World J Microbiol Biotechnol
DOI 10.1007/s11274-015-1911-5
ORIGINAL PAPER
Ecotoxic heavy metals transformation by bacteria and fungi
in aquatic ecosystem
Amiy Dutt Chaturvedi1 • Dharm Pal2 • Santhosh Penta3 • Awanish Kumar4
Received: 24 April 2015 / Accepted: 24 July 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract Water is the most important and vital molecule
of our planet and covers 75 % of earth surface. But it is
getting polluted due to high industrial growth. The heavy
metals produced by industrial activities are recurrently
added to it and considered as dangerous pollutants.
Increasing concentration of toxic heavy metals (Pb2?,
Cd2?, Hg2?, Ni2?) in water is a severe threat for human.
Heavy metal contaminated water is highly carcinogenic
and poisonous at even relatively low concentrations. When
they discharged in water bodies, they dissolve in the water
and are distributed in the food chain. Bacteria and fungi are
efficient microbes that frequently transform heavy metals
and remove toxicity. The application of bacteria and fungi
may offer cost benefit in water treatment plants for heavy
metal transformation and directly related to public health
and environmental safety issues. The heavy metals transformation rate in water is also dependent on the enzymatic
capability of microorganisms. By transforming toxic heavy
metals microbes sustain aquatic and terrestrial life.
Therefore the application of microbiological biomass for
heavy metal transformation and removal from aquatic
ecosystem is highly significant and striking. This paper
reviews the microbial transformation of heavy metal,
& Awanish Kumar
drawanishkr@gmail.com; awanik.bt@nitrr.ac.in
1
Department of Biochemistry, Shri Shankaracharya
Mahavidyalaya, Bhilai, India
2
Department of Chemical Engineering, National Institute of
Technology Raipur, Raipur, India
3
Department of Chemistry, National Institute of Technology
Raipur, Raipur, India
4
Department of Biotechnology, National Institute of
Technology Raipur, Raipur, Chhattisgarh 492010, India
microbe metal interaction and different approaches for
microbial heavy metal remediation from water bodies.
Keywords Heavy metals Ecotoxic Microbes
Transformation Remediation
Introduction
Bacteria and fungus are unique microorganisms that play a
major role in the biotransformation of heavy metals. Water
bodies contribute significantly for both aquatic as well as
terrestrial life but they become polluted progressively due
to mass development of industries and this is harmful for
the surrounding life (Congeevaram et al. 2007). Municipal
water and industrial waste discharge in water bodies
damage the quality of water and affects the aquatic life.
Due to globalization, increase in population and industrial
development causes deposition of heavy metals in lakes
and rivers. These are major issues that have been discussed
by the developing and developed countries at world scenario (Souza and Tundisi 2003). Scientific approach may
lead to solve these problems at in vitro to in vivo level as
suggested by Hoppe (1993). A large number of populations
were affected by mercury pollution in Minamata, Japan. It
was caused by the release of mercury from chemical
industry in Minamata Bay. This highly toxic mercury
accumulated in the fish which was later eaten by the local
population and resulted into mercury poisoning (Chang and
Guo 2009). Loss of appetite, nausea, irritability, and
muscular stiffness are common due to minor exposure of
heavy metal to human. Prolonged exposure to different
heavy metals such as cadmium, copper, lead, nickel and
zinc can cause injurious effects on aquatic life as well as
humans (Yan and Viraraghavan 2000). Despite the adverse
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World J Microbiol Biotechnol
effect of heavy metals, slight attention has been given for
their presence in the aquatic ecosystem. Aquatic microorganisms especially bacteria and fungi secrete numerous
amounts of extracellular enzymes that could efficiently do
the transformation of heavy metal (Edwards et al. 2013).
Bacteria and fungi transform heavy metals and make them
available as micronutrients for the use of flora and fauna in
water (Alexander 1994). This paper provides the emphasis
on heavy metal effects on life and discusses the biotransformation of heavy metals specifically by bacteria and
fungi in waste water so that there would be reduction of the
heavy metal toxicity and concentration in aquatic
ecosystem.
Heavy metal microbe interaction, equilibrium
and kinetics
Biotransformation is the conversion of compounds by
microorganisms through enzymatic reactions like oxidation, reduction and hydrolysis. It is the most vital process
for the removal of the heavy metals from water, soil and
sediment. Bacteria and fungus are good for removing
heavy metal contamination of water naturally therefore
biotechnologists have explained in vitro techniques to
biotransform heavy metals using microorganism (Glazer
and Nikaido 2007). Pollution from paper pulp, distillery,
leather, petroleum, pesticide and beverage industries are
the sources of heavy metals pollution but they can be
remediated through microbial treatment (Thakur 2006).
Fungi are also important for this purpose because they
metabolize dissolved heavy metal from water bodies.
Heavy metals can be precipitated as insoluble sulphides by
the metabolic activity of sulphate reducing bacteria. Heavy
metal ions can also be captured in the cellular organization
of microbes and subsequently reduce the concentration. In
the biosphere microorganisms transform carbon and play a
symbiotic role to produce renewable energy and nutrient
support for aquatic biodiversity (Park et al. 2010). In the
same way microbial biomass helps to transform heavy
metals by their food metabolism and make it available as
nutrient in the food chain. Different heavy metals disposed
in water and their regulatory limits (mg/l) as per Comprehensive Environmental Response Compensation and
Liability Act (CERCLA), USA is summarized in Table 1.
Heavy metals can be transformed from one oxidation
state to another by bacteria and fungi. They are also able to
tolerate harmful effects of heavy metals. Metals (Cu2?,
Cd2?, Pb2? and Ca2?) are recurrently found as soluble
cationic forms. They are found as precipitates (CuS, PbS,
and CdCO3) in reduced conditions. Metal bioavailability
increases at low pH (due to its free ionic metal species) and
decreases at high pH (Rzymski et al. 2014). However, in
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Table 1 Heavy metals and their regulatory limits (as per CERCLA,
USA)
S. No.
Heavy metals
Maximum concentration
limit (mg/l)
1.
Antimony (Sb)
0.006
2.
Arsenic (As)
0.01
3.
4.
Cadmium (Cd)
Chromium (Cr)
0.005
0.01
5.
Copper (Cu)
1.3
6.
Iron (Fe)
0.3
7.
Lead (Pb)
0.015
8.
Manganese (Mn)
0.05
9.
Mercury (Hg)
0.002
10.
Nickel (Ni)
0.2
11.
Radium (Ra)
5
12.
Selenium (Se)
0.05
13.
Silver (Ag)
0.05
14.
Thallium (Tl)
0.002
15.
Uranium (U)
16.
Zinc (Zn)
30
5.0
contrast to other heavy metal bio availabilities, nickel form
complexes with inorganic ligands (OH-, SO42-, Cl and
NH3) at wide pH range (5–9). The fate of heavy metals in
water depends mainly on the initial concentration and some
edaphic conditions such as pH (Katsoyiannis and Zouboulis 2004). Metal resistance mechanisms have been also
identified in bacteria and fungi. Due to their strong ionic
nature, heavy metals bind to many cellular ligands and
displace native essential metals from their normal binding
sites (Valls et al. 2000). Once metal binds with cell ligands,
it is taken up by the microbes and transformation of toxic
heavy metal starts. Microbes have the great ability to
reduce the toxicity of the heavy metals or they convert
insoluble toxic cations of heavy metals into less toxic
soluble form. Archaea and Eubacteria are capable of oxidizing Mn(II), Fe(II), Co (III), AsO2, SeO or decrease
concentration of Mn(IV), Fe(III), Co (II), AsO24-, SeO3
and make them less or non toxic. Bacterial species (Alcaligenes, Bacillus, Pseudominas) do the reduction of
Cr(VI) to Cr(III), reduction of Hg(II) to Hg(0), reduction of
Se(VI) to elemental Se, reduction of U(VI) to U(IV).
Several researcher showed that yeasts are also capable of
accumulating heavy metals such as Cu(II), Ni(II), Co(II),
Cd(II) and Mg(II) and are superior metal accumulators
compared to certain bacteria (Rajkumar et al. 2012). Fungi
are known to tolerate and detoxify heavy metals by active
uptake, extracellular and intracellular precipitation and
valence transformation. Many species of fungi can absorb
some heavy metals (Cd, Cu, Hg, Pb, and Zn) into their
mycelium and spores (Rajkumar et al. 2012). Cell surface
functional groups of the fungus might act as ligands for
World J Microbiol Biotechnol
metal sequestration resulting in the removal of the metals
(Pal et al. 2010). Many bacteria and fungi produce some
cellular secretions that transform toxic metals from the
food chain of aquatic ecosystem by being bound to the
particular cellular secretions (Colberg et al. 1995).
Microbes cannot degrade heavy metals directly but they
can change the valence states of metals which may convert
them into less toxic forms. Some reactions are explained
below where microbes interact with metals and change the
valency of heave metals that causes biotransformation/
detoxification.
Eubacteria and Archaea
Mn(II) ! Mn(IV)
Oxidation
qe ¼
Eubacteria and Archaea
Fe(II) ! FeðIIIÞ
Oxidation
ðCs
BQo Ce
Ce Þ½1 þ ðB 1ÞCe =Cs
ð3Þ
where, Cs is the saturation concentration of the adsorbed
component, B is a constant which is a measure of the
energy of interaction between the solute and the adsorbate
surface, and Qo is the constant indicating the amount of
solute adsorbed forming a monolayer.
To visualize multi-metal ions biosorption system, several extended Langmuir models such as Langmuir multicomponent model (Eq. 4) has been developed and studied
(Langmuir 1918; Volesky 2003; Pagnanelli et al. 2002)
Eubacteria and Archaea
Co(IIIÞ ! Co(II)
Reduction
Eubacteria and Archaea
AsO2 ! AsO42
Reduction
Eubacteria and Archaea
Se0 ! SeO23
Oxidation
CrO24
where, qe and Ce are the equilibrium metal sorption
capacity and equilibrium concentration of adsorbate and K
and n are Freundlich co-efficients.
Both the basic models (Eqs. 1 and 2) are not able to
explain any mechanisms of sorbate uptake and scarcely
have a meaningful physical interpretation for biosorption.
The above mentioned two empirical models do not include
the external variable, even though they are found suitable
in biosorption. Any wastewaters generally contain multiple
metal ions so to visualize multi-layer biosorption, other
models such as BET model (Eq. 3) and have been developed (Brunauer et al. 1938)
Pseudomonas fluorescens
! Cr(OH)3
Reduction
Equilibrium and kinetics studies are essential to visualize
the mechanism of biosorption. Therefore equilibrium and
kinetic models for heavy metals sorption were developed
by considering the effect of the contact time, effect of
temperature, initial heavy metal ion concentrations, and
initial pH. Few commonly used equilibrium and kinetic
models for biosorption have been described below.
qei ¼
bi qmax;i cei
P
1 þ Ni¼1 bi cei
ð4Þ
where, cei and qei are the unadsorbed concentration of each
component at equilibrium and the adsorbed quantity of
each component per g of dried adsorbent at equilibrium,
respectively. bi and qmax,i are derived from corresponding
individual Langmuir isotherms.
Equilibrium models
Kinetic models
Several empirical models and the mechanistic models
(based on mechanism of metal ion biosorption) have been
proposed for biosorption to estimate metal uptake capacity
of different species. To predict the experimental behavior
mechanistic models are recommended (Pagnanelli et al.
2002; Volesky 2003). The Langmuir model (Eq. 1) which
is based on monolayer adsorption of solute and the Freundlich model (hetero-geneous surfaces) (Eq. 2) are the
two widely utilized equilibrium isotherms.
KL Ce
) qe ¼
1 þ KL Ce
ð1Þ
where, qe is the adsorbent capacity at equilibrium concentration Ce for the formation of monolayer and KL is the
adsorption coefficient.
qe ¼ KCne
ð2Þ
For design of biosorption process, kinetics studies (determining rate of the sorption and hydrodynamic parameters)
are very important. Kinetic models (based on the capacity
of the adsorbent) mostly used to investigate the biosorption
phenomenon are the Lagergren’s first-order equation
(Eq. 5) and pseudo second-order equation (Eq. 6) (Ho
2006).
dq
¼ k 1 ð qe
dt
qÞ
ð5Þ
where, q is the amount of adsorbed pollutant on the
biosorbent at time t and k1 is the rate constant of Lagergren
first-order biosorption.
dq
¼ k 2 ð qe
dt
qÞ 2
ð6Þ
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World J Microbiol Biotechnol
where k2 is the rate constant of pseudo second-order
biosorption.
Another model used for biosorption is Intra particle
diffusion model (Eq. 7). The initial rate of the intra particle
diffusion is;
q ¼ kp t1=2 þ D
ð7Þ
where kp is the intraparticle diffusion rate constant and D is
a constant that gives idea about the thickness of the
boundary layer. In most of the cases the pseudo secondorder equation is found more appropriate for biosorption
(Ho 2006).
Effective microbial processes for heavy metal
transformation
Unlike organic pollutants, heavy metals cannot be
destroyed, but must either be converted to a non toxic
stable form or removed. Heavy metals are readily
biodegradable at one place, but not at another because of
different capacities, modes, and rates of biodegradation
(Miyata et al. 1998). There are several conventional processes to eliminate heavy metals from contaminated water
but that require instrumentation, manpower and technical
inputs. A number of methods (adsorption, evaporation,
electroplating, ion exchange, membrane filtration, precipitation) have been developed for removal/transformation of
toxic metal ions from wastewaters. These conventional
technologies are also expensive due to non-regenerable
materials used. Microbial based methods using bacteria and
fungus are natural, cost effective and safe for heavy metal
removal from water bodies (Fig. 1). Some efficient heavy
metal transformer bacterial and fungal species in aquatic
ecosystem are listed in Table 2. High industrial growth and
capricious human activities resulted in the accumulation of
heavy metals in the aquatic system. Decontamination of
heavy metals from wastewater has been a challenged for a
long time. Some reports on microbial transformation of
heavy metals came out in successive years via biological
methods (Biofiltration, Bioabsorbtion, and Bioremediation). These processes represent a biotechnological
advancement as well as a cost efficient tool for the removal
of heavy metals from aquatic ecosystem and are discussed
below briefly.
Biofiltration
Biofiltration do the capture of heavy metals by microbes
and transform it in less or non toxic substance. This process
is able to remove high proportion of toxic heavy metals
from effluents without production of toxicity (Srivastava
123
and Majumder 2008). This is an important technique and
highly recommendable for tropical wastewater where
sewage is mixed with industrial effluents. Tripathi and
Tripathi (2011) reported the high efficiency of biofiltration
by modifying it with ozone to improve the quality of secondary effluent treatment for heavy metal removal. Specific
chemical modifications can be done with some oxidizing
agents like peroxide, or ozone to increase the biofiltration
efficiency for heavy metal removal. Biofiltration is shown
to be very effective by Tripathi and Tripathi (2011) in the
significant removal of not only heavy metal but also
organic and inorganic contents present in the secondary
effluent. Microbes do rapid biofiltration of heavy metals as
compared to conventional or mechanical processes. Some
microorganisms have been identified to possess strong
heavy metals removal potential from waste water
(Table 2). Bacteria and fungi are accomplished at utilizing
the heavy metals rapidly due to their small size and high
surface to volume ratio. Bacteria dominate over fungi in
biofiltration process because they are smaller and more
active than fungi (Scragg 2005). But a few fungi can
transform some compound heavy metals which are beyond
the metabolic abilities of bacteria (Iram et al. 2015; Sasek
et al. 1993). Fungal specie like Micrococcus and Aspergillus tolerate high concentrations of chromium and nickel
from industrial wastewater (Congeevaram et al. 2007).
Therefore both are very important for this process. In a few
cases microbes may develop a biofilm for rapid biofiltration (Hoppe 1993). Hoppe (1993) have suggested Mn
removal on large scale by re-circulating batch cultures of
Leptothrix discophora SP-6. It would be possible to seed a
new manganese biofilter in water. The percentage removal
of Mn was very high ([97 %) in many cases and utilized
for water treatment effectively. Moreover, chemical modification of adsorbents can also improve the filter efficiency
and bacteria and fungi are susceptible to develop their
capabilities.
Biosorption
Biosorption is an asset of certain types of dormant microbial biomass to bind and concentrate heavy metals from
even very dilute aqueous solutions. Bacteria and fungi have
proven to be a potent heavy metal biosorbents (Table 3).
The mechanisms by which metal ions bind to the bacterial
and fungal cell surface include covalent bonding, electrostatic interactions, extracellular precipitation, redox interactions, Van der Waals forces or the combination of these
processes (Zhang and Li 2011; Mohite and Patil 2014). The
negatively charged groups (carboxyl, hydroxyl, and phosphoryl) of the bacterial cell wall adsorb metal cations,
which are then retained by mineral nucleation. Biosorption
studies of some heavy metals showed that the extent of
World J Microbiol Biotechnol
Fig. 1 Microbial processing of toxic heavy metal biotransformation/removal on aquatic ecosystem of the earth
Table 2 Bacteria and fungi used for degradation of heavy metals
Microorganisms
Heavy metals
removal
References
Arthrobacter viscosus, E. coli, Klebsiella sp., Lactobacillus sp.,
Proteus sp., Pseudomonas sp., Staphylococcus sp., Vibrio sp.
Cd, Fe, Pb, Ni and
Zn
Blanquez et al. (2006), Sri-Kumaran et al. (2011),
Quintelas et al. (2013), Yan and Viraraghavan
(2000)
Bacilli Sp., Chrollea vulgaria, Enterobacteria, P. fluorescence,
Rhizopus archizus, Utrobacter
As, Cd, Co, Cr, Cu,
Ni, and Zn
Thakkar et al. (2006), Rzymski et al. (2014)
Leptrospirillum ferrooxidase, P. aevurginasa, P. thremophillus,
P. ferroxidanse
Cd, Cr, Cu, Fe, Ni,
Pb and Zn
Miyata et al. (2000), Sand et al. (1992), Osman and
Bandyopadhyay (1999)
Aspergillus niger, Coriolus hersutus, Mucor rouxi, Penicillium
chrysogenum, Tea fungus, Trametes versicolo
As, Cd, Co, Cr, Cu,
Fe, Ni, Pb and Zn
Rzymski et al. (2014), Dursun et al. (2003),
Mamisahebei et al. (2007)
Table 3 Heavy metal adsorption capacities of some bacteria and fungi
Heavy metals adsorbed
Microorganism involved
References
Ni2?
Arthrobacter viscosus
Quintelas et al. (2013)
Cd2?, Ni2?, Pb2?
Gluconoacetobacter hansenii
Mohite and Patil (2014)
?
Cr6 , U6
?
Serratia marcescens, S. rubidaea
Kumar et al. (2011), Zhang and Li (2011)
Cr6?, Ni2?
Cd2?, Hg2?, Zn2?
Mycelial and cocus form of bacteria and fungi
Pseudomonas putida
Congeevaram et al. (2007)
Vallas et al. (2000)
Cu2?, Pb2?
Aspergillus flavus, A. niger
Iram et al. (2015)
Cu2?
Candida utilis
Zu et al. (2006)
Cu2?
Ganoderma lucidum
Muraleedharan and Venkobachar (1990)
Cd , Ni , Pb , Zn
Mucor rouxii
Yan and Viraraghavan (2000)
Pb2?
Pleurotus ostreatus
Osman and Bandyopadhyay (1999)
Ni2?
Rhodotorula glutinis
Suazo-Madrid et al. (2011)
Hg2?
Trametes versicolor and Pleurotus sajur-caj
Arica et al. (2003)
2?
2?
2?
2?
sorption varies markedly with the metal, chemicals adsorbents/precipitants and the microorganisms (Tyagi et al.
2000). It is also flexible for removal of toxic metals and has
easy adaptability for in situ and ex situ application in a
range of bioreactor configuration. Quintelas et al. (2013)
have shown the bioadsorption of Ni(II) by a bacteria
Arthrobacter viscosus supported on zeolite in batch and
continuous mode at laboratory scale as well as pilot scale.
Yan and Viraraghavan (2000) studied the effect of pretreatment of Mucor rouxii biomass on bioadsorption of
Pb2?, Cd2?, Ni2? and Zn2?. Iram et al. (2015) have shown
the efficient biosorption and bioaccumulation capability of
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World J Microbiol Biotechnol
fungal isolates against Cu and Pb. Different parameters
affect the adsorption processes such as contact time, initial
metal ions concentration and pH but the role of pH is very
significant for the selective adsorption of various heavy
metal ions. Basaldella et al. (2007) have used NaA zeolite
for Cr(III)removal at neutral pH, while Barakat (2008)
used 4A zeolite at high pH for removal of Cr(III). Both
group have also reported that Cu(II) and Zn(II) were
adsorbed at neutral and alkaline pH respectively, and
Cr(VI) was adsorbed at acidic pH while the adsorption of
Mn(IV) was achieved at high alkaline pH values. These
studies further indicated that the adsorption capacities of
the metal ions were found to be strongly dependent on pH.
Low cost and higher efficiency even at low metal concentrations make it very attractive in comparison to other
physicochemical methods for heavy metal removal.
Biosorption strategies for metal removal using microorganisms can reduce the bioavailability and biotoxicity of
heavy metals in the environment.
Bioremediation
Bioremediation is the degradation or transformation of
pollutants into non-hazardous or less hazardous material.
Recently, it is gaining high importance as an alternate
technology for removal of heavy metal pollutants from
water. Bacteria are generally used for bioremediation, but
fungi have also been used for this purpose. Bioremediation
by microbes could be an efficient method to reduce the
heavy metal load in aquatic environments (Blanquez et al.
2006). Biostimulation or bioaugmentation is applied for the
removal of the contaminant from the waste water. The
biostimulation is the addition of nutrients, oxygen or other
electron donors/acceptors to the coordinated site to
increase the population of naturally occurring microorganisms available for remediation, while the bioaugmentation is the addition of microorganisms that may
biodegrade the contaminants (Dursun et al. 2003). Bioremediation technology involves the use of microorganisms
to reduce or transform contaminants present in soils, water,
and air (Alexander 1994). Alcaligens, Bacillus, Citrobacteria, Escherichia, Klebsilla, Pseudomonas, Rhodococcus,
Staphylococcus are the microorganisms that are frequently
used in bioremediation (Chikere et al. 2012). This process
involves biochemical reactions in an organism that result in
activity, growth and reproduction of that organism.
Chemical processes involved in microbial metabolism
consist of contaminants, oxygen, and reactants that convert
metabolites to well defined products (Miyata et al. 2000). A
key factor for the remediation of heavy metals is that
metals are non-biodegradable, but can be transformed
through sorption, methylation, and changes in valence state
(Arica et al. 2003). Although bioremediation is an
123
attractive solution, quite often the heavy metals are toxic to
the microbes actively involved in the bioremediation,
making it hard to maintain a high rate of filtration. One
possible solution to this problem is genetically engineered
microbes, that are resistant to the extreme conditions of the
contaminated site and also has bioremediation potential.
Genetically engineered microorganisms for heavy
metal removal
The conventional method for the removal of the heavy
metals from the polluted site is time consuming, costly,
dangerous and may be effective at one site, but not at
another because of different derivative capacities, equilibrium, kinetic, and thermodynamic properties. Modification
of microbial genomics is a comprehensive approach for the
removal of heavy metals and it has been applied on various
microorganisms (Sri-Kumaran et al. 2011). Bacterial and
fungal genes encoding catabolic enzymes for complex
compounds started to be cloned and characterized in early
1980s to prepare a genetically engineered microorganism
(GEM).
Once GEMs became a reality, much effort was spent on
transformation of heavy metals by bacteria and fungi.
GEMs have useful and desired properties for different bioremedial pathway or enzyme with novel biotransformation
features. The genetic modification has been done on various bacteria and fungi for the removal of toxic heavy
metals from aquatic environment (Table 4). Researchers
have successfully produced a multiplasmid containing
bacterial strain for transformation of many heavy metals at
a time. It is evident that the engineered bacteria and fungi
show more removal efficiency versus natural ones (Deng
et al. 2003). The engineered bacteria and fungi are more
selective with high removal efficiency of heavy metals
(Kostal et al. 2004; Valls et al. 2000).
GEMs are useful in reduction of metal toxicity and
transform microbes in the natural biodiversity (Scragg
2005). Genes responsible for transformation of heavy
metals like As, Cd, Cr, Ni, and Pb have been first identified.
Then plasmids have been designed where gene responsible
for transformation of these heavy metals were cloned and
transformed in bacteria and fungi. Finally potent engineered microbial strains were generated by biotechnologist
those are able to degrade a variety of heavy metals and are
more selective for heavy metal biotransformation. Modified gene of microorganism occupies high surface volume
as compare to older once for biotransformation of toxic
heavy metal substances (Rzymski et al. 2014). Sriprang
et al. (2003) introduced the phytochelatin synthase gene of
Arabidopsis thaliana into Mesorhizobium huakuii bacteria
subsp. rengei (strain B3). They have also established the
World J Microbiol Biotechnol
Table 4 Heavy metals removal efficiency of some genetic modified bacteria and fungi
Bacteria and fungi
used for genetic
modification
Improvement in the performance
of heavy metal removal
Specific modification carried out on the microorganisms
Reference
Escherichia coli
Six folds improved
transformation and removal for
Ni
Two compatible plasmids, pSUNI and pGPMT3 were used for
the expression of a Ni2? transport system in E. coli
Deng et al. (2003)
Escherichia coli
Five folds enhanced
biotransformation arsenate
degradation and removal
Better biotransformation of Cd
The metalloregulatory protein ArsR, was overexpressed in
E. coli which offers high affinity and selectivity toward
arsenite
Phytochelatin synthase gene was expressed under the control of
the nifH promoter that increased the ability of cells to bind
Cd2?
Kostal et al.
(2004)
Pseudomonas putida
Threefolds enhanced for Cd
Recombinant expression of Metallothioneins protein (strong
metal-binding capacity) was expressed
Valls et al. (2000)
Pseudomonas putida
20-folds improved for Cr
Chromium resistance properties encoded by a natural plasmid
and transformed in P. putida
Mondaca et al.
(1998)
Aspergillus fumigatus
Enhanced Fe transformation and
removal
Ganoderma lucidum
Transformation for Cu removal
Modification of G. lucidum destroys autolytic enzymes that
cause putrification of biomass which finally leads to
transformation or adsorption of heavy metals
Muraleedharan
and
Venkobachar
(1990)
Tea fungus
Better adsorption and removal of
As and Fe
Chemically pre-treated and modified for adsorption of As and
Fe
Mamisahebei
et al. (2007)
Mesorhizobium
huakuii
Abrogation of extracellular siderophore biosynthesis following
inactivation of the acyl transferase SidF or nonribosomal
peptide synthetase SidD was done for Fe transformation and
removal
symbiotic association between M. huakuii subsp. rengei
(strain B3) and an herb Astragalus sinicus for better biotransformation of Cd(II). Deng et al. (2003) constructed a
genetically engineered E. coli (Strain JM109) which
simultaneously expresses metallothionein enzyme and a
nickel transport system to remove and recover Ni(II) from
waste water. Metalloregulatory protein ArsR (offers high
affinity and selectivity toward arsenite) was overexpressed
in E. coli by Kostal et al. (2004) for arsenic removal from
water bodies. Pseudomonas putida was engineered for over
expression of metallothioneins gene to treat heavy metal
pollution in industrial sewage (Valls et al. 2000). A 200 kb
natural plasmid having chromium resistant property was
transferred to Pseudomonas putida KT 2441 strain to
confer a chromate resistance phenotype and the resistant
strain took up 50 % less Cr than the susceptible strain
(KT2441) of P. putida (Mondaca et al. 1998). Aspergillus
fumigatus excretes triacetylfusarinine C and fusarinine C to
confine extracellular iron found in industrial waste water.
Schrettl et al. (2007) used this fungus to capture iron and
also show the uses of ferricrocin for fungal hyphal iron
storage. Arsenic contamination in ground water poses a
severe threat to health. Tea fungus (a symbiont of two
yeasts viz., Pichia sp. NRRL Y-4810 and Zygosaccharomyces sp. NRRL Y-4882 and a bacterium Acetobacter
Sriprang et al.
(2003)
Schrettl et al.
(2007)
sp. NRRL B-2357) was exploited by Mamisahebei et al.
(2007) for removal of As(III), As(V) and Fe(II) from
ground water samples. The biosorption rate of tea fungus
reported against heavy metals (As and Fe) tend to increase
with the increase in contact time and adsorbent dosage
(Mamisahebei et al. 2007). Muraleedharan and Venkobachar (1990) have enhanced the biosorptive capacity
for Cu(II) with the help of engineered mushrooms.
Based on the above discussed research reports, genetically modified bacteria and fungi are the promising recommendations for the transformation or/removal of heavy
metals from wastewater. These findings reveal the suitability and distinct use of engineered microbes for heavy
metal removal. Depending on the ambient conditions, the
GEM mediated biotransformation processing of heavy
metal is highly effective and safe. Natural occurring and
GEMs are unique for remediation of municipal sewage
waste. Use of mixed microbial cultures would be certainly
beneficial in multi-contaminated heavy metal solution. The
application of genetic engineering in microbes for heavy
metal removal has awakened great interest. It is hoped that
the above discussed microbial processes for heavy metal
transformation would be effective and improve biotransformation efficiency of heavy metals in aquatic ecosystem
(Fig. 1).
123
World J Microbiol Biotechnol
Conclusion
Living organisms of aquatic ecosystem are directly
exposed to toxic heavy metals that are commonly present
in ionized form. These heavy metal ions exert adverse
effects on aquatic life. Bacteria and fungi can significantly
decrease the toxic heavy metal concentration and distribution in water bodies by biotransformation. From the
above discussion it is concluded that there is a high possibility of large scale application of biofilter and biosorption in heavy metal biotransformation. It may be utilized by
the microbial population as a growth substrate for the
removal of toxic heavy metals from polluted water
(Rzymski et al. 2014). More information is needed about
microbes mediated transformation of heavy metals to make
eco-friendly environment. Heavy metals seem to be more
toxic for aquatic ecosystem and their high range in the
environment has proven hazardous (Sharma and Kuhad
2010). More efforts should be made to prevent toxic sewage waste discharge into water.
Microbial activities are very important for removal of
toxic heavy metals. The metal removal capacity of bacteria
and fungi is far better, more beneficial and eco friendly
than other conventional methods and various studies suggest that microbial process are most promising for biotransformation of toxic heavy metals in aquatic
ecosystems. Biosorption has a good potential of heavy
metal biotransformation. For example, Quintelas et al.
(2013) have reported high adsorption capacities of Ni(II)
by Arthrobacter viscosus supported on zeolite 13 X. High
sorption capacity (44.45 mg/g at 25 °C and 63.53 mg/g at
70 °C) by acetone pretreated Rhodotorula glutinis biomass
was shown by Suazo-Madrid et al. (2011). The adsorption
capacity of nickel(II) on Parthenium hysterophorous ash
was also high and percent removal of Ni increased from
67.30 to 97.54 % (Singh et al. 2009). Many researchers
have also shown the adsorption capacity of other microbes
using different adsorbents for different heavy metals (Yan
and Viraraghavan 2000; Basaldella et al. 2007; Barakat
2008).
Heavy metal biotransformation done by natural and
genetically modified bacteria and fungi is also very effective and provides a promising and spontaneous approach
for the removal of a wide variety of ecotoxic heavy metals.
The accomplishments in microbial cloning techniques
improve the heavy metal removal efficiency including the
reduction in the treatment cost of contaminated water.
Genetically modified microbes are capable of removing
heavy metals up to ppb level and are also cheaper for the
treatment of industrial wastewater. GEMs have improved
heavy metal biotransformation ability, and their application
could be used for bioremediation of heavy metals from
123
aquatic environments. Optimization and operational conditions could limit the practical and fast application of
microbes in biotransformation because a huge number of
heavy metals having toxicological effects on environment
could be targeted. Current understanding on this particular
topic is not sufficient and there is a gap between existing
knowledge and its application. More research in this area is
needed to reduce environmental heavy metal toxicity and
enjoy a more sustainable future.
Acknowledgments Authors are thankful to Department of Biochemistry, Shri Shankaracharya Mahavidyalaya, Bhilai (CG), and
National Institute of Technology (NIT), Raipur (CG), India for providing facility, space and resources for this review.
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