volatilized into the air or can precipitate because of its low solubility in water
eliminates harmful Hg
2+ (Wagner-Dobler 2003). Microbial as well as abiotic
transformations can regulate the production of methyl mercury in the environment.
Hg
2+ methylation and MeHg degradation are involved in direct transformation.
Reduction of Hg
2+ to Hg
0 as well as its cyclic oxidation influences formation of
MeHg indirectly through the regulation of Hg
2+ which is the substrate for methylation (Barkay and Wagner-Dobler 2005). In fact, these days, an eco-friendly and
cheaper treatment technology is employed for cleaning mercury-polluted wastewater
through mercury-tolerant microbes. This was proved through the manufacturing of
pilot plants to eliminate mercury which remediates 100 m
3 of 50% wastewater
having mercury per day (Wagner-Dobler 2003). In addition to enzymatic transformation, metallothioneins and polyphosphates can also be employed for
bioremediating mercury through sequestration of mercury ions in a form which is
biologically inactive. Polyphosphate kinase is coded by ppk gene and participates in
polyphosphate biosynthesis. These are orthophosphate polymers with negative
charges and can bind with mercury ions (Kornberg 1995). ppk genes expressed in
several transgenic bacteria express ppk genes to tolerate as well as store about 16 μM
of mercury from solutions (Pan-Hou et al. 2002). An additional strategy is the
production of mercuric sulphide (HgS) through directly reacting Hg
2+ with H 2 S
generated anaerobically in Clostridium cochlearium (Pan-Hou and Imura 1981). It
was observed in the same type of studies that Klebsiella aerogenes NCTC418
generated HgS in continuous aerobic culture with mercury chloride. Biological
elimination of mercury was done through a mercury-reducing biofilm having natural
as well as engineered mercuric reductase (Brunke et al. 1993). Sulphate-reducing
bacteria is also utilized as a source of H 2 S which precipitates metal as sulphides. The
HgS generates high-solubility products which eliminate mercury through H 2 S
(Hakansson et al. 2008).
12.7.5 Nickel
Nickel is present in five stable isotopes:
58 Ni,
60 Ni,
61 Ni,
62 Ni, and
64 Ni. Of these,
58 Ni is highly abundant metal in the environment. Nickel plays a very significant
function in the biochemistry of microbes as well as plants. The enzyme urease
consists of nickel. Other enzymes such as hydrogenases, superoxide dismutase and
glyoxalase enzyme possess Ni-Fe clusters or employ nickel like a co-factor. Nickel
is extremely harmful to animals and humans because of its ability to cross the
placenta and influence the fetus. Nickel tolerance in bacteria is usually mediated
through efflux pumps. One such process of resistance has been investigated by Grass
et al. (2000) in C. (Ralstonia) metallidurans CH34. They observed that
cnrYHXCBAT gene system codes cnrCBA efflux pump. Transcription begins at
the cnr promoter through cnrY and cnrC after nickel gets an entry into the periplasm.
The products of the three genes cnrYXH control the induction of complete gene
cluster. cnrH is actually an extracytoplasmic function (ECF) sigma factor (Grass
et al. 2000) that continuously expresses cnrCBA. Membrane-bound proteins cnrX
340
N. Srivastava
eliminates harmful Hg
2+ (Wagner-Dobler 2003). Microbial as well as abiotic
transformations can regulate the production of methyl mercury in the environment.
Hg
2+ methylation and MeHg degradation are involved in direct transformation.
Reduction of Hg
2+ to Hg
0 as well as its cyclic oxidation influences formation of
MeHg indirectly through the regulation of Hg
2+ which is the substrate for methylation (Barkay and Wagner-Dobler 2005). In fact, these days, an eco-friendly and
cheaper treatment technology is employed for cleaning mercury-polluted wastewater
through mercury-tolerant microbes. This was proved through the manufacturing of
pilot plants to eliminate mercury which remediates 100 m
3 of 50% wastewater
having mercury per day (Wagner-Dobler 2003). In addition to enzymatic transformation, metallothioneins and polyphosphates can also be employed for
bioremediating mercury through sequestration of mercury ions in a form which is
biologically inactive. Polyphosphate kinase is coded by ppk gene and participates in
polyphosphate biosynthesis. These are orthophosphate polymers with negative
charges and can bind with mercury ions (Kornberg 1995). ppk genes expressed in
several transgenic bacteria express ppk genes to tolerate as well as store about 16 μM
of mercury from solutions (Pan-Hou et al. 2002). An additional strategy is the
production of mercuric sulphide (HgS) through directly reacting Hg
2+ with H 2 S
generated anaerobically in Clostridium cochlearium (Pan-Hou and Imura 1981). It
was observed in the same type of studies that Klebsiella aerogenes NCTC418
generated HgS in continuous aerobic culture with mercury chloride. Biological
elimination of mercury was done through a mercury-reducing biofilm having natural
as well as engineered mercuric reductase (Brunke et al. 1993). Sulphate-reducing
bacteria is also utilized as a source of H 2 S which precipitates metal as sulphides. The
HgS generates high-solubility products which eliminate mercury through H 2 S
(Hakansson et al. 2008).
12.7.5 Nickel
Nickel is present in five stable isotopes:
58 Ni,
60 Ni,
61 Ni,
62 Ni, and
64 Ni. Of these,
58 Ni is highly abundant metal in the environment. Nickel plays a very significant
function in the biochemistry of microbes as well as plants. The enzyme urease
consists of nickel. Other enzymes such as hydrogenases, superoxide dismutase and
glyoxalase enzyme possess Ni-Fe clusters or employ nickel like a co-factor. Nickel
is extremely harmful to animals and humans because of its ability to cross the
placenta and influence the fetus. Nickel tolerance in bacteria is usually mediated
through efflux pumps. One such process of resistance has been investigated by Grass
et al. (2000) in C. (Ralstonia) metallidurans CH34. They observed that
cnrYHXCBAT gene system codes cnrCBA efflux pump. Transcription begins at
the cnr promoter through cnrY and cnrC after nickel gets an entry into the periplasm.
The products of the three genes cnrYXH control the induction of complete gene
cluster. cnrH is actually an extracytoplasmic function (ECF) sigma factor (Grass
et al. 2000) that continuously expresses cnrCBA. Membrane-bound proteins cnrX
340
N. Srivastava