Study On Microbes For Heavy Metal Remediation Biology Essay

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It is an expensive non-ecofriendly approach, while biological treatment of metal is an eco-friendly, less combustive, cheaper and easily adaptable measure for the treatment of liquid

as well as solid waste. Several microorganisms (fungi, algae and bacteria) have been

frequently used and exploited for the treatment of various metals like aluminium, copper,

nickel, cobalt, zinc, etc.



The continued rising rate of human activities is posing unprecedented threats, which would

ultimately lead to a disturbing imbalance in the biosphere. Various industrial activities like

chemical works, service stations, metal fabrication shops, tanneries, textile plants, paper mills,

waste disposal sites and intensive agriculture have polluted the environment to a great extent

(Wong, 2003; Wong et al.,2002). Heavy metal contamination has become a serious problem in

areas of intense industry and agriculture. The contamination of the environment with heavy

metals has become a worldwide problem affecting crop yields, soil biomass and fertility,

contributing to bioaccumulation in the food chain. Various efficient clean up techniques are

available, but most of them are costly, labour intensive and cause ecological disturbances due to

which they have found limited acceptability among the communities. These heavy metals need

to be removed from the biosphere for agro-ecological sustainability and human benefit (EU,

2002; USDOL, 2004).

168 Microbial Applications

Heavy metal pollution in the aquatic systems is increasing by leaps and bounds and is

becoming a serious threat today. The chemical processes for removal of these heavy metals that

exist today are not economical for treating large volume of water bodies containing enormous

amount of heavy metals. The conventional processes used for effluent treatment are precipitation

as hydroxides/sulphides, oxidation/reduction and ion exchange. These processes are expensive

and not eco-friendly. Further, the major disadvantage with conventional treatment techniques is

the production of sludge. As a result, an aquatic problem is changed into solid disposal problem.

The increasing demand of eco-friendly technologies has led to the search of low-cost alternatives,

which could be safely and effectively used for heavy metal remediation. In this endeavour, the

use of microbes has emerged as an option for developing economic and eco-friendly wastewater

treatment processes (Ferner, 2001; INECAR, 2001).


Metals are important for the living beings since they are the active centres of many enzymes.

The chemical properties of the metal have been recruited for catalyzing key reactions and for

maintaining protein structure. The metals are therefore required in minute amounts for normal

cell metabolism, and their intake is subject to intricate homeostatic mechanisms. However, many

other metals seem to serve no biologically relevant function (Robinson et al., 2001; Rosenfeld

and Beath, 1964). Instead, they cause damage, mostly due to their avidity for the sulfhydryl

groups of proteins, which they block and inactivate (Nalon, 2003). From a physiological point,

metals fall into three main categories:

(i) Essential and basically non-toxic (Ca and Mg)

(ii) Essential, but harmful at high concentrations (Fe, Mn, Zn, Cu, Co, Ni and Mo)

(iii) Toxic (Hg or Cd)

Arsenic (As), though a non-metal, is often studied in heavy metal contamination since most

living forms react to this element as they react to metal ions.

Heavy metals are one of the major pollutants of the lithosphere and hydrosphere and need to

be removed from the contaminated sites. Heavy metals are detrimental to the growth of plants as

well as survival of animal life (Ohlendorf et al., 1986). In plants, these are absorbed through the

roots, and once inside the plant, they induce chlorosis of leaves, deficiency of essential elements

and inhibition of root growth (Kim et al., 2003). The accumulation of heavy metals in plants and

their subsequent release during decomposition represents a recycling of heavy metals in the

ecosystem that has an important effect on the level of toxic metals in the biosphere. Apart from

plants, heavy metals have a detrimental effect on other forms of life. Livestock and wildlife have

long been reported to be suffering from heavy metal poisoning (Bhargava et al., 2008a; 2008b).


Several metals are considered hazardous wastes that can accumulate in human body and have a

relatively large half-life. Some of the heavy metals are of importance to man, but their dietary

intake has to be maintained since excesses may lead to toxicity resulting in clinically diagnosable

Microbes for Heavy Metal Remediation 169

symptoms (McLaughlin et al., 1999). However, metals like Cd, Pb, As and methylated forms of

Hg have no known importance in human physiological processes and are toxic even at low

concentrations. The nature of effects could be toxic (chronic, subchronic or acute), neurotoxic,

mutagenic, teratogenic or carcinogenic (Richards, 2007). This toxicity is compounded by the

fact that several heavy metals accumulating in the human body have a large half life (Duruibe

et al.2007; Ogwuegbu and Ijiuma, 2003).

Cadmium (Cd) is one of the most important pollutants in terms of food chain contamination.

Cadmium is known to accumulate in the human kidney for a relatively long time, from 20 to 30

years, and, at high doses, is also known to produce health effects on the respiratory system.

Subchronic inhalation exposure to Cd leads to pulmonary effects like emphysema, bronchiolitis

and alveolitis, while high exposure leads to cadmium pneumonitis, an obstructive lung disease

characterized by chest pain, bloody sputum and death of lung tissues. Cd accumulates in kidney

cortex and causes renal tubular dysfunction. Cd exposure also leads to bone defects like

osteomalacia, osteoporosis and spontaneous fractures (McKenna and Chaney, 1991; Strehlow

and Barltrop, 1988; Young, 2005).

Lead contamination is one of the greatest concerns for human health. Exposure to Pb occurs

through multiple agents that includes inhalation of air and ingestion of Pb in food, water or soil.

The danger of Pb is aggravated by low mobility even under high precipitation. Lead toxicity

causes dysfunction of the kidneys, reproductive and cardiovascular systems; inhibition of

haemoglobin synthesis; and damage to the central nervous systems (Kantor, 2006; Ogwuegbu

and Muhanga, 2005).

Zinc (Zn) causes the same signs of illness as does lead and is often mistaken as lead

poisoning. Common signs of Zn toxicosis are diarrhoea, vomiting, anaemia, liver and kidney

failure. Excess amount of Zn causes several dysfunctions resulting in impairment of growth and

reproduction. Mercury has no known function in human physiology. Toxicity by inorganic forms

of Hg causes abortion, corrosive oesophagitis, haematochezia and congenital malformation. On

the contrary, poisoning with organic forms of Hg causes erethism, acrodynia, gingivitis, stomatitis,

neurological disorders and extensive damage to brain and central nervous system (Fosmire 1990).

Arsenic (As) is known to form complexes with coenzymes leading to inhibition of production

of adenosine triphosphate, the main energy yielding molecule in the body. Arsenic toxicity causes

an immune disorder wherein the body’s immune system attacks part of its own peripheral nervous

system resulting in muscle weakness. Arsenic is carcinogenic in its oxidation states and high

exposure often causes death.


Anthropogenic mobilization from metal ores has created novel, metal-loaded niches with a strong

selective pressure for endurance of metals, especially heavy metals. Several prokaryotes show

specific resistance determinants, tolerating a wide range in concentration of these elements by a

variety of mechanisms. Microorganisms have been successfully exploited to deal with heavy

metal pollution in a variety of schemes. Emerging technologies in this area rely on enhancing

the biosorption of metals into biomass, or the precipitation of ions by exploiting some metal-related facet of bacterial metabolism (Gadd et al., 2001).

170 Microbial Applications

Different microorganisms organisms exhibit diverse responses to heavy metals, which confer

upon them a certain range of metal tolerance. Eukaryotes are more sensitive to heavy metal toxicity

than bacteria and their typical mechanism for regulating intracellular metal ion concentrations is

the expression of metallothioneins (MTs), a family of metal-chelating proteins. Prokaryotic MTs

have been studied in detail, in the cyanobacterium Synechococcus, where they confer resistance to



and Cd


. A number of specific resistance mechanisms including active efflux and sequestration

or transformation to other chemical species become functional at concentrations above the

homeostatic or non-toxic levels. These tolerance mechanisms are often plasmid-borne, which

facilitates dispersion from cell to cell. A good example of heavy metal resistance is that of Ralstonia

metalliduransCH34 (b-proteobacterium) that contains resistance determinants that allow it to grow

at millimolar concentrations of nine different toxic metal ions. The organisms that thrive at the

highest metal concentration are the iron and the sulphur oxidizing bacteria, including the thiobacilli

and thermophilic archaea. One such bacterium, Thiobacillus ferrooxidansis highly resistant to Al,

Cu, Ni, Co and Zn. Sulphate-reducing bacteria display a certain degree of metal tolerance as a

secondary outcome of their metabolism. They are anaerobes that produce sulphide and immobilise

toxic ions as metal sulphides although enzymatic metal reduction might also be common in these

bacteria (Collard et al., 1994; Labrenz et al., 2000).

Thus, bacteria show diverse responses to metal ions and different bacterial groups have

developed abilities to cope with these toxic elements in a variety of environments be it anaerobic,

aerobic or thermophilic. With respect to pollution control, these activities show promise since

they help in hastening the mobilization of metals, designing metal-tolerant strains that are better

adapted to performing biodegradation of organic pollutants and metal bioremediation or mitigation

through the breeding of natural or engineered strains (Taghavi et al., 1997).


Biosorption is the uptake of organic and inorganic metal species, both soluble and insoluble, by

physico-chemical mechanisms such as adsorption. It is a rapid phenomenon of passive metal

sequestration by the non-growing biomass. Biosorption mainly involves cell surface complexation,

ion exchange and microprecipitation. Biosorbents are natural ion-exchange materials that primarily

contain weakly acidic and basic groups, the chelation process being unspecific. The metals are

stripped from the matrix after loading by sulphuric or hydrochloric acid, sodium hydroxide or

complexing agents, whether dead biomass or live bacteria are used. Different microbes differ in

their metal binding capacities and have been found to vary in their affinity for different heavy

metals. A large number of microorganisms belonging to various groups, viz. bacteria, fungi,

yeasts, cyanobacteria and algae have been reported to bind a variety of heavy metals to different

extents (Gupta et al., 2000)

Metal Binding Sites

Biosorbents can be prepared with different ionic forms such as protonated (H


forms) or saturated

with cations such as Na+

, Ca


and Mg


by pretreating the biomass with mineral acids, bases

or salts. Metal binding appears to be a two-step process where the first step involves a

Microbes for Heavy Metal Remediation 171

stoichiometric interaction between the metal and the reactive chemical groups in the cell wall, and

the second step is an inorganic deposition of increased amounts of metal. All the metal ions before

gaining access to the plasma membrane and cell cytoplasm come across the cell wall, which

consists of a variety of polysaccharides and proteins and hence offers a number of active sites

capable of binding metal ions. Difference in cell wall compositions among different groups of

microorganisms cause significant differences in the type and amount of metal ion binding to them.

The potential metal binding groups in the cellulosic cell wall of eukaryotic algae are carboxylate,

amine, imidazole, phosphate, sulfhydryl, sulphate and hydroxyl. Of these, amine and imidazoles

are positively charged when protonated and may build negatively charged metal complexes. Brown

algae contain fucoidin and alginic acid in their cell wall of which alginic acid offers anionic

carboxylate and sulfate sites at neutral pH. The fresh water forms contain galacturonic acid and its

polymer pectin, which also has anionic sites to which metals can bind by electrostatic attractions.

The amino and carboxyl groups, and nitrogen and oxygen of the peptide bonds are also available

for coordination bonding with metal ions such as lead, copper or chromium. This bond formation

could be accompanied by displacement of protons and is partly dependent on the extent of

protonation that is determined by the pH. Cell walls of bacteria and cyanobacteria are principally

composed of peptidoglycans consisting of linear chains of the disaccharide N-acetylglucosamine-b1,4-Nacetylmuramic acid with peptide chains. As compared to Gram-positive bacteria, the cell

walls of Gram-negative bacteria are thinner and not heavily cross-linked. They have an outer

membrane, which is composed of an outer layer of lipopolysaccharide (LPS), phospholipids and

proteins. A comparison of the Cd


biosorption capacities of Gram-positive and Gram-negative

bacteria revealed that the glycoproteins present on the outer side of Gram-positive bacterial cell

walls have more potential binding sites for Cd2+

than the phospholipids and LPS and hence are

responsible for the observed difference in capacity. The possible mechanisms of heavy metal binding

in some microorganisms have been provided in Table 11.1.

Table 11.1:The mechanisms involved in the binding and subsequent remediation of various heavy metals in microbes.

Microorganism Metal binded Mechanism

Rhizopus arrhizus Uranium Utilization of amine nitrogen of chitin crystallites

Citrobacter Uranium, Cadmium, Lead, Copper Phosphatase mediated cleavage of glycerol


Spirulina platensis Copper, Chromium, Lead, Zinc, Ion exchange mechanism

Nickel, Cadmium

Oscillatoria anguistissima Copper Zinc Ion exchange mechanism

Ecklonia radiata Copper Ion exchange mechanism

Ecklonia maxima Nickel Ion exchange mechanism

Sargassum fluitans Zinc, Copper, Cadmium Ion exchange mechanism

Once inside cells, metal species may be bound, precipitated, localized within intracellular

structures or organelles, or translocated to specific structures depending on the element concerned

and the organism. Freely-suspended and immobilized microbial biomass has received attention

with immobilized systems possessing much greater advantages of higher mechanical strength

and easier biomass/liquid separation. Immobilized living biomass has mainly taken the form of

bacterial biofilms on inert supports, and is used in a variety of bioreactor configurations including

rotating biological contactors, fixed bed reactors, fluidized beds, trickle filters and air-lift


172 Microbial Applications

Advantages of Biosorption

Biosorption is an ideal alternative for decontamination of metal containing effluents. Biosorbents

are attractive, since naturally occurring biomass or spent biomass can be effectively utilized.

Besides this, biosorption offers advantages of low operating cost, minimizes the volume of

chemical and/or biological sludge to be disposed, is highly efficient in dilute effluents and has

no nutrient requirements. These advantages have served as potential incentives for promoting

biosorption as a viable clean-up technology for heavy metal pollution (Vieira and Volesky, 2000).

The main drawback in the use of biomaterials is that existing ion-exchange synthetic resins

provide a similar performance and have least ecological problem. However, the high cost of

synthetic resins and increasing demand for eco-friendly technologies has led to the development

of low cost alternatives that could be considered as single use materials. Biosorption methods

seem to be more effective than their physico-chemical counterparts in removing dissolved metals

at low concentrations. In addition to this, higher specificity of biosorbents may avoid an important

problem encountered with ion-exchange resins: the overloading of binding sites by alkaline-earth metals present in the polluted effluents. Finally, biological systems offer the potential of

genetic modification to further increase the specificity towards certain metal ions or the

bioaccumulation yield.

Modifying Microorganisms for Better Biosorbents

Cell wall modification greatly alters the binding of metal ions, since biosorption mainly involves

cell surface sequestration. A number of methods have been employed for cell wall modification

of microbial cells in order to enhance the metal binding capacity of biomass and to elucidate

the mechanism of biosorption. The modifications can be introduced either during the growth

of the microbe or in the pre-grown biomass. The effect of culture conditions of cells on their

biosorptive capacity has given mixed results. Very high biosorption due to change in cell wall

composition has been reported inAspergillus nigergrown in the presence of large amounts of

potassium hexacyanoferate. Likewise, in Gram-positive bacteria, Cd


biosorption reportedly

increased by about 14% after addition of nutrients in 2 hrs of incubation, but in Gram-negative

bacteria, nutrient addition did not cause any significant increase in Cd


uptake. Pre-grown

biomass could be given several chemical and physical treatments to modify the metal-binding

properties of biomass to specific requirements. The physical treatments include heating or

boiling, freezing or thawing, lyophilization and drying. The various chemical treatments include

washing the biomass with detergents, cross-linking with organic solvents, and alkali or acid


Genetic Engineering for Better Biosorbents

The first studies in the genetic engineering of biosorbents involved the cloning of eukaryotic

MTs for their intracellular expression in bacteria. Cytoplasmic production of human MT fused

to araB in Escherichia colibrought about a 3-5-fold increase in Cd and Cu bioaccumulation. It

was observed that the chelating efficiency of MT was higher when targeted to the periplasmic

Microbes for Heavy Metal Remediation 173

space. Therefore, in order to circumvent the problems associated with cytoplasmic expression,

later studies directed MTs to the periplasmic space or to outer membrane compartments of E.

coli. An E. colistrain expressing MT fused to the outer membrane maltose protein (LamB)

showed a 15-20-fold rise in Cdþ


binding, as compared to the wild-type. A similar Cdþ



performance was obtained through the expression of MT on the surface of E. coli, Ralstonia

metalliduransand Pseudomonas putidaas a fusion to the L domain of the IgA protease

autotransporter. An alternative approach is the cytoplasmic expression combined with the

introduction of specific heavy metal membrane transporters. This approach overcomes metal

uptake limitations across the cell membrane, but is restricted to those metals for which there are

active import systems. The cloning of pea or yeast MTs fused to glutathione S-transferase in E.

coli, together with a nickel transporter from Helicobacter pylori, produced a significant increase

in Ni accumulation with respect to cells expressing MT but not the transporter. Similarly,

genetically engineered bacteria coexpressing the MerT-MerP mercury transporter with MTs or

metal-binding peptides in the cytoplasm showed an Hg bioaccumulation comparable to that of

cells directly expressing the binding peptides on the cell surface. Cytoplasmic expression of

metal-binding polypeptides is also an effective system for cellular detoxification of some metals.

Only a few reports describe the importance of engineering lipopolysaccharides with a major

role in metal biosorption. It has been reported that Pseudomonas aeruginosaPAO1 A-band and

B-band lipopolysaccharide mutants differ in their binding specificities towards iron and lanthanum,

suggesting that the design of biosorbents with higher metal specificity is feasible (Basnakova

et al., 1998; Gutnick and Bach, 2000; Wall and de Lorenzo, 2002).


Apart from their use as biosorbents, bacteria can also be used to efficiently immobilize certain

heavy metals through their capacity to reduce these elements to a lower redox state, producing

less bioactive metal species. Microbiological metal precipitation is a widespread activity that is

either the result of a dissimilatory reduction or the secondary consequence of metabolic processes

unrelated to the transformed metals. Dissimilatory processes refer to processes in which the

transformation of the target metal is unrelated to its intake by the microbial catalyst and, thus,

the chemical species that result from the cognate biological activity generally end up in the

extracellular medium. Dissimilatory reduction of uranium, selenium, chromium and other metals

is performed by a number of microorganisms under various environmental conditions and could

be used in waste treatment.

Sulphate reducing bacteria (SRB), believed to have the best known natural meta precipitation

mechanisms, have received attention because they enzymatically mediate the reductive

precipitation of toxic heavy metals such as U, Cr, Tc and As. Desulfovibrio desulfuricansis one

such example of a bacterium immobilizing heavy metals by promoting their precipitation at the

cell periphery as an insoluble low-valence oxide and suggests the possible use of this organism

in the treatment of metal contaminated wastewaters. SRB play a crucial role in metal sulphide

immobilization in anaerobic sediments that contain high concentrations of metals. By examining

bacterial biofilms grown in continuous culture, this phenomenon could be attributed to the

deposition of metal sulphides at the biofilms surface or in the liquid phase, followed by entrapment

174 Microbial Applications

of the precipitated sulphides by the exopolymer. SRB have been successfully used in the treatment

of waters and leachates in pilot laboratory surveys and large-scale bioreactors. Mixed sulphate-reducing bacterial consortia are more effective than pure cultures in the removal of heavy metals

from solution. SRB-containing reactors have also been used in the removal of heavy metals

from soil in a microbial integrated decontamination process. However, the fact that even low

levels (20-200 μM) of free Cd (II), Zn (II) or Ni (II) ions are toxic to SRB limits their use. A

suitable alternative for improving sulphide-dependent metal removal might be the transference

of dissimilatory sulphate reduction pathway to environmental bacteria. The first attempt in this

direction was the expression of the thiosulphate reductase gene from Salmonella entericapv.

typhimuriumin E. coli. One of the recombinant strains removed 98% of the available cadmium

(from up to 200 mM solutions) under anaerobiosis (De Luca et al., 2001).

The metal precipitation is also mediated by the liberation of inorganic phosphate from

organic phosphate donor molecules. Most of the work has been carried out with Citrobacter

isolated from metal-polluted soil. This bacterium could accumulate high levels of uranium,

nickel and zirconium through the formation of highly insoluble metal phosphates, opening the

way for future applications in bioremediation. The precipitation process was catalyzed by the

crystallization of inorganic phosphate liberated via the activity of a periplasmic acid

phosphatase, with a role for phosphate groups within the lipopolysaccharide. Although this

phenomenon could be considered an example of metal biosorption, metal phosphate formation

by bacterial metabolism plays a pivotal role in this mechanism. Similarly, Cd immobilization

by Bacillus thuringiensisDM55 cells is proposed to be phosphate-dependent. The transfer of

metal phosphate precipitation activity from Citrobacterto other bacteria is possible and has

been successfully carried out in E. coli. Another tool to counteract metal contamination is the

generation of bacteria with engineered polyphosphate pathways. This is the possibility based

on the apparent relationship between depletion of polyphosphate reserves in the cell and heavy

metal resistance.


Microorganisms exhibit a number of enzymatic activities that transform metal species through

oxidation, reduction, methylation and alkylation. Apart from the enzymatic transformations that

lead to metal precipitation and immobilization, other biological reactions can be applied to

bioremediation because they generate less poisonous metal species. Mercury and arsenic are the

metals for which such reactions have been best studied. Natural isolates of some bacteria often

show broad-spectrum mercury determinants, encoding the capacity to degrade organomercurials

such as the highly poisonous methylmercury to Hg


, which is subsequently transformed to Hg



a reaction catalyzed by mercuric reductase, a product of merAgene. This property shows future

prospects for effective exploitation in agricultural systems. The merAreductase activity has been

effectively introduced into diverse engineered strains. An E. colivariant containing simultaneously

the merA and glutathione S-transferase genes has been reported to withstand high mercury

concentrations (30 mg/l) and to reduce some of the metal present in the solution to Hg

0. A

combined method of chemical leaching and subsequent volatilisation of mercury by bacteria has

been developed that removed more than 60% of mercury from polluted Minamata Bay sediments.

Microbes for Heavy Metal Remediation 175

In spite of this, the volatilisation of mercury remains a matter of controversy, which arose after

the mersystem was introduced into plants designed for phytoremediation and a public concern

that this might eventually contribute to global atmospheric pollution in the long run.


Symbiosis between plant and microbes in the rhizoshpere, an area encircling the plant root

system, has long been studied by microbial ecologists. Researchers have exploited this symbiotic

relationship between bacteria and plants for in situbioremediation of a wide range of organic

pollutants such as parathion, atrazine, trichloroethylene, toluene, etc. However, the use of this

symbiotic relationship for remediation of heavy metals is still in its infant stage with research in

this field gradually picking up. The Arabidopsis thalianaphytochelatin synthase gene has been

expressed in a microsymbiont, Mesorhizobium huakuiisubsp. rengei, which resides in the nodules

of Astragalus sinicus. This symbiont expressing PC synthase was able to increase cadmium by

1.5 fold in the nodules. Another example involves the modification of Pseudomonas putida

06909, a moderately cadmium resistant rhizoshpere bacterium that has an efflux pump in the

metalloregulatory cadoperon. This bacterium has been engineered to produce MBP-EC20, a

metal binding peptide that has high affinity for cadmium. Production of MBP-EC20 in P. putida

06909 enables enhanced cadmium binding as well as protects the engineered strain and the

colonized sunflower plants against the toxic effects of cadmium. The results have demonstrated

that a combination of enhanced microbial biosorption and plant-bacterium symbiosis is a promising

strategy for heavy metal remediation (Wu et al., 2006).


For effective bioremediation, further genetic improvement of strains should be carried out for

the adaptation of existing methodologies for large-scale and in situdecontamination processes.

Increasing efforts are being made to isolate new heavy metal resistant bacteria capable of

accumulating increased amount of heavy metals. Very less is currently known about the

transformation of metals by Archaea, which often colonizes extreme environments and may be

advantageous for future research. Access may also be gained to new, enticing activities through

genomic and proteomic approaches. The availability of the genome sequences for some of the

strains that inhabit niches polluted with heavy metals will prove very advantageous for future

research efforts. For technical improvement of bacterial immobilization and the biosorption of

heavy metals, processes taking place in the cell microenvironment need to be better understood

if the mineralization process is to be improved. The physico-chemical conditions around the cells

may play an important role in metal immobilization for effective biosorption. The combination

of genetic engineering of the bacterial catalysts with judicious eco-engineering of the polluted

sites will be of paramount importance in future bioremediation strategies.


Microorganisms play important roles in the environmental fate of toxic metals and metalloids

with physico-chemical and biological mechanisms affecting transformations between soluble and

insoluble phases. Such mechanisms are important components of natural biogeochemical cycles

176 Microbial Applications

with some processes being of potential application for the treatment of contaminated materials.

Remediation technologies using microorganisms are feasible alternatives to the cleansing of

soils or the concentration of metals in polluted waters by physical or chemical means. Exploitation

of biological processes will depend on a number of scientific, economic and political factors,

including the availability of a market. Molecular approaches may enable the design of biomass

with specific metal-binding properties through the expression of metal-chelating proteins and

peptides, the improvement of metal precipitation processes and the introduction of metal

transformation activities in robust environmental strains. Bacterially mediated precipitation of

metals as well as the use of biosurfactants are the objects of large-scale commercial development

and hold great promise that other biotechnologies could be used in the field of metal