Article Index

1.1.1. Microbe-Based Clean Up System (Microbial Bioremediation)

Microorganisms uptake heavy metals actively (bioaccumulation) and/or passively (adsorption). The microbial cell walls, which mainly consist of polysaccharides, lipids and proteins, offer many functional groups that can bind heavy metal ions, and these include carboxylate, hydroxyl, amino and phosphate groups. Among various microbe-mediated methods, the biosorption process seems to be more feasible for large scale application compared to the bioaccumulation process, because microbes will require addition of nutrients for their active uptake of heavy metals, which increases the biological oxygen demand or chemical oxygen demand in the waste. Further, it is very difficult to maintain a healthy population of microorganisms due to heavy metal toxicity and other environmental factors. Fungi of the genera Penicillium, Aspergillus and Rhizopus are potential microbial agents for the removal of heavy metals from aqueous solutions. Endophytic bacteria that are known to be beneficial to plants also enhance the ability of host plants accumulating higher levels of heavy metals.

Microorganisms are ubiqutious in heavy metal-contaminated environments and can easily convert heavy metals into non-toxic forms. In bioremediation processes, microorganisms mineralize the organic contaminants to end-products such as CO2 and H2O, or to metabolic intermediates which are used as primary substrates for cell growth. Different mechanisms of bioremediation are known, including biosorption, metal-microbe interactions, bioaccumulation, biomineralisation, biotransformation and bioleaching. Microorganisms are capable of dissolving metals and reducing or oxidizing transition metals. Different methods by which microbes restore the environment are oxidizing, binding, immobilizing, volatizing and transformation of heavy metals. Bioremediation can be made successful in a particular location by the designer microbe approach, and by understanding the mechanism controlling growth and activity of microorganisms in the contaminated sites, their metabolic capabilities and their response to environmental changes.

Microbial bioremediation by adsorption

Heavy metals can be biosorbed by microbes at binding sites present in cellular structure without the involvement of energy. Among the various reactive compounds associated with bacterial cell walls, the extracellular polymeric substances (high-molecular weight compounds secreted into their environment) are of particular importance and are well known to have significant effects on acid-base properties and metal adsorption. Secreted extracellular polymeric substances have a great ability to complex heavy metals through various mechanisms including proton exchange and micro-precipitation of metals.

Microbial bioremediation by physio-bio-chemical mechanism

Biosorption is the process which involves higher affinity of a biosorbent towards sorbate (metal ions), continued until equilibrium is established between the two components. Saccharomyces cerevisiae acts as a biosorbent for the removal of Zn (II) and Cd (II) through the ion exchange mechanism. Cunninghamella elegans is a promising sorbent against heavy metals released by textile wastewater. Fungi are potential biocatalysts to access heavy metals and transform them into less toxic compounds. Some fungi such as Klebsiella oxytoca, Allescheriella sp., Stachybotrys sp., Phlebia sp., Pleurotus pulmonarius, Botryosphaeria rhodina have metal binding potential. Pb (II) contaminated soils can be remediated by fungal species like A. parasitica and Cephalosporium aphidicola with biosorption process. Hg resistant fungi (Hymenoscyphus ericae, Neocosmospora vasinfecta and Verticillium terrestre) were able to biotransform a Hg (II) state to a nontoxic state. Many of the contaminants are hydrophobic, and they are taken up by microbes through the secretion of some biosurfactant and direct cell-contaminant association. Biosurfactants form stronger ionic bonds with metals and form complexes before being desorbed from soil matrix to water phase due to low interfacial tension.

Bioremediation may also involve aerobic or anaerobic microbial activities. Aerobic degradation often involves introduction of oxygen atoms into the reactions mediated by monooxygenases, dioxygenases, hydroxylases, oxidative dehalogenases, or chemically reactive oxygen atoms generated by enzymes such as ligninases or peroxidases. Anaerobic degradations of contaminants involve initial activation reactions followed by oxidative catabolism mediated by anoxic electron acceptors. The immobilization technique is used to reduce the mobilization of heavy metals from contaminated sites by changing the physical or chemical state of the toxic metals. Solidification treatment involves mixing of chemical agents at the contaminated sites or precipitation of hydroxides. In the contaminated sites, microbes mobilize the heavy metals by leaching, chelation, methylation and redox tansformation of toxic metals. It is not possible to destroy heavy metals completely, but the process transforms their oxidation state or organic complex, making them water-soluble, precipitated and less toxic. In the bioremediation of contaminated environments, microbes use heavy metals and trace elements as terminal electron acceptors or reduce them through the detoxification mechanism. Microorganisms remove heavy metals through the mechanisms which they employ to derive energy from metals redox reactions, to deal with toxic metal through enzymatic and non-enzymatic processes. Two main mechanisms for development of resistance in bacteria are detoxification (transformation of the toxic metal state and making it unavailable) and active efflux pumping of the toxic metal from cells. The basic redox (oxidation and reduction) reaction takes place in the soil between toxic metals and microorganisms; microorganisms act as an oxidizing agent for heavy metals and cause them to lose electrons, which are accepted by alternative electron acceptors (nitrate, sulphate and ferric oxides).

In aerobic conditions, oxygen acts as an electron acceptor, while in anaerobic conditions microbes oxidize organic contaminants by reducing electron acceptors. The microorganism takes energy for growth by oxidizing the organic compound with Fe (III) or Mn (IV) as an electron acceptor. Anaerobic degradation of organic contamination is stimulated with the higher availability of Fe (III) for microbial reduction. Biodegradation of chlorines from contaminants takes place through reductive dechlorination, where contaminants as chlorinated solvents act as an electron acceptors in respiration. Microorganisms reduce the state of metals and change their solubility, like the Geobacter (anaerobic respiration bacterial species found in anaerobic conditions in soils and aquatic sediment), and reduce the Uranium soluble state (U6+) to insoluble state (U4+). Different defense systems (exclusion, compartmentalization, complex formation and synthesis of binding protein and peptides) reduce the stress developed by toxic metals. These metal binding protein transcription factors are known to mediate in hormone and redox signaling process in the context of toxic metal (Cd, Zn, Hg, Cu, Au, Ag, Co, Ni and Bi) exposure.

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