Their elimination from waste water before being released into the environment is
important for the maintenance of the ecosystem and from an economic point of view.
There are many techniques such as sludge filtration, adsorption processes, chemical
oxidation or reduction reactions, chemical precipitation, ion exchange, electrochemical treatment and reverse osmosis which are used to remediate contaminated
environments with heavy metals (Siddiquee et al. 2015). However, these techniques
are costly, particularly when the metal concentrations are extremely low. As most of
the heavy metal salts have high solubility in solution, the separation by chemical and
physical techniques is also challenging. Hence, there is a need to evaluate alternative
techniques applicable, and it should be appropriate and suitable for the local
conditions.
In this perspective, some microorganisms have developed resistance mechanism
to adapt to these pollutants and could be promising for bioremediation processes
(Giovanlla et al. 2017). Bioremediation is an innovative technique for the removal
and recovery of heavy metals ions from contaminated sites. This method involves
using living organisms such as bacteria, fungi and algae to reduce and/or recover
heavy metal pollutants into less hazardous form. This technique has been used for
the removal of heavy metals from polluted soil and wastewater. These microorganisms help to detoxify hazardous components in the environment by the process
which occur naturally or can be improved through the addition of nutrients and
electron acceptors. Metals whose different valence transformations states vary in
toxicity can be detoxify through the valence transformation mechanism. For
instance, methyl mercury is converted to less toxic Hg(II) by the enzyme organomercurial lyase produced by mercury-resistant bacteria (Wang et al. 2010a). Similarly, Cr(VI) is reduced to Cr(III) having less mobility and toxicity by
microorganisms used in bioremediation. Heavy metals can also be detoxified by
other mechanisms such as volatilization, vacuole compartmentalization and metal
binding. Metal binding involves chelators such as phytochelatin (e.g. glutathione
derived peptides), metal binding peptides and metallothein which bind to heavy
metals and facilitate microbial absorption and transportation of metal ions. Volatilization mechanism takes place only in metals which have volatile states such as Hg
and Se and involve turning metal ions into a volatile state. The MerA enzyme is
utilized by mercury-resistant bacteria to reduce Hg(II) to the volatile form Hg(0) and
Se(V) can be reduced to elemental Se(0) to remediate polluted soil and waters
(Wu et al. 2010). Thus, bioabsorption, bioaccumulation, biotransformation and
biomineralization are some techniques used by microorganisms for their survival
in metal-polluted environment. These mechanisms have been exploited for bioremediation technology (Gadd 2000; Lin and Lin 2005).
Various factors influencing the microbial remediation of heavy metals include the
concentration of pollutants, bioavailability of metals to the microbe, electron acceptors, pH, oxygen, redox potential, soil structure, temperature, moisture content,
nutrient, osmotic pressure and water capacity. Hence, the choice of microorganisms
may be native to the contaminated environments or isolated from another environment and brought to the polluted site (Sharma et al. 2000). One such approach is to
search for new enzymes from extremophilic microorganisms. Extremophiles are
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S. Kaushik et al.
important for the maintenance of the ecosystem and from an economic point of view.
There are many techniques such as sludge filtration, adsorption processes, chemical
oxidation or reduction reactions, chemical precipitation, ion exchange, electrochemical treatment and reverse osmosis which are used to remediate contaminated
environments with heavy metals (Siddiquee et al. 2015). However, these techniques
are costly, particularly when the metal concentrations are extremely low. As most of
the heavy metal salts have high solubility in solution, the separation by chemical and
physical techniques is also challenging. Hence, there is a need to evaluate alternative
techniques applicable, and it should be appropriate and suitable for the local
conditions.
In this perspective, some microorganisms have developed resistance mechanism
to adapt to these pollutants and could be promising for bioremediation processes
(Giovanlla et al. 2017). Bioremediation is an innovative technique for the removal
and recovery of heavy metals ions from contaminated sites. This method involves
using living organisms such as bacteria, fungi and algae to reduce and/or recover
heavy metal pollutants into less hazardous form. This technique has been used for
the removal of heavy metals from polluted soil and wastewater. These microorganisms help to detoxify hazardous components in the environment by the process
which occur naturally or can be improved through the addition of nutrients and
electron acceptors. Metals whose different valence transformations states vary in
toxicity can be detoxify through the valence transformation mechanism. For
instance, methyl mercury is converted to less toxic Hg(II) by the enzyme organomercurial lyase produced by mercury-resistant bacteria (Wang et al. 2010a). Similarly, Cr(VI) is reduced to Cr(III) having less mobility and toxicity by
microorganisms used in bioremediation. Heavy metals can also be detoxified by
other mechanisms such as volatilization, vacuole compartmentalization and metal
binding. Metal binding involves chelators such as phytochelatin (e.g. glutathione
derived peptides), metal binding peptides and metallothein which bind to heavy
metals and facilitate microbial absorption and transportation of metal ions. Volatilization mechanism takes place only in metals which have volatile states such as Hg
and Se and involve turning metal ions into a volatile state. The MerA enzyme is
utilized by mercury-resistant bacteria to reduce Hg(II) to the volatile form Hg(0) and
Se(V) can be reduced to elemental Se(0) to remediate polluted soil and waters
(Wu et al. 2010). Thus, bioabsorption, bioaccumulation, biotransformation and
biomineralization are some techniques used by microorganisms for their survival
in metal-polluted environment. These mechanisms have been exploited for bioremediation technology (Gadd 2000; Lin and Lin 2005).
Various factors influencing the microbial remediation of heavy metals include the
concentration of pollutants, bioavailability of metals to the microbe, electron acceptors, pH, oxygen, redox potential, soil structure, temperature, moisture content,
nutrient, osmotic pressure and water capacity. Hence, the choice of microorganisms
may be native to the contaminated environments or isolated from another environment and brought to the polluted site (Sharma et al. 2000). One such approach is to
search for new enzymes from extremophilic microorganisms. Extremophiles are
312
S. Kaushik et al.