application of Biotechnology in Oil and Gas Industries
117