• Phytodegradation (phytotransformation): Certain enzymes, viz. nitroreductases
(degradation of nitroaromatic compounds), dehalogenases (degradation of chlorinated solvents and pesticides) and laccases (degradation of anilines) degrade or
metabolize the organic contaminants or mineralize inside the plant cells. For
example, Populus sp., Myriophyllum spicatum, Algae and stonewart (Katyayan
2019; Rylott and Bruce 2008; Schnoor et al. 1995).
• Phytostabilization (phytoimmobilization): Plant roots reduce the movement of
contaminants (organic or inorganic) off-site.
Root exudates act by precipitating the metals as insoluble forms and thus are
subsequently trapped in the soil matrix. In this manner, the mobilization and
diffusion of contaminants are restricted in the soil, e.g. Haumaniastrum,
Eragrostis, Ascolepis, Gladiolus, Alyssum, Indian mustard (Ali et al. 2013;
Berti and Cunningham 2000; Domínguez et al. 2009; Katyayan 2019; Prasad
2004).
• Phytovolatilization: Some plants have the ability to both absorb and volatilize
certain metals and metalloids. Certain metals like Hg, Se and As can be absorbed
by roots and transformed into non-toxic forms, thereby released into the atmosphere. Se can be absorbed and degraded by Astragalus bisulcatus and Stanleya
pinnata. Plant species like Nicotiana tabacum, Liriodendron tulipifera or Brassica napus for reducing the toxic effect of Hg (Brooks 1998; Katyayan 2019;
Pilon-Smits and LeDuc 2009; Pilon-Smits and Pilon 2000; Poschenrieder and
Barceló 2004; Ruiz and Daniell 2009).
• Phytoextraction (phytoaccumulation, phytoabsorption or phytosequestration): In
this method, plants accumulate metals and radionuclides and transport them to
their harvest, i.e. aerial parts. Application of this technique can be applied to the
metals, viz. Cd, Ni, Cu, Zn, Pb, Se, As etc. and other organic compounds.
Elsholtzia splendens, Alyssum bertolonii, Thlaspi caerulescens and Pteris vittata
are the hyperaccumulator plants which are known to carry out this process, which
are known to store high concentrations of these metals in their aerial plants (this
may vary from 0.01% to 1% dry weight, depending on the metal) (Blaylock and
Huang 2000; Hernández-Allica et al. 2008; Ma et al. 2001; McGrath 1998;
McGrath and Zhao 2003; Pedron et al. 2009; Van der Ent et al. 2013; Xie et al.
2009).
3.5 Parameters Affecting Bioremediation
Many factors combine in such a manner that the process of bioremediation can be
taken care of systematically. The availability of the contaminants to the microbes
possessing degradation abilities along with favourable conditions, viz., soil type,
temperature, pH, O 2 or other electron acceptors and nutrient availability is essential
(Abatenh et al. 2017).
Various chemical and physical wastes produced due to numerous human activities are degraded, removed, altered, immobilized and detoxified from the
3 Microbial Indicators of Bioremediation: Potential and Success
93
(degradation of nitroaromatic compounds), dehalogenases (degradation of chlorinated solvents and pesticides) and laccases (degradation of anilines) degrade or
metabolize the organic contaminants or mineralize inside the plant cells. For
example, Populus sp., Myriophyllum spicatum, Algae and stonewart (Katyayan
2019; Rylott and Bruce 2008; Schnoor et al. 1995).
• Phytostabilization (phytoimmobilization): Plant roots reduce the movement of
contaminants (organic or inorganic) off-site.
Root exudates act by precipitating the metals as insoluble forms and thus are
subsequently trapped in the soil matrix. In this manner, the mobilization and
diffusion of contaminants are restricted in the soil, e.g. Haumaniastrum,
Eragrostis, Ascolepis, Gladiolus, Alyssum, Indian mustard (Ali et al. 2013;
Berti and Cunningham 2000; Domínguez et al. 2009; Katyayan 2019; Prasad
2004).
• Phytovolatilization: Some plants have the ability to both absorb and volatilize
certain metals and metalloids. Certain metals like Hg, Se and As can be absorbed
by roots and transformed into non-toxic forms, thereby released into the atmosphere. Se can be absorbed and degraded by Astragalus bisulcatus and Stanleya
pinnata. Plant species like Nicotiana tabacum, Liriodendron tulipifera or Brassica napus for reducing the toxic effect of Hg (Brooks 1998; Katyayan 2019;
Pilon-Smits and LeDuc 2009; Pilon-Smits and Pilon 2000; Poschenrieder and
Barceló 2004; Ruiz and Daniell 2009).
• Phytoextraction (phytoaccumulation, phytoabsorption or phytosequestration): In
this method, plants accumulate metals and radionuclides and transport them to
their harvest, i.e. aerial parts. Application of this technique can be applied to the
metals, viz. Cd, Ni, Cu, Zn, Pb, Se, As etc. and other organic compounds.
Elsholtzia splendens, Alyssum bertolonii, Thlaspi caerulescens and Pteris vittata
are the hyperaccumulator plants which are known to carry out this process, which
are known to store high concentrations of these metals in their aerial plants (this
may vary from 0.01% to 1% dry weight, depending on the metal) (Blaylock and
Huang 2000; Hernández-Allica et al. 2008; Ma et al. 2001; McGrath 1998;
McGrath and Zhao 2003; Pedron et al. 2009; Van der Ent et al. 2013; Xie et al.
2009).
3.5 Parameters Affecting Bioremediation
Many factors combine in such a manner that the process of bioremediation can be
taken care of systematically. The availability of the contaminants to the microbes
possessing degradation abilities along with favourable conditions, viz., soil type,
temperature, pH, O 2 or other electron acceptors and nutrient availability is essential
(Abatenh et al. 2017).
Various chemical and physical wastes produced due to numerous human activities are degraded, removed, altered, immobilized and detoxified from the
3 Microbial Indicators of Bioremediation: Potential and Success
93