Metal Bio Remediation by Fungi

8/7/2019 Metal Bio Remediation by Fungi

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Fungi in Metal BioremediationBy: Witiyasti Imaningsih


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Metal transformations

Fungi are of fundamental importance as decomposerorganisms and plantsymbionts (mycorrhizas) and cancomprise the largest pool of biomass (including othermicroorganisms and invertebrates) in the soil

(Wainwright, 1988; Metting, 1992). They can be dominant in acidic conditions, where the

mobility of toxic metals may be increased (Morley et al., 1996), and this, combined with their explorative

filamentous growth habit and high surface area to

mass ratio, ensures thatfungi are integral bioactivecomponents of major environmental cycling processes

for metals and other elements including carbon,nitrogen, sulfur and phosphorus


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adaptive and constitutive mechanisms of metalresistance are well known in free-living (Gadd,1993a; Gadd & Sayer, 2000) and mycorrhizalfungi (Meharg & Cairney,

Metals and their compounds, derivatives andradionuclides, interact with fungi in a variety ofways depending on the metal species, organismand environmental conditions, while fungal

metabolism can dramatically influence speciationand, therefore, mobility and toxicity (Gadd,1993a; Gadd & Sayer, 2000).


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As in many other areas of bioremediation,

much research on fungal metal

bioremediation is laboratory based, with

fewer developments to pilot/ demonstration

scale, and little commercial operation


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Solubilization

Mechanisms of metal solubilization

Fungal solubilization of insoluble metalcompounds, including certain oxides,phosphates, sulfides and mineral ores, occurs by

several mechanism Solubilization can occur by protonation of the

anion of the metal compound, decreasing itsavailability to the cation (kation), with the proton-

translocating ATPase of the plasma membraneand the production of organic acids being sourcesof protons


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In addition, organic acid anions are frequentlycapable of soluble complex formation withmetal cations, thereby increasing mobility

The incidence ofmetal-solubilizing abilityamong natural soil fungal communitiesappears to be high; in one studyapproximately one-third of the isolates testedwere able to solubilize at least one Co3 (PO4)2 , ZnO or Zn3 (PO4 ) 2


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A further mechanism of metal solubilization is

the production of low-molecular-weight iron-

chelating siderophores which solubilize

iron(III). Siderophores are the most common

means of acquisition of iron by bacteria and

fungi, the most common fungal siderophore

being ferrichrome (Crichton, 1991).


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Environmental significance of metal

solubilization by fungi

Solubilization of insoluble metal compounds is animportant aspect of fungal physiology for the releaseof anions, such as phosphate, and essential metalcations into forms available for intracellular uptakeand into biogeochemical cycles.

As well as phosphate, soil fungi, including mycorrhizas,may increase inorganic nutrient availability to plantsand other microorganisms by increasing the mobility

of essential metal cations, and other anions, such assulfate (Gharieb, Sayer & Gadd, 1998; Gharieb & Gadd,1999)


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the presence of citric acid in the terrestrial

environment will leach potentially toxic metals

from soil (Francis, Dodge & Gillow, 1992)


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Significance of fungalmetal

solubilization for bioremediation

Although heterotrophic leaching by fungi can

occur as a result of several processes,

including the production of siderophores (in

the case of iron), in most fungal strains,

leaching appears to occur mainly by the

production of organic acids.


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Pyromorphite (Pb5 (PO4 ) 3 Cl) is a stable lead

mineral and can form in urban and industrially

contaminated soils. Since such insolubility

reduces lead (timbal) bioavailability, the

formation of pyromorphite has been

suggested as a remediation technique for lead

contaminated land, if necessary by means ofphosphate addition


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Pyromorphite can be solubilized byphosphatesolubilizing fungi, for exampleAspergillus niger, and plants grown with

pyromorphite as a sole phosphorus sourceaccumulated

Related to heterotrophic solubilization is fungaltranslocation, for example of caesium, zinc and

cadmium, which can lead to metal orradionuclide concentration in the mycelium and/or in fruiting bodies.


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Whether the concentration factors observed

in vitro can be reproduced in the field and

whether such amounts can contribute to soil

bioremediation remains uncertain


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Immobilization

Metal immobilization by fungi may be

metabolism independent, occurring whether

the biomass is dead or alive, or metabolism

dependent, comprising processes that

sequester, precipitate, internalize or transform

the metal species as well as the production of

extracellular metabolites, both organic andinorganic (Gadd, 1993a; Morley et al ., 1996).


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Physico-chemical mechanisms of

metal immobilization

Fungal cell walls are complex macromolecularstructures

Predominantly consisting ofchitin, chitosan and

glucans, but also containing otherpolysaccharides, proteins, lipids and pigmentssuch as melanin (Peberdy, 1990; Gadd, 1993a).This variety of structural components ensuresmany diferent functional groups are able to bind

metal ions to varying degrees depending on theirchemical proclivities (kecenderungan)(Gadd,1990; Bardi et al ., 1999).


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Fungi and their by-products have receivedconsiderable attention as possible biosorbentmaterials for metal-contaminated aqueous

solutions, because of the ease with whichthey are grown and the availability of fungalbiomass as an industrial waste product, forexample A. niger(citric acid production) and

Saccharomyces cerevisiae (brewing) (Gadd,1990; Kapoor, Viraraghavan & Cullimore,1999).


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Many studies have shown their eficacy in

sorbing metal contaminants either as living or

dead biomass, in pelleted whole-cell or

dissembled forms, and as freely suspended or

immobilized sorbents in batch and continuous

processes (Tobin et al ., 1994; Mogollon et al .,

1998; Yetis et al ., 1998; Karamushka & Gadd,1999; Lo et al ., 1999; Yin et al ., 1999; Zhou,

1999).


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Metal transformations

Physiological mechanisms of metal

immobilization Transport and intracellular fate

Many metals are essential for fungal growth

and metabolism, and for metals such as

sodium, magnesium, potassium, calcium,manganese, iron, cobalt, nickel, copper and

zinc, mechanisms exist for their acquisition

from the external environment by transport

systems of varying specificity (Gadd, 1993a;

Gadd & Sayer, 2000).


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Biosorption


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Transmission electron microscopy (TEM) shows that Pb(II) is

associated in the cell wall and membrane after 3 minutes

and cytoplasm after 2 hours inS

accharomyces cerevisiae(Suh et al., 1998). A three-step mechanism ofPb(II)

accumulation is advocated. Therst step is metabolism

independent, the second step is metabolism dependent, and

the third step is metabolism dependent or independent after

24 hours. Two mechanisms govern the removal of Cr(VI)from the aqueous solution by dead biomass ofAspergillus

niger(Park et al., 2005). During mechanism I, Cr(VI) is

reduced directly to Cr(III) by contact with the biomass.

Mechanism II consists of three steps: the binding of Cr(VI) to

positively charged groups in the cell wall, reduction of Cr(VI)

to Cr(III) by adjacent functional groups, and release of Cr(III)

by electron repulsion (tolakan elektron).


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Fungi can remove both soluble and insoluble

metals from solution and can leach metals from

solid wastes. Fungi produce protons, organic

acids, phosphatases, and other metabolites forsolubilization. Many heterotrophic fungi produce

organic acids that assist in solubility and

complexing of metal cations.

Several fungi are known to produce large

amounts of different kinds of acids that assist in

metal leaching purposes


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The cultural ltrate ofAspergillus nigercan render

the solubility of 18% Cu, 7% Ni, and 4% Co, and theseamounts are enhanced by the addition ofHCl(Sukla

et al., 1992)

The mechanisms offungal transformation are

reduction, methylation, and dealkylation of metals(Table 11.1). Certain species ofPenicillium are also

known to remove ironfrom alloys (Siegel et al.,

1990). Alternaria alternata causes volatilization of

substantial amounts ofselenium to thedimethylselenide form (Thompson-Eagle et al., 1991).

The volatilization process is optimized and used in the

fungal bioremediation of contaminated water and

land(Thompson-Eagle and Frankenberger, 1992)

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Metal Bio Remediation by Fungi

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