214
A. Naz et al.
2003; Gonzalez et al. 2005). Reactive oxygen species are known to exert harmful
effects on the cellular protein and Deoxyribonucleic acid (DNA) and maybe one
of the main causes for the development of cellular-level genetic mutation that can
eventually lead to malignancy or cancer (Cheung and Gua 2007).
Microorganisms can adapt to metal-polluted environments by transforming their
cellular metabolic processes via adsorption uptake, metal efflux, metal biotransformation or DNA methylation to avoid metal toxicity. Generally, these processes operate either by direct enzymatic reduction of Cr
6+ to less toxic forms Cr
3+ or indirectly
by making metabolic complexes (Camargo et al. 2003; Pei et al. 2009). Microorganisms naturally growing in metal-contaminated environment are hence resistant
to heavy metal toxicity. Works on chromite mine also suggest that microbes isolated
from mine spoils, and effluents are resistant toward Cr-induced toxicity (Das et al.
2013). It is observed that during the biotransformation of Cr
6+ to Cr
3+ , bacterial proteins are generated that protect the cells from oxidative stress due to the production of
ROS. Cellular reduction of hexavalent chromium is an active process that produces
redox intermediate Cr species with (+5) or (+6) oxidation states along with stable
(+3) state, that forms Cr− DNA ducts and is the major cause of toxicity-induced
mutation and chromosomal breaks (Zhitkovich 2011).
According to advances in molecular biology, several chromosomal genes impart
resistance to the toxicity of Cr
6+ in bacteria, and due to this reason, microbial species
are able to proliferate in Cr
6+ contaminated environment showing Cr− resistance
(Thatoi et al. 2014). For example, ChrR chromosomal gene conferred Cr resistance to
Pseudomonas aeruginosa (Aguilar-Barajas et al. 2008; Thatoi et al. 2014). Bacterial
species and Archean are known to evolve various metabolic biochemical processes
for tolerating toxic compounds in both aerobic and anaerobic systems. However, the
biochemical mechanism of the transformation of Cr
6+ differs between aerobic and
anaerobic microorganisms.
9.3.1 Aerobic Microbial Reduction
Aerobic microbes detoxify Cr
6+ by oxygen-dependent processes in which carbonrich substrate acts as electron donor while O
− is the electron acceptor. Cheung et al.
(2006) reviewed that, aerobic microbes reduce Cr
6+ in two or three steps; the first step
is being Cr
6+ to Cr
5+ followed by Cr
4+ and then finally Cr
3+ . In this way, microbial
cells release ChrR and SR enzymes which catalyze electron transfer to reduce Cr
6+
and the formation of intermediate Cr
5+ , which is again reduced to Cr
3+ . The formation
of Cr
4+ intermediate in this process is still mysterious (Fig. 9.1).
Codd et al. (2001) investigated the formation of Cr
4+ during the reduction of Cr
4+
to Cr
3+ and opined that the study of Cr
4+ formation is very difficult because of its
instability may be because of the absence of suitable detecting method (Lay and
Levina 1998). Further, Chirwa and Molokwane (2011), reported that the presence of
NADH, NADPH, and endogenous electrons in the microbial cell serves as electron
donors which provide electrons for the reduction of Cr
6+ .
A. Naz et al.
2003; Gonzalez et al. 2005). Reactive oxygen species are known to exert harmful
effects on the cellular protein and Deoxyribonucleic acid (DNA) and maybe one
of the main causes for the development of cellular-level genetic mutation that can
eventually lead to malignancy or cancer (Cheung and Gua 2007).
Microorganisms can adapt to metal-polluted environments by transforming their
cellular metabolic processes via adsorption uptake, metal efflux, metal biotransformation or DNA methylation to avoid metal toxicity. Generally, these processes operate either by direct enzymatic reduction of Cr
6+ to less toxic forms Cr
3+ or indirectly
by making metabolic complexes (Camargo et al. 2003; Pei et al. 2009). Microorganisms naturally growing in metal-contaminated environment are hence resistant
to heavy metal toxicity. Works on chromite mine also suggest that microbes isolated
from mine spoils, and effluents are resistant toward Cr-induced toxicity (Das et al.
2013). It is observed that during the biotransformation of Cr
6+ to Cr
3+ , bacterial proteins are generated that protect the cells from oxidative stress due to the production of
ROS. Cellular reduction of hexavalent chromium is an active process that produces
redox intermediate Cr species with (+5) or (+6) oxidation states along with stable
(+3) state, that forms Cr− DNA ducts and is the major cause of toxicity-induced
mutation and chromosomal breaks (Zhitkovich 2011).
According to advances in molecular biology, several chromosomal genes impart
resistance to the toxicity of Cr
6+ in bacteria, and due to this reason, microbial species
are able to proliferate in Cr
6+ contaminated environment showing Cr− resistance
(Thatoi et al. 2014). For example, ChrR chromosomal gene conferred Cr resistance to
Pseudomonas aeruginosa (Aguilar-Barajas et al. 2008; Thatoi et al. 2014). Bacterial
species and Archean are known to evolve various metabolic biochemical processes
for tolerating toxic compounds in both aerobic and anaerobic systems. However, the
biochemical mechanism of the transformation of Cr
6+ differs between aerobic and
anaerobic microorganisms.
9.3.1 Aerobic Microbial Reduction
Aerobic microbes detoxify Cr
6+ by oxygen-dependent processes in which carbonrich substrate acts as electron donor while O
− is the electron acceptor. Cheung et al.
(2006) reviewed that, aerobic microbes reduce Cr
6+ in two or three steps; the first step
is being Cr
6+ to Cr
5+ followed by Cr
4+ and then finally Cr
3+ . In this way, microbial
cells release ChrR and SR enzymes which catalyze electron transfer to reduce Cr
6+
and the formation of intermediate Cr
5+ , which is again reduced to Cr
3+ . The formation
of Cr
4+ intermediate in this process is still mysterious (Fig. 9.1).
Codd et al. (2001) investigated the formation of Cr
4+ during the reduction of Cr
4+
to Cr
3+ and opined that the study of Cr
4+ formation is very difficult because of its
instability may be because of the absence of suitable detecting method (Lay and
Levina 1998). Further, Chirwa and Molokwane (2011), reported that the presence of
NADH, NADPH, and endogenous electrons in the microbial cell serves as electron
donors which provide electrons for the reduction of Cr
6+ .