organisms that are able to thrive at extreme environmental conditions (salinity, pH,
temperature, pressure, dryness, radiations or concentrations of heavy metals). Most
of the extremophilic microorganisms belong to the Achaea domain, and their
enzymes known as extremozymes have unique structure-function properties such
as stability at high temperature, extreme pH, high ionic strength, in the presence of
organic solvents and heavy metals (Cabrera and Blamey 2018; Koga and Moril
2007; Cavicchioli 2011).
Bacteria and archaea that live in extreme conditions have been reported as great
microbial resources of heavy metal bioremediation. Sequencing of the genome of
extremophilic microorganisms such as Metallosphaera sedula (Aurenik et al. 2008),
Leptospirillum ferriphilum (Mi et al. 2011) and Sulfolobus solfataricus (Schelert
et al. 2013) has identified clusters containing the Hg-resistance gene merA. Takeuchi
et al. (2001) reported the isolate Acidithiobacillus ferrooxidans SUG 2-2 to volatilize
mercury from acidic soils polluted by this metal. Figueroa et al. (2018) have
reviewed about the extremophiles focussing on heavy metal and radionuclide pollution. Some halophilic archaea have developed tolerance to heavy metals. Halophilic microorganisms are often able to absorb heavy metals (Zhuang et al. 2010).
Wang et al. (2012) reported that the Halobacterium sp. NRC-1 showed high
resistance to arsenic due to the presence of genes for arsenite and antimonite
extrusion system on plasmid. Kaur et al. (2006) studied the haloarchaeal strategies
of adaptation to high metal concentration of iron, zinc, manganese, copper, cobalt,
nickel using Halobacterium sp. NRC-1 as a model organism. Srivastava et al. (2013)
have reported the intracellular synthesis of silver nanoparticles by the haloarchaeal
isolated Halococcus salifodinae BK 3 when the cells were grown in the medium
containing silver nitrate. Similarly, selenium nanoparticles are synthesized when
these cells are grown in the presence of sodium selenite. Cadmium tolerance has
been reported in haloarchaeal strains from salterns of Ribandar and Siridao in India
(Chaudhary et al. 2014). Biosorption of metals by the organism at the surface or by
the exopolysaccharides (EPS) secreted to form the biofilms enables organism to
tolerate metals (Srivastava and Kowshik 2013). Kawakami et al. (2007) found that
Halobacterium salinarum CCM 2090 has a Ca(II)-dependent aggregation system.
Calcium ion is adsorbed on the surface of the cells and induces ionic cross-bridging
between the EPS, resulting in aggregation of the haloarchaeal cells. Cations such as
Zn
2+ , Cu
2+ , Fe
2+ , Mn
2+ , Co
2+ and Ni
2+ could replace Ca
2+ , enabling organisms to
tolerate these metals. Popescu and Dumitru (2009) reported the two Haloferax stains
having the capacity to reduce the concentration of Zn, Ni, Cr and Pb ions by
biosorption process from the media with high salinity. Halobacterium sp. GUSF
was reported to be able to absorb Mn at high concentration and high rates (Naik and
Furtado 2014). Halobacterium noricense was found to adsorb Cd (Showalter et al.
2016) while Haloferax st. BBK2 was found to accumulate Cd intracellularly (Das
et al. 2014). Methanobacterium bryantii was found to produce extracellular proteins
to chelate Cu (Kim et al. 1995).
Hence, the extremophiles belonging to the haloarchaea group can be used in the
treatment of hypersaline heavy metals polluted sites and wastewaters for heavy
metals removals. However, developing technologies for exploring for microbial
12 Potential of Extremophiles for Bioremediation
313
temperature, pressure, dryness, radiations or concentrations of heavy metals). Most
of the extremophilic microorganisms belong to the Achaea domain, and their
enzymes known as extremozymes have unique structure-function properties such
as stability at high temperature, extreme pH, high ionic strength, in the presence of
organic solvents and heavy metals (Cabrera and Blamey 2018; Koga and Moril
2007; Cavicchioli 2011).
Bacteria and archaea that live in extreme conditions have been reported as great
microbial resources of heavy metal bioremediation. Sequencing of the genome of
extremophilic microorganisms such as Metallosphaera sedula (Aurenik et al. 2008),
Leptospirillum ferriphilum (Mi et al. 2011) and Sulfolobus solfataricus (Schelert
et al. 2013) has identified clusters containing the Hg-resistance gene merA. Takeuchi
et al. (2001) reported the isolate Acidithiobacillus ferrooxidans SUG 2-2 to volatilize
mercury from acidic soils polluted by this metal. Figueroa et al. (2018) have
reviewed about the extremophiles focussing on heavy metal and radionuclide pollution. Some halophilic archaea have developed tolerance to heavy metals. Halophilic microorganisms are often able to absorb heavy metals (Zhuang et al. 2010).
Wang et al. (2012) reported that the Halobacterium sp. NRC-1 showed high
resistance to arsenic due to the presence of genes for arsenite and antimonite
extrusion system on plasmid. Kaur et al. (2006) studied the haloarchaeal strategies
of adaptation to high metal concentration of iron, zinc, manganese, copper, cobalt,
nickel using Halobacterium sp. NRC-1 as a model organism. Srivastava et al. (2013)
have reported the intracellular synthesis of silver nanoparticles by the haloarchaeal
isolated Halococcus salifodinae BK 3 when the cells were grown in the medium
containing silver nitrate. Similarly, selenium nanoparticles are synthesized when
these cells are grown in the presence of sodium selenite. Cadmium tolerance has
been reported in haloarchaeal strains from salterns of Ribandar and Siridao in India
(Chaudhary et al. 2014). Biosorption of metals by the organism at the surface or by
the exopolysaccharides (EPS) secreted to form the biofilms enables organism to
tolerate metals (Srivastava and Kowshik 2013). Kawakami et al. (2007) found that
Halobacterium salinarum CCM 2090 has a Ca(II)-dependent aggregation system.
Calcium ion is adsorbed on the surface of the cells and induces ionic cross-bridging
between the EPS, resulting in aggregation of the haloarchaeal cells. Cations such as
Zn
2+ , Cu
2+ , Fe
2+ , Mn
2+ , Co
2+ and Ni
2+ could replace Ca
2+ , enabling organisms to
tolerate these metals. Popescu and Dumitru (2009) reported the two Haloferax stains
having the capacity to reduce the concentration of Zn, Ni, Cr and Pb ions by
biosorption process from the media with high salinity. Halobacterium sp. GUSF
was reported to be able to absorb Mn at high concentration and high rates (Naik and
Furtado 2014). Halobacterium noricense was found to adsorb Cd (Showalter et al.
2016) while Haloferax st. BBK2 was found to accumulate Cd intracellularly (Das
et al. 2014). Methanobacterium bryantii was found to produce extracellular proteins
to chelate Cu (Kim et al. 1995).
Hence, the extremophiles belonging to the haloarchaea group can be used in the
treatment of hypersaline heavy metals polluted sites and wastewaters for heavy
metals removals. However, developing technologies for exploring for microbial
12 Potential of Extremophiles for Bioremediation
313