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120 S. Chatterjee et al.

widely, ranging from low-molecular weight aliphatic, aromatic, amino acids, and
fulvic acids (soluble portion) and thus organic complexes with metals is poorly
defined (McLean and Bledsoe 1992). The transport of metals in the soil solution is
considerably affected by the complexes formed with the soil matrix. Binding of
metals with organic matters like plant exudates or humus occupies a continuum of
reactive sites, ranging from weak to strong chemical bonds.
Traditional methods of mitigating metal contamination in soils and water include
various isolation, extraction, immobilization, and toxicity reduction methods,
including isolation or physical barrier (i.e., concrete, steel); chemical solidification
or stabilization; hydrocyclone, fluidized bed, or flotation processes; electro kinetic
processes; soil washing; and pump-and-treat systems (Mulligan et al. 2001). These
methods for metal sequestration are prohibitively expensive (around $400 to $750
billion in the USA alone), energy intensive, and can reduce the fertility and bioac-
tivity of soils. The tremendous economic costs of technology-based environmental
remediation are not a viable option for most of the developing countries to go for
such expensive outlay (Mulligan et al. 2001). Moreover, there is no effective way to
deactivate radioactive materials, except to allow them to decay in a site. Unfortu-
nately, many of radionuclides have very long half-lives (e.g., Sr-90: 28 years;
Cs-137: 30 years; Pu-239: 24,100 years; Tc-97: 2.6 million years; and U-235: 7.13
million years). Further methods like incineration and land-filling also raise several
questions like, air/soil/groundwater pollution, and translocation of contaminants
from one site to another. The problem of heavy metal contamination persists even
with the disposal of incineration residues like land filling. Though the rate of heavy
metals mobility in landfills is very low, however, landfills are not the permanent
solution to contain heavy metals for long times. The high cost and other limitations
of technology-based remediation is perhaps the driving factor in the development of
alternative remediation technologies (Korda et al. 1997; Brim et al. 2000).
Natural biodegradation can reduce waste and help in cleaning up of varied
types of environmental contaminants. By definition, bioremediation (includes
phytoremediation) is the use of living organisms (bacteria and fungi or plants) for
degrading or detoxifying the hazardous environmental pollutants into less toxic
forms (Robles-Gonza ´lez et al. 2008; Cozzarelli et al. 2010). Specific contaminants
may be targeted for bioremediation like degradation of chlorinated hydrocarbons or
such other compounds by indigenous or exogenous bacteria. Nevertheless, biodeg-
radation is a complex process involving orchestrated actions of a string of
organisms (Cho et al. 2000).
Microorganisms have the capacity to remove many contaminants from the
environment by a diversity of enzymatic process. Oxidation of toxic, organic
components to non-toxic product is one of the common types of bioremediation
process taking place by microorganism having wide phylogenetic diversity. Aro-
matic hydrocarbons, xenobiotics and pesticides, and range of organic contaminants
(Landmeyer et al. 2010; Landmeyer 2011) are usually aerobically degraded, as
oxygen is the most commonly preferred electron acceptor in microbial respiration.
However, a number of microorganisms along with plants (phytoremediation), as a
result of their versatility, adaptability, and diversity in the environment,