Bioleaching Mineral Oksida & SilicateMekanisme Bioleaching oleh fungi

Oleh

Siti K ChaerunGeomicrobiology-Biomining & Biocorrosion Laboratory, Microbial Culture Collection

Laboratory, Biosciences & Biotechnology Research Center (BBRC)Department of Metallurgical Engineering, Faculty of Mining & Petroleum Engineering

Institut Teknologi Bandungskchaerun@metallurgy.itb.ac.id; skchaerun@gmail.com

Cell phone: 087878590709 (WhatsApp)

monomers• Sugars• Fatty acids• Nucleotides• Amino acids

macromolecules• Polysaccharides• Lipids• Nucleic Acids (DNA, RNA)• Proteins

Structure of a fungal cell wall

Chitin (C8H13O5N)n (/ˈkaɪtɨn/ KY-tin) is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world. It is the main component of the cell walls of fungi,

The main organic components comprising algal and fungal walls

The mitochondrion (plural mitochondria) is a membrane bound organelle found in most

eukaryotic cells

The ribosome is a large and complex molecular machine, found within all living cells, that serves as the primary site of

biological protein synthesis (translation). Ribosomes link amino acids together in the order specified by messenger RNA (mRNA)

molecules

Extracellular polymeric substances (EPS)i.e., exopolysaccharides

Oil dropletC,H,O,N,S

Na+

K+

Cl-

Mg2+

Ca2+

Co2+

Br-

Sr2+

SiO2

Ni2+

Ti2+Cr3+

Cu2+Zn2+

0.01~10 mg/l Ti, Cr, Ni, Cu, Zn0.01~ 0.5 mg/l Co0.01~25 mg/l Ca0.01~100 mg/l Si(stimulate bacterial growth)

Aerobic conditionpH 5-8

Bacteria were identified as:Pseudomonas aeruginosa, Bacillus cereus or Bacillus thuringiensis,Paracoccus seriniphilus or Paracoccus marcusii

transmembrane transport

Seawate

r

Fungi• Fungi are heterotrophic eukaryotes which lack

of chlorophyll and require pre-formed organic carbon and energy source

• It contains a real nucleus and normally forms mycelia.

• Fungi typically reproduce asexually and/or sexually by producing spores

• They are aerobic organisms and thus oxygen supply is necessary.

• Heterotrophic fungi can withstand a much wider pH range compared to chemolithoautotrophic bacteria

• Filamentous fungi which are capable of bioleaching include the genus Aspergillus which have black, brown or green spores, and the genus Penicillium which is well-known in producing Penicillin as an effective treatment of infectious diseases

Asexual spores called conidia

• Fungi are capable of oxidizing substrate only partially and then secreting it. This incomplete oxidation causes the accumulation of organic acids, which are able to extract metals from solid materials.

• These organic acids may be categorized into two groups:– those derived from sugars by simple oxidation

(gluconic acid = C6H12O7, kojic acid = C6H6O4), – and those which are related to tricarboxylic acid

intermediates (citric acid, oxalic acid and malic acid).

the general metabolic relationship between organic acids (gluconic, citric and oxalic acids) produced by A. niger

• Gluconic acid results from the conversion of glucose by glucose oxidase

• the biosynthesis of citric acid in fungi involves:– glycolysis –tricarboxylic acid cycle (TCA

cycle).

Glycolysis• Is the conversion of glucose to pyruvate by:

– The Embden-Meyerhof Parnas (EMP) Pathway – Pentose Phosphate (PPP) Pathway or Enter-

Doudoroff (ED) Pathway. (Of these, EMP is the most common and important pathway)

• Pyruvate is then oxidized to carbon dioxide and water in the TCA cycle and at the same time, accumulation of citric acid occurs within the cycle (Figure 2.2b).

The oxalate biosynthesis in fungi• Three possible pathways may be involved:

– formation of oxalate by splitting of the oxaloacetate which does not enter the TCA cycle

– formation of oxalate by splitting of the oxaloacetate which arises from the TCA cycle

– Formation of oxalate from glyoxylate via the glyoxylate cycle

[Gadd, 1999]

Fungal Leaching Mechanisms• Metal leaching by heterotrophic microbes

generally involves an indirect process with microbial production of organic acids (e.g. lactic acid, oxalic acid, citric acid, gluconic acid), amino acids, and other metabolites via:– Acidolysis– Complexolysis– Alkalolysis– bioaccumulation

Acidolysis• Acidolysis is a fast and dominant leaching mechanism in fungi

bioleaching• It is a process which involves the protonation of oxygen atoms

in the metal compound.• The protonated oxygen then combine with water, resulting in

the metal oxide being detached from the solid surface and being solubilised

• The amount of metal oxides solubilized results from the maximum amount of protons obtained from the organic acids.

• Acidolysis reaction in general:

• The common acids secreted by heterotrophs are lactic, oxalic, gluconic, acetic, citric, succinic, pyruvic and formic acids.

• These assist in creating a low pH environment which enhances the bioleaching of metals

• Organic acids produced which decrease the availability of anions to the cations in metal compounds, thus causing the solubilization of metal ions

Complexolysis• While the organic acids formed from the heterotrophs

participate in acidolysis, the relatively slower mechanism of complexolysis also takes place because the organic acids, some of which are powerful natural chelating agents, form a metallic complex with the metals from the material to be bioleached.

• The solubilization of metal ions is based on the complexing capacity of a molecule.

• If the bonds between metal ions to ligands are stronger than the lattice bonds between metal ions with solid particles, metal will be successfully leached out from solid particles

Chelation describes a particular way that ions and molecules bind metal ions

a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex

• Complexolysis adalah mekanime pelarutan logam yang diinduksi oleh ligan. Dalam mekanisme ini terjadi mikroba membentuk pengompleks atau chelating agents yang menyebabkan peningkatan mobilitas logam.

• Complexolysis, which often combines with acidolysis, stabilize the metal ions that are solubilized into solution by acidolysis.

• The stability of metal complexes also reduces the toxicity of metal ions to the microbes

• Stable complexes could be formed between the organic ligands with metal ions, such as oxalic acid and iron, citric acid and magnesium, phenol derivatives and some amino acids with metal ions.

• The production of low molecular weight iron chelating siderophores by fungi or bacteria also enables solubilization of iron (III)

Bacterial CellAerobic condition

Neutral pH

Fe+3 (Insoluble form in solution)

PrecipitatesMicrobes

Soluble form in solution

Fe+3-Siderophores

Transport into cell

Fe+2-Siderophores (enzymatically)

Desferrisiderophore Fe+2 ---assimilated into protein

Siderophores (Greek: "iron carrier") are small, high-affinity iron chelating compounds secreted by microorganismssuch as bacteria, fungi and grasses. Siderophores are amongst the strongest soluble Fe3+ binding agents known.

Chelation describes a particular way that ions and molecules bind metal ions

a ligand /lɪɡənd/ is an ion or molecule (functional group) that binds to a central metal atom to form a

coordination complex

• It has been found that amino acids are released together with organic acids.

• Both groups of compounds provide protons and complexing capacity for metal solubilization

• In most cases of metal solubilisation by heterotrophic microorganisms, the organic acids are the lixiviant (leaching agent) and excreted amino acids are also able to solubilize metals.

• However, it is found that leaching through amino acids is not of great importance for fungi in general as amino acid efflux is rare among filamentous fungi

• The complexation of metal ions with citrate was found to result in the formation of highly mobile species and therefore allows transport and activity of toxic metals at a distance from their source.

• Metals interaction with oxalic acid could ultimately lead to the formation of insoluble oxalates which could immobilize toxic metal species.

• Most metal oxalates are immobile and resistant to further solubilization, with only a few species of anaerobic bacteria, aerobic actinomycetes, bacteria and fungi able to degrade them readily

Alkalolysis• The enzymatic hydrolysis of urea or

deamination of amino acids by microbes, when these compounds are used as the energy source, results in the production of ammonia which is able to leach metals by the alkalolysis process.

• This mechanism is very effective in mobilizing metals in silicates or aluminosilicates

• This mechanism enables bioleaching to take place at high pH.

Bioaccumulation• Bioaccumulation is the only mechanism that does not involve

the excretion of metabolites• Bioaccumulation occurs when the solubilised metal ions

accumulate within the mycelia of the heterotrophic fungi through passive adsorption and active metabolic reactions, thereby enhancing further metal solubilisation into the solution

solubilised metal ions

• This could be interpreted as the mycelium functioning somewhat as a sink for metal ions.

• Fungal cell wall contains many different functional groups (e.g. hydroxyl, amine, carboxyl, phosphate and sulfate groups) which are able to bind metal ions to a greater or lesser extent.

• Among the filamentous fungi, the genera of Aspergillus and Penicillium have been reported to have high ability to accumulate heavy metals and radionuclides from their external environment.

• The fourth advantage is energy source. Fungi leach metals by the excretion of metabolites, and involve several indirect leaching mechanisms such as acidolysis and complexolysis of metal ions. Energy for growth and organic acid excretion is externally supplied through the organic substrates in the medium.

• The main drawback on the use of heterotrophic fungi is the need for a significant amount of organic carbon source for growth and for the production of leaching agents. Thus, bioleaching using heterotrophs needs a higher operating cost compared with chemolithoautotrophs.

Bioaccumulation

The accumulation process involves the biological sequestering of substances that enter the organism through respiration, food intake, epidermal (skin) contact with the substance, and/or other means.

The level at which a given substance is bioaccumulated depends on: the rate of uptake the mode of uptake how quickly the substance is eliminated from the organism transformation of the substance by metabolic processes the lipid (fat) content of the organism the hydrophobicity of the substance environmental factors other biological and physical factors

Biosorption

The ability of certain types of inactive, dead, microbial biomass to bind and accumulate heavy metals from aqueous solutions through non metabolically mediated or physico-chemical pathways of uptake.

Materials which exhibit biosorptive behaviour (biosorbents) include certain algae, fungi and bacteria.

Interactions of metal and fungal cellsMetallothioneins (MTs) are a family of small, highly conserved, cysteine-rich metal-binding proteins that are important for zinc and copper homeostasis, protection against oxidative stress, and buffering against toxic heavy metals.

Phytochelatins are oligomers of glutathione, produced by the enzyme phytochelatin synthase. They are found in

plants, fungi, nematodes and all groups of algae including cyanobacteria. Phytochelatins act as chelators, and are

important for heavy metal detoxification. They are abbreviated PC2 through PC11

oligomer is (chemistry) a compound intermediate between a monomer and a polymer, normally having a specified number of units between about five and a hundred while polymer is (organic chemistry) a long or larger molecule consisting of a

chain or network of many repeating units, formed by chemically bonding together many identical or similar small

molecules called monomers a polymer is formed by polymerization, the joining of many monomer molecules.

Methylation: the addition of a methyl-group (-CH3) to a compound

Dealkylation: the removal of a methyl-group from a compound

A vacuole• The fungal vacuole is a large, membrane-bounded organelle

that functions as a reservoir for the storage of small molecules (including polyphosphate, amino acids, several divalent cations (e.g. calcium), other ions, and other small molecules) as well as being the primary compartment for degradation.

• It is an acidic compartment, containing an ensemble of acid hydrolases (An acid hydrolase is an enzyme that works best at acidic pHs. It is commonly located in lysosomes, which are acidic on the inside).

• A vacuole is a membrane-bound organelle which is present in plant and fungal cells and some animal and bacterial cells.

• A space or vesicle within the cytoplasm of a cell, enclosed by a membrane and typically containing fluid.

Uptake nutrisi secara extraselular (oleh fungi)

Fig 1

Fig 2

• Fungi are in direct contact with their nutrients in the environment.

• Smaller molecules (such as simple sugars and amino acids) in solution in the watery film surrounding the hyphae can be directly absorbed by the hyphae.

• Larger insoluble polymers such as cellulose, starch, and proteins must undergo a preliminary digestion before they can be used.

• Molecules that are too large to be absorbed by the fungus are attacked by extracellular enzymes (Figs. 1 and 2).

• Like all digestive enzymes, the digestive enzymes of fungi-control hydrolysis reactions that cleave the large molecules into simpler components.

• The digestive enzymes are highly specific and are able to control hydrolysis of particular molecules only.

• Complete digestion of a large polymer is a stepwise process involving different enzymes until finally a simple, soluble molecule is released. It is this simple molecule that is taken up by the fungus. Once it is absorbed into the cell, this small molecule is further acted upon by intracellular enzymes.

• The ability to utilize large molecules ultimately depends on the ability of the fungus to digest them, which in turn depends on the enzymes with which the fungus is equipped.

• Fungi typically have a large number of enzymes but for the most part, many of them are inactive until the fungus comes into contact with a substrate on which particular enzymes can act.

• Growth of the fungus occurs equally well on a medium containing either complex or simple nutrients.

• The necessary enzymes may be entirely lacking, however, and the fungus may be unable to grow on a medium that contains an undigestible substrate.

• All ions and molecules entering the fungal cell must pass through both the cell wall and the plasmalemma.

• The wall itself is somewhat porous, allowing ions and molecules to pass through if perhaps through minute pores or channels.

• The plasmalemma is a semipermeable membrane that can regulate the movement of solutes into the cell