9 An Insight into Microbial Remediation of Hexavalent Chromium …
211
E. cloacae HO1 strain and Bacillus sp. QC1-2 (Campos et al. 1995; Thatoi and
Pradhan 2017). However, their metal-reducing potential depends on many factors
such as their own genetic configuration in addition to various environmental factors
where they grow.
It is a topic of debate among researchers that microbes belong to the same species
may not give same Cr
6+ reduction efficiency or have similar Cr
6+ tolerance, and the
nature of growth medium can play a vital role in this differentiation. Mergeay (1995),
Gogoi et al. (2015) reported that, the metal resistant capacity of microbes depends
on providing nutrient medium. Bacillus sphaericus was isolated, investigated by
many researchers and different reducing potential of the same species under different environmental conditions have been reported. These studies reveal that reducing efficiencies of microbes not only depend on the microbial species, but also the
factors like source site of isolation, nutrient medium, initial Cr
6+ concentration,
contact/reaction time, and similar other environmental factors. The summary of the
microbial species studied by many researchers has been classified depending upon
the environmental conditions (aerobic, anaerobic, or both), where they have grown
their tolerance level, contact time (HRT) and provided nutrient medium, along with
their respective Cr
6+ reduction efficiencies are categorized and described in Table 9.1.
Studies show that, maximum numbers of microbes that have a high tolerance level
are mainly aerobic and belong to Bacillus sp. (Table 9.1). However, the tolerance level
varied from 49,650–500 mgL
−1 that may be due to various environmental conditions
used in the studies. Sau et al. (2008) reported that among all the Cr
6+ -resistant
microbes, Bacillus firmus demonstrated maximum tolerance (49,650 mgL
−1 ) to Cr
6+
and it is reduced to 104 mgL
−1 of Cr
6+ within 144 h of incubation time.
Hexavalent chromium tolerance and reduction have been observed by different microbial genus of aerobic and anaerobic group, for example, Bacillus,
Pseudomonas, Nesterenkonia, Cellulosimicrobium, Staphylococcus, Alcaligenes,
Burkholderia, Arthrobacter, Acinetobacter, Desulfovibrio, Pantoea, Methanothermobacter, Desulfovibrio, and Desulfotomaculum. But if reduction efficiency and
utility of microbes for the reduction of Cr
6+ to Cr
3+ are considered, aerobic and facultative microbes are shown better efficacy (Narayani and Shetty 2013; Kumar et al.
2009).
9.3 Cellular Mechanism of Chromium Reduction
Scientific literatures elaborated on the microbial transformation mechanism for
the reduction of Cr
6+ , which suggests that the hydrogenase, nitroreductase, quinine reductase, ChrR, YieF, membrane associate reductase (MR), soluble associate
reductase (SR) enzymes are responsible for catalyzing electron transfer process in
the transformation of Cr
6+ to Cr
3+ (Park et al. 2000; Kwak et al. 2003; Ackerley
et al. 2004; Thatoi et al. 2014). Electron donating species like NAD(P)H mediates
the enzymatic transformation of hexa (+6) to tri (+3) species of chromium but generates Reactive Oxygen Species (ROS) in the process (Park et al. 2001; Chardin et al.
211
E. cloacae HO1 strain and Bacillus sp. QC1-2 (Campos et al. 1995; Thatoi and
Pradhan 2017). However, their metal-reducing potential depends on many factors
such as their own genetic configuration in addition to various environmental factors
where they grow.
It is a topic of debate among researchers that microbes belong to the same species
may not give same Cr
6+ reduction efficiency or have similar Cr
6+ tolerance, and the
nature of growth medium can play a vital role in this differentiation. Mergeay (1995),
Gogoi et al. (2015) reported that, the metal resistant capacity of microbes depends
on providing nutrient medium. Bacillus sphaericus was isolated, investigated by
many researchers and different reducing potential of the same species under different environmental conditions have been reported. These studies reveal that reducing efficiencies of microbes not only depend on the microbial species, but also the
factors like source site of isolation, nutrient medium, initial Cr
6+ concentration,
contact/reaction time, and similar other environmental factors. The summary of the
microbial species studied by many researchers has been classified depending upon
the environmental conditions (aerobic, anaerobic, or both), where they have grown
their tolerance level, contact time (HRT) and provided nutrient medium, along with
their respective Cr
6+ reduction efficiencies are categorized and described in Table 9.1.
Studies show that, maximum numbers of microbes that have a high tolerance level
are mainly aerobic and belong to Bacillus sp. (Table 9.1). However, the tolerance level
varied from 49,650–500 mgL
−1 that may be due to various environmental conditions
used in the studies. Sau et al. (2008) reported that among all the Cr
6+ -resistant
microbes, Bacillus firmus demonstrated maximum tolerance (49,650 mgL
−1 ) to Cr
6+
and it is reduced to 104 mgL
−1 of Cr
6+ within 144 h of incubation time.
Hexavalent chromium tolerance and reduction have been observed by different microbial genus of aerobic and anaerobic group, for example, Bacillus,
Pseudomonas, Nesterenkonia, Cellulosimicrobium, Staphylococcus, Alcaligenes,
Burkholderia, Arthrobacter, Acinetobacter, Desulfovibrio, Pantoea, Methanothermobacter, Desulfovibrio, and Desulfotomaculum. But if reduction efficiency and
utility of microbes for the reduction of Cr
6+ to Cr
3+ are considered, aerobic and facultative microbes are shown better efficacy (Narayani and Shetty 2013; Kumar et al.
2009).
9.3 Cellular Mechanism of Chromium Reduction
Scientific literatures elaborated on the microbial transformation mechanism for
the reduction of Cr
6+ , which suggests that the hydrogenase, nitroreductase, quinine reductase, ChrR, YieF, membrane associate reductase (MR), soluble associate
reductase (SR) enzymes are responsible for catalyzing electron transfer process in
the transformation of Cr
6+ to Cr
3+ (Park et al. 2000; Kwak et al. 2003; Ackerley
et al. 2004; Thatoi et al. 2014). Electron donating species like NAD(P)H mediates
the enzymatic transformation of hexa (+6) to tri (+3) species of chromium but generates Reactive Oxygen Species (ROS) in the process (Park et al. 2001; Chardin et al.