and Wang 2008). Various approaches are developed through recombinant DNA
technology to get genetically superior strains to employ in the biosorption mechanism. The bacterial cell wall is geared up by these various approaches to perform as
anchors through metal ion-binding polypeptides. Of these, one investigation was in
connection with the fusion protein of the α-domain of IgA protease of Neisseria
gonorrhoeae and metallothionein (MT) from rats (Valls et al. 2000) and lpp-ompAvarious sizes of peptides (EC20) (Bae et al. 2000). The major biosorption processes
developed by microorganisms are also regarded as resistance or tolerance
mechanisms towards heavy metals.
12.5.4.1 Transport Across Cell Membranes
Transfer of heavy metals through microbial cell membranes is probably conducted
by similar processes which are employed to pass on metabolically significant ions
like potassium, magnesium and sodium. The metal transfer systems may get puzzled
because of the existence of similarly charged heavy metal ions as well as ionic radius
linked with necessary ions. This type of process is not related to metabolic activity.
Principally, biosorption by living organisms includes two stages: the first one is
metabolism-independent, where metals are attached to the cell walls, and the second
one is intracellular intake, which is metabolism-dependent and metal ions are
transferred through the cell membrane (Costa and Leite 1991; Gadd 1988; Huang
et al. 1990; Nourbakhsh et al. 1994).
12.5.4.2 Ion Exchange
The cell wall of a microorganism possesses several types of polysaccharides and has
a substitution of bivalent metal ions through the counter ions of the polysaccharides
as alginates in marine algae are found as K
+, Na
+
, Ca
2+ and Mg
2+ salts. Heavy metal
biosorption can be done by replacement through counter ions like CO
2+ , Cu
2+ , Cd
2+
and Zn
2+ (Kuyucak and Volesky 1988). Copper biosorption through Ganoderma
lucidium (Muraleedharan and Venkobachar 1990) and Aspergillus niger fungi is one
of the examples of this phenomena.
12.5.4.3 Complexation
Electrostatic interaction of metallic ion chelating agent with a polymer produced by a
viable or unviable microorganism results in an extracellular complexation or coordination. This could be due to the biosurfactants, polysaccharides, proteins and
nucleic acids. Electrostatic attraction is because of electron pairs of chelating agents,
and no electron transfer will take place if they adhere to the metallic ions. The final
composition has the electric charge which is the total of all individual charges of
every member of the complex (Veglio and Beolchini 1997; Davis et al. 2003).
“Oxidative stress” in the cell will happen if this detoxification/complexation system
is filled to its full capacity (Gavrilescu 2004). A complex is formed over the cell
surface by the interaction of metals with active groups which removes metal from the
solution. According to Aksu (1992), Chlorella vulgaris and Zoogloea ramigera
carry out biosorption of copper by adsorption as well as by creating coordination
bonds amongst metals and amino and carboxylic groups present in polysaccharides
326
N. Srivastava
technology to get genetically superior strains to employ in the biosorption mechanism. The bacterial cell wall is geared up by these various approaches to perform as
anchors through metal ion-binding polypeptides. Of these, one investigation was in
connection with the fusion protein of the α-domain of IgA protease of Neisseria
gonorrhoeae and metallothionein (MT) from rats (Valls et al. 2000) and lpp-ompAvarious sizes of peptides (EC20) (Bae et al. 2000). The major biosorption processes
developed by microorganisms are also regarded as resistance or tolerance
mechanisms towards heavy metals.
12.5.4.1 Transport Across Cell Membranes
Transfer of heavy metals through microbial cell membranes is probably conducted
by similar processes which are employed to pass on metabolically significant ions
like potassium, magnesium and sodium. The metal transfer systems may get puzzled
because of the existence of similarly charged heavy metal ions as well as ionic radius
linked with necessary ions. This type of process is not related to metabolic activity.
Principally, biosorption by living organisms includes two stages: the first one is
metabolism-independent, where metals are attached to the cell walls, and the second
one is intracellular intake, which is metabolism-dependent and metal ions are
transferred through the cell membrane (Costa and Leite 1991; Gadd 1988; Huang
et al. 1990; Nourbakhsh et al. 1994).
12.5.4.2 Ion Exchange
The cell wall of a microorganism possesses several types of polysaccharides and has
a substitution of bivalent metal ions through the counter ions of the polysaccharides
as alginates in marine algae are found as K
+, Na
+
, Ca
2+ and Mg
2+ salts. Heavy metal
biosorption can be done by replacement through counter ions like CO
2+ , Cu
2+ , Cd
2+
and Zn
2+ (Kuyucak and Volesky 1988). Copper biosorption through Ganoderma
lucidium (Muraleedharan and Venkobachar 1990) and Aspergillus niger fungi is one
of the examples of this phenomena.
12.5.4.3 Complexation
Electrostatic interaction of metallic ion chelating agent with a polymer produced by a
viable or unviable microorganism results in an extracellular complexation or coordination. This could be due to the biosurfactants, polysaccharides, proteins and
nucleic acids. Electrostatic attraction is because of electron pairs of chelating agents,
and no electron transfer will take place if they adhere to the metallic ions. The final
composition has the electric charge which is the total of all individual charges of
every member of the complex (Veglio and Beolchini 1997; Davis et al. 2003).
“Oxidative stress” in the cell will happen if this detoxification/complexation system
is filled to its full capacity (Gavrilescu 2004). A complex is formed over the cell
surface by the interaction of metals with active groups which removes metal from the
solution. According to Aksu (1992), Chlorella vulgaris and Zoogloea ramigera
carry out biosorption of copper by adsorption as well as by creating coordination
bonds amongst metals and amino and carboxylic groups present in polysaccharides
326
N. Srivastava