oxidizing nitrate reducing bacteria (so-NRB) which oxidize sulfide into sulfate
coupled with nitrate reduction. Hence, the sulfate reduction into sulfide by SRB
and re-oxidation into sulfate by so-NRB are complementary metabolic cycles.
The reduction of nitrate by hNRB and so-NRB forms metabolic intermediate, i.e.,
nitrite which further inhibits the SRB growth. Since the structure of nitrite acts as a
sulfide analog, it therefore easily binds with dissimilatory sulfite reductase (Dsr)
enzyme, which is responsible for sulfate reduction in SRB. This interaction further
inhibits the Dsr enzyme thereby limiting the sulfide production. Also, excess accumulation of nitrite can chemically oxidize the sulfide into sulfur, polysulfide, and
sulfate (Sanders and Sturman 2005; Kaster et al. 2007; Lin et al. 2009). In such
reaction, nitrite is also reduced in N 2 and NH 4
+
. Besides, an increase in redox
potential by conversion of nitrite to NO and N 2 O also inhibits the SRB growth
(from À400 mV to +100 mV) (Nemati et al. 2001; Hubert et al. 2009). Moreover,
limiting nitrate concentrations during nitrite reduction continuously leads to end
products N 2 and NH 4
+ formation (Hubert and Judd 2010). NRBs are known to
catalyze the reduction of nitrate via two main pathways in the presence of carbon and
energy sources. Conversion of ammonium by dissimilatory nitrate reduction or
de-nitrification of nitrogen results in the reduction of nitrate. In the oil reservoir,
both these processes of nitrate reduction have been observed (Gevertz et al. 2000;
Hubert and Voordouw 2007).
The use of nitrate for controlling souring has different advantages. In general, the
biocide treatment has its efficacy against all the resident microbial population present
in the reservoir. However, nitrate is known to target SRB specifically and thus can
have an added advantage over biocides. Though nitrate can also have some negative
effects such as accumulation of biomass leads to pipeline bio-fouling and downstream corrosion due to the change in oxidizing potential (Martin 2008). Dissimilatory nitrate reduction present in the facultative organism also utilizes nitrate as an
alternative to aerobic or fermentative growth. Some of the SRB are known to
upregulate the nitrite reductase (nrf) gene which detoxifies nitrite to ammonium
whereby down-regulating Dsr gene (Haveman et al. 2004). This strategy is not much
common among SRB, though nrf transiently inhibits the sulfate reduction by nitrite.
Some SRB, for example, Desulfovibrio gracilis has the ability to reduce nitrate in the
presence of high sulfate concentration.
In comparison to nitrate, nitrite acts as a good inhibitor because it directly inhibits
the Dsr enzyme. Some successful field trials have also demonstrated the nitrite
application as a biocide for the inhibition of microbial souring, e.g., New Mexico
field (Sturman et al. 1999). However, nitrate is easier to handle and is less toxic than
nitrite. Additional researches have also proposed the synergistic effect of biocide
with nitrate injection. Nitrite and other biocide synergy have also been tested with
Desulfovibrio sp. SRB consortium. Few reports have mentioned, injection of biocide
with a nitrite combination or mixture of different biocides can inhibit the SRB
community successfully (Greene et al. 2006).
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G. Prajapat et al.
coupled with nitrate reduction. Hence, the sulfate reduction into sulfide by SRB
and re-oxidation into sulfate by so-NRB are complementary metabolic cycles.
The reduction of nitrate by hNRB and so-NRB forms metabolic intermediate, i.e.,
nitrite which further inhibits the SRB growth. Since the structure of nitrite acts as a
sulfide analog, it therefore easily binds with dissimilatory sulfite reductase (Dsr)
enzyme, which is responsible for sulfate reduction in SRB. This interaction further
inhibits the Dsr enzyme thereby limiting the sulfide production. Also, excess accumulation of nitrite can chemically oxidize the sulfide into sulfur, polysulfide, and
sulfate (Sanders and Sturman 2005; Kaster et al. 2007; Lin et al. 2009). In such
reaction, nitrite is also reduced in N 2 and NH 4
+
. Besides, an increase in redox
potential by conversion of nitrite to NO and N 2 O also inhibits the SRB growth
(from À400 mV to +100 mV) (Nemati et al. 2001; Hubert et al. 2009). Moreover,
limiting nitrate concentrations during nitrite reduction continuously leads to end
products N 2 and NH 4
+ formation (Hubert and Judd 2010). NRBs are known to
catalyze the reduction of nitrate via two main pathways in the presence of carbon and
energy sources. Conversion of ammonium by dissimilatory nitrate reduction or
de-nitrification of nitrogen results in the reduction of nitrate. In the oil reservoir,
both these processes of nitrate reduction have been observed (Gevertz et al. 2000;
Hubert and Voordouw 2007).
The use of nitrate for controlling souring has different advantages. In general, the
biocide treatment has its efficacy against all the resident microbial population present
in the reservoir. However, nitrate is known to target SRB specifically and thus can
have an added advantage over biocides. Though nitrate can also have some negative
effects such as accumulation of biomass leads to pipeline bio-fouling and downstream corrosion due to the change in oxidizing potential (Martin 2008). Dissimilatory nitrate reduction present in the facultative organism also utilizes nitrate as an
alternative to aerobic or fermentative growth. Some of the SRB are known to
upregulate the nitrite reductase (nrf) gene which detoxifies nitrite to ammonium
whereby down-regulating Dsr gene (Haveman et al. 2004). This strategy is not much
common among SRB, though nrf transiently inhibits the sulfate reduction by nitrite.
Some SRB, for example, Desulfovibrio gracilis has the ability to reduce nitrate in the
presence of high sulfate concentration.
In comparison to nitrate, nitrite acts as a good inhibitor because it directly inhibits
the Dsr enzyme. Some successful field trials have also demonstrated the nitrite
application as a biocide for the inhibition of microbial souring, e.g., New Mexico
field (Sturman et al. 1999). However, nitrate is easier to handle and is less toxic than
nitrite. Additional researches have also proposed the synergistic effect of biocide
with nitrate injection. Nitrite and other biocide synergy have also been tested with
Desulfovibrio sp. SRB consortium. Few reports have mentioned, injection of biocide
with a nitrite combination or mixture of different biocides can inhibit the SRB
community successfully (Greene et al. 2006).
122
G. Prajapat et al.