24
K. KirupaSree et al.
• The third mechanism includes plasmid mediated bacterial metal ion resistance, which involves highly specific anion or cation efflux systems encoded by
resistance genes associated with plasmids.
• The fourth and widely known mechanism involves detoxification of the toxic
anion or cation by enzymatically converting it from a more toxic to a less toxic
form. This mechanism does essentially occur in detoxification of inorganic and
organomercurials.
Some other major mechanisms of microbial metal transformations between
soluble and insoluble metal species include chemolithotrophic leaching,
chemoorganotrophic leaching, rock and mineral bioweathering and biodeterioration, biocorrosion, redox mobilization, methylation, complexation (For example,
complex formation with microbial products such as metallothionein like proteins and
extracellular polymers (EPS) in case of soluble metal species whereas for insoluble
metal species the process includes biosorption, accumulation, biomineral formation, redox immobilization, metal sorption to biogenic minerals and formation of
metalloid nanoparticlesare well notable processes (Tunali 2006)). The key factors in
controlling these mechanisms include:
• The nature of the biomass i.e. living or non-living;
• The type of biological ligands available for heavy metal sequestration;
• The chemical, stereochemical and coordination characteristics of the targeted
metals and metalloid species (Remoudaki et al. 1999).
• The hysic-chemical characteristics of the metal solution such as pH, and presence
of competing co-ions (Esposito 2002).
4.2 Interaction Between the Heavy Metals and Microbes
Numerous mechanisms have been executed by bacteria to detoxify and resist heavy
metals by which the metal ions bind to the cell surface and incorporate electrostatic interactions, covalent binding, Van der Waals forces, redox interactions and
extracellular precipitation or by means of all these as combined processes (Blanco
et al. 2000). In general the response of bacteria may fall into two categories: (1)
mechanisms dependent on activation by specific metals, and (2) general mechanisms, which convey resistance but do not depend on metal stress for their activation. Uptake of heavy metals and detoxification through Heavy metal tolerant—plant
growth promoting microbes is largely responsible for the Bioaccumulation. There
are two methods involved in bioaccumulation of Heavy metals: one is passive uptake
which is also known as biosorption, a metabolism independent accumulation of heavy
metals by inactive non-living biomass or living cells/biological materials. Another
one is active uptake that occurs only in alive cells, it requires energy and metabolism
for the exchange of metals (Gutierrez-Corona et al. 2016). One or a blend of different
processes involved in the biosorption includes coordination, complexation, chelation,
ion exchange, entrapment and micro precipitation (Pokethitiyook and Poolpak 2016).
K. KirupaSree et al.
• The third mechanism includes plasmid mediated bacterial metal ion resistance, which involves highly specific anion or cation efflux systems encoded by
resistance genes associated with plasmids.
• The fourth and widely known mechanism involves detoxification of the toxic
anion or cation by enzymatically converting it from a more toxic to a less toxic
form. This mechanism does essentially occur in detoxification of inorganic and
organomercurials.
Some other major mechanisms of microbial metal transformations between
soluble and insoluble metal species include chemolithotrophic leaching,
chemoorganotrophic leaching, rock and mineral bioweathering and biodeterioration, biocorrosion, redox mobilization, methylation, complexation (For example,
complex formation with microbial products such as metallothionein like proteins and
extracellular polymers (EPS) in case of soluble metal species whereas for insoluble
metal species the process includes biosorption, accumulation, biomineral formation, redox immobilization, metal sorption to biogenic minerals and formation of
metalloid nanoparticlesare well notable processes (Tunali 2006)). The key factors in
controlling these mechanisms include:
• The nature of the biomass i.e. living or non-living;
• The type of biological ligands available for heavy metal sequestration;
• The chemical, stereochemical and coordination characteristics of the targeted
metals and metalloid species (Remoudaki et al. 1999).
• The hysic-chemical characteristics of the metal solution such as pH, and presence
of competing co-ions (Esposito 2002).
4.2 Interaction Between the Heavy Metals and Microbes
Numerous mechanisms have been executed by bacteria to detoxify and resist heavy
metals by which the metal ions bind to the cell surface and incorporate electrostatic interactions, covalent binding, Van der Waals forces, redox interactions and
extracellular precipitation or by means of all these as combined processes (Blanco
et al. 2000). In general the response of bacteria may fall into two categories: (1)
mechanisms dependent on activation by specific metals, and (2) general mechanisms, which convey resistance but do not depend on metal stress for their activation. Uptake of heavy metals and detoxification through Heavy metal tolerant—plant
growth promoting microbes is largely responsible for the Bioaccumulation. There
are two methods involved in bioaccumulation of Heavy metals: one is passive uptake
which is also known as biosorption, a metabolism independent accumulation of heavy
metals by inactive non-living biomass or living cells/biological materials. Another
one is active uptake that occurs only in alive cells, it requires energy and metabolism
for the exchange of metals (Gutierrez-Corona et al. 2016). One or a blend of different
processes involved in the biosorption includes coordination, complexation, chelation,
ion exchange, entrapment and micro precipitation (Pokethitiyook and Poolpak 2016).