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Microbial Enhanced Oil Recovery: Microbiology and Fundamentals 409

adopted themselves to almost all the possible ecosystems on the earth such as oil field
environments [310]. The first microorganism isolated from the oil reservoirs were
SRB and were recovered by Bastin et al. [67].
In 2009, Barton and Fauque [310] specified that 220 species of 60 genera of SRB
have been described until now. They belong to four phyla within the bacteria and
two phyla within the archaea [315,321 325]:
1. Proteobacteria;
2. Firmicutes;
3. Thermodesulfobacteria;
4. Nitrospirae; and
And two phyla of archaea:
1. Euryarchaeota;
2. Crenarchaeota;

10.5.2.3 Fermentative Microorganisms
Fermentation is defined as an anaerobic catabolism of a reduced carbon source to pro-
duce ATP within a strict internal redox balance [326]. A diversity of end-products can
be generated by microorganisms through fermentation such as carbon dioxide, etha-
nol, lactate, butyrate, acetate, and propionate [326]. Two major forms of microbial
catabolism are fermentation and respiration [301]. In the former, all the electrons
from the organic substrates are put back onto the organic products, while in the latter,
the removed electrons are finally transferred to an inorganic electron acceptor such as
oxygen or nitrate [301]. Several respiratory microorganisms have been recovered from
oil fields [177,226,228,286,327 334]. The main difference between the fermentation
and respiration (the electron transport chain-dependent processes) is that the former is
less energetically efficient due to retainment of lots of potential chemical energy in
most of the end products [326]. Fermentation is associated with a negative ΔG due
to breakdown of a large molecule to some more stable smaller products [301].
Youssef et al. [6] mentioned about isolation of some fermentative microorganisms
from oil reservoirs. These authors specified dual metabolic abilities, i.e., fermentative
and respiratory for many microorganisms in this group. The majority of the thermo-
philic fermentative microorganisms isolated from oil fields are belonging to phylum
Thermotogae [263 265,267,335 338] and also the order Thermoanaerobacteriales
[125,200,202,203,339,340] within the class Clostridia, phylum Firmicutes [6]. Acetate
and hydrogen are the end products of the Thermotogae phylum. Moreover, Youssef
et al. [6] mentioned other isolated species capable of fermenting organic acid [258]
and amino acid [259]. In addition some other fermentative bacteria have been
isolated from the oil fields [147,171,194,204,341 344]. More details are available in
literature [6,345].