the C. vulgaris alga (Aksu 1992) and chromium biosorption through G. lucidum and
A. niger fungi.
12.5.4.6 Siderophores
Certain microorganisms are having low molecular weight similar to heavy metal
chelating agents and are known as siderophores. Certain particular iron chelators
siderophores are formed in the medium, when microorganisms are cultured in the
iron-lacking medium. They have a significant function in the complexation of toxic
metals as well as radionuclides through increase in their solubility. Siderophores are
of low molecular weight and consist of catecholate, phenolate or hydroxamate for
attachment which are produced by various bacteria like Actinomycetes Azotobacter
and genus Pseudomonas to confine the iron ions needed for their metabolic activity
as well as for biosorption (Pattus and Abdallah 2000; Gazsó 2001; Das et al. 2007).
12.5.4.7 Biosurfactants
Most surfactants utilized for bioremediation are manufactured industrially from
petroleum, but microorganisms can also produce them. Naturally produced
biosurfactants are saponins made in plants, glycolipids in microorganism and bile
salts in the bodies of organisms. They contain various advantages in comparison to
industrially produced surfactants, like less toxicity towards decomposing
microorganisms and low recalcitrance in the environment, more varieties of chemical structures and ability to work on a wider setting at diverse temperatures and pHs
(Bognolo 1999). Biosurfactants are metabolically produced by aerobic microbes like
bacteria, fungi and yeasts and are released into the medium. The hydrophilic part is
made from amino acids, peptides or saccharides, while the hydrophobic fraction
generally contains saturated or unsaturated fatty acids (Inamuddin et al. 2021).
Biosurfactants have the capability to form numerous structures like micelles,
vesicles, sphere-shaped or irregular lamellar structures, amongst others (Champion
et al. 1995; Mulligan 2005; Li and Li 2011). When challenged by insoluble organic
as well as inorganic pollutants like hydrocarbons, oil, pesticides and heavy metals
such as uranium, cadmium and lead, microorganisms remove their shortcomings
naturally through the excretion of these structures in culture medium attached to their
cell walls. This helps in transporting and translocating insoluble substrates which
ultimately assist in biosorption. In heavy metal ion bioremediation, a complicated
harmless biosurfactant/metallic ion structure is produced which emulsify and solubilize ions and ultimately compartmentalize them physically. Rhamnolipids formed
by P. aeruginosa is amongst highly employed natural biosurfactants in bioremediation (Bognolo 1999; Champion et al. 1995; Mulligan 2005; Tabak et al. 2005; Zhang
et al. 2005).
12.5.4.8 Oxidation-Reduction (Redox)
Metal ions, metalloid and organometal compounds can be mobilized or immobilized
by microorganisms which ultimately promotes redox processes. Prokaryotes alone
can gain energy from oxidation of metals Mn
2+ , Fe
2+ , Co
2+ , Cu, AsO
2À , Se0 or
SeO 3
2À or by reducing Mn
4+ , Fe
3+ , Co
3+ , AsO 4
2À
, SeO 4
2À or SeO 3
2 (Gavrilescu
328
N. Srivastava
A. niger fungi.
12.5.4.6 Siderophores
Certain microorganisms are having low molecular weight similar to heavy metal
chelating agents and are known as siderophores. Certain particular iron chelators
siderophores are formed in the medium, when microorganisms are cultured in the
iron-lacking medium. They have a significant function in the complexation of toxic
metals as well as radionuclides through increase in their solubility. Siderophores are
of low molecular weight and consist of catecholate, phenolate or hydroxamate for
attachment which are produced by various bacteria like Actinomycetes Azotobacter
and genus Pseudomonas to confine the iron ions needed for their metabolic activity
as well as for biosorption (Pattus and Abdallah 2000; Gazsó 2001; Das et al. 2007).
12.5.4.7 Biosurfactants
Most surfactants utilized for bioremediation are manufactured industrially from
petroleum, but microorganisms can also produce them. Naturally produced
biosurfactants are saponins made in plants, glycolipids in microorganism and bile
salts in the bodies of organisms. They contain various advantages in comparison to
industrially produced surfactants, like less toxicity towards decomposing
microorganisms and low recalcitrance in the environment, more varieties of chemical structures and ability to work on a wider setting at diverse temperatures and pHs
(Bognolo 1999). Biosurfactants are metabolically produced by aerobic microbes like
bacteria, fungi and yeasts and are released into the medium. The hydrophilic part is
made from amino acids, peptides or saccharides, while the hydrophobic fraction
generally contains saturated or unsaturated fatty acids (Inamuddin et al. 2021).
Biosurfactants have the capability to form numerous structures like micelles,
vesicles, sphere-shaped or irregular lamellar structures, amongst others (Champion
et al. 1995; Mulligan 2005; Li and Li 2011). When challenged by insoluble organic
as well as inorganic pollutants like hydrocarbons, oil, pesticides and heavy metals
such as uranium, cadmium and lead, microorganisms remove their shortcomings
naturally through the excretion of these structures in culture medium attached to their
cell walls. This helps in transporting and translocating insoluble substrates which
ultimately assist in biosorption. In heavy metal ion bioremediation, a complicated
harmless biosurfactant/metallic ion structure is produced which emulsify and solubilize ions and ultimately compartmentalize them physically. Rhamnolipids formed
by P. aeruginosa is amongst highly employed natural biosurfactants in bioremediation (Bognolo 1999; Champion et al. 1995; Mulligan 2005; Tabak et al. 2005; Zhang
et al. 2005).
12.5.4.8 Oxidation-Reduction (Redox)
Metal ions, metalloid and organometal compounds can be mobilized or immobilized
by microorganisms which ultimately promotes redox processes. Prokaryotes alone
can gain energy from oxidation of metals Mn
2+ , Fe
2+ , Co
2+ , Cu, AsO
2À , Se0 or
SeO 3
2À or by reducing Mn
4+ , Fe
3+ , Co
3+ , AsO 4
2À
, SeO 4
2À or SeO 3
2 (Gavrilescu
328
N. Srivastava