sites is also problematic. On the surface of the bacterial cell wall, many functional
groups are present containing a vast range of polysaccharides and proteins
performing as active sites for binding various metal ions. The binding is based on
the living or non-living biomass status, a variety of biomaterials, metal solution
chemistry characteristics, and surrounding environmental setting (Gee and Dudeney
1988). Bacteria can detoxify metals through reduction to a different oxidation state.
General reduction mechanisms change Hg
2+ to Hg
0 , Cr
6+ to Cr
3+ and AsO 4
3À to
AsO 3
3À . Metal precipitation can be done through metallic phosphate precipitation as
a result of dissimilarity reduction or secondary outcome of metabolic mechanisms
concerned with the transformed metals (Valls and de Lorenzo 2002). The process of
chemical modification of compounds through biological agents is known as biotransformation, while formation of CO 2 , NH 4
+ and H 2 O is called mineralization.
Reduction of inorganic mercury (Hg
2+ ) to elemental mercury (Hg
0
) mediated
through mercuric ion reductase is one example of this mechanism (Dash and Das
2012). Some other similar reactions are the change of arsenate to arsenite and
chromium (VI) to its less harmful form chromium (III). But, unsafe waste sites are
usually polluted with organic compounds and metals. Media composition, soil type
and pH decide bioavailability of metals and controls the degree to which metals
oppose biodegradation. In an environment where a lot of different pollutants are
present, the non-biodegradable metal component is removed or stabilized by different processes like mobilization, separation and collection, off-site transfer and
dumping. Metals hinder the mechanism of organic pollutant degradation through
specific degradation enzymes as well as enzymes concerned with general metabolism. The metal bioavailability is controlled through their interaction with organic
compounds. Rosner and Aumercier (1990) reported that common intermediate of
aromatic hydrocarbons, salicylate enhanced uptake of cadmium and toxicity in
Escherichia coli. Biodegradation rate of compounds is decreased, and the acclimation period is increased by the metals which are present as soluble complexed species
and ionic solutes in an environment (Sandrin and Maier 2003). In an environment
which is co-contaminated, the energy supplies to preserve simultaneous metal
resistance and organic degradation are elevated; thus, the instantaneous activity of
bioremediation via microbes is required to execute both the activities under environmental conditions. Therefore, organic contaminant-degrading metal-tolerant bacteria
with biosorption and biotransformation activities are necessary for eliminating both
metal toxicity and organic pollution (Das et al. 2016).
12.5.1 Microorganisms as Biosorbents of Heavy Metals
Various reports show that several prokaryotes and eukaryotes consist of diverse
natural capabilities to biosorb harmful heavy metal ions which provide them dissimilar built-in tolerance especially in diluted solutions (between 10 and 20 mg/L
À1 )
because of their mobility, solubility and bioavailability powers of these metal ions
(Malik 2004; Tabak et al. 2005; Kim et al. 2007; Chen and Wang 2008). Being
eukaryotic or prokaryotic decides the way by which microorganisms interact with
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N. Srivastava
groups are present containing a vast range of polysaccharides and proteins
performing as active sites for binding various metal ions. The binding is based on
the living or non-living biomass status, a variety of biomaterials, metal solution
chemistry characteristics, and surrounding environmental setting (Gee and Dudeney
1988). Bacteria can detoxify metals through reduction to a different oxidation state.
General reduction mechanisms change Hg
2+ to Hg
0 , Cr
6+ to Cr
3+ and AsO 4
3À to
AsO 3
3À . Metal precipitation can be done through metallic phosphate precipitation as
a result of dissimilarity reduction or secondary outcome of metabolic mechanisms
concerned with the transformed metals (Valls and de Lorenzo 2002). The process of
chemical modification of compounds through biological agents is known as biotransformation, while formation of CO 2 , NH 4
+ and H 2 O is called mineralization.
Reduction of inorganic mercury (Hg
2+ ) to elemental mercury (Hg
0
) mediated
through mercuric ion reductase is one example of this mechanism (Dash and Das
2012). Some other similar reactions are the change of arsenate to arsenite and
chromium (VI) to its less harmful form chromium (III). But, unsafe waste sites are
usually polluted with organic compounds and metals. Media composition, soil type
and pH decide bioavailability of metals and controls the degree to which metals
oppose biodegradation. In an environment where a lot of different pollutants are
present, the non-biodegradable metal component is removed or stabilized by different processes like mobilization, separation and collection, off-site transfer and
dumping. Metals hinder the mechanism of organic pollutant degradation through
specific degradation enzymes as well as enzymes concerned with general metabolism. The metal bioavailability is controlled through their interaction with organic
compounds. Rosner and Aumercier (1990) reported that common intermediate of
aromatic hydrocarbons, salicylate enhanced uptake of cadmium and toxicity in
Escherichia coli. Biodegradation rate of compounds is decreased, and the acclimation period is increased by the metals which are present as soluble complexed species
and ionic solutes in an environment (Sandrin and Maier 2003). In an environment
which is co-contaminated, the energy supplies to preserve simultaneous metal
resistance and organic degradation are elevated; thus, the instantaneous activity of
bioremediation via microbes is required to execute both the activities under environmental conditions. Therefore, organic contaminant-degrading metal-tolerant bacteria
with biosorption and biotransformation activities are necessary for eliminating both
metal toxicity and organic pollution (Das et al. 2016).
12.5.1 Microorganisms as Biosorbents of Heavy Metals
Various reports show that several prokaryotes and eukaryotes consist of diverse
natural capabilities to biosorb harmful heavy metal ions which provide them dissimilar built-in tolerance especially in diluted solutions (between 10 and 20 mg/L
À1 )
because of their mobility, solubility and bioavailability powers of these metal ions
(Malik 2004; Tabak et al. 2005; Kim et al. 2007; Chen and Wang 2008). Being
eukaryotic or prokaryotic decides the way by which microorganisms interact with
322
N. Srivastava