Dark Septate Endophytes (DSE) in Polluted Areas 127Given the stable nature of these compounds, heavy metals are a group of
very persistent pollutants in the soil. Of the 118 elements of the periodic table,
49 are considered as heavy metals. Not all these high-density metals are
particularly toxic in normal concentrations; for instance, iron (Fe),
molybdenum (Mo), Zn and manganese (Mn) are essential micronutrients.
However, others are toxic at low concentrations and have no known function,
such as silver (Ag), arsenic (As), Hg, Cd, Pb and antimony (Sb) (Niess, 1999).
The risk of heavy metals resides in the fact that they can be chemically or
biologically degraded (Kabata-Pendias, 2000), and in addition, they tend to
bioaccumulate provoking diverse toxic effects. High concentrations of these
metals (essential or not) in the soil can produce symptoms of toxicity in plants,
since they displace the essential elements and disable multifaceted activity
causing, for example, inhibition of growth (Van Assche & Clijster, 1990).
Among its negative effects on plant biology, it can result in growth inhibition
or in oxidative stress-related damage, due to the formation of reactive oxygen
species (ROS) (Mudipalli, 2008). Moreover, these metals can also bind to
organic molecules, such as pigments or enzymes, replacing some essential
metals, altering their specific function (Malayeri et al., 2008). In this regard,
heavy metals show a remarkably high affinity for the sulfhydryl, amino,
phosphate, carboxyl and hydroxyl groups. For example in the carbonic
anhydrase enzyme, the Zn atom present in its active center can be replaced by
a heavy metal, thereby reducing Calvin cycle efficiency (Navarro-Aviñó et al.,
2007).
A wealth of physical (cancer, hurts in the kidney, autoimmunity, etc) and
psychical problems (anxiety, passiveness, etc) have been described for
humans. Hence, their persistence, progressive accumulation and the
transference likelihood to other systems pose a threat both to human health and
ecosystems (Becerril et al., 2007). For these reasons, the adoption of necessary
measures to enable the restoration of soil becomes essential.
SOIL (BIO)REMEDIATION
Soil remediation represents a technological challenge both for industries
and governmental institutions. Contaminated soil can be remediated by
chemical, physical or biological techniques (Mc Eldeowney et al., 1993). In
turn, technologies can be classified in two categories: (a) ex-situ, which
requires removal of the polluted soil to treat it (inside/outside) and (b) in situ,
in which remediation is performed without removing the contaminated soil.