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Bioremediation is possible if the biological species can be identified correctly. The organic toxic materials can be simplified into harmless chemicals by the process of immobilization, but inorganic toxins cannot be simplified. The only possible way is to convert them into a harmless form. Bioremediation is the process which completely depends upon the metabolic capability of the organisms and different organisms react differently to the heavy metals. So, the identification of the proper microbial species for the clearance of heavy metal pollution is an important task. Some of them act as an essential micronutrient for the growth and development of the microorganisms like non. The mechanisms involved like adsorption, conversion of the redox state and formation of the insoluble forms depend upon the enzymatic activity, the population of the microorganisms and the geochemistry.

The exact cellular mechanisms about bioremediation is not understood properly, which limits its successful application. The native microorganisms should be explored for the detoxification of the environment (Garbisu et al. 2001). The capable microorganisms should be identified with their genes helping in bioremediation. The study of these genes could help us to develop genetically engineered microorganisms for bioremediation in the fimire.

Mechanisms of bioremediation

The things which are important for bioremediation are the enzymatic activity of the microorganisms and the resistance of the microorganisms towards heavy metal pollution. Many microbes can develop a defense mechanism from organic pollutants by forming hydrophobic outer cell membranes (Sikkema et al. 1995) or cells have the mechanisms of energy-driven ion/proton pump to release heavy metal cations. The enzymatic activity of the microbes is capable of dissolving or volatilizing the metals or transforming them into one redox state to another state. Some common mechanisms related to the bioremediation are discussed below.

5.1 Bioadsorption

Microbes adsorb different heavy metals in then extracellular structures without any expenditure of energy. So, this process can be called a passive process. The cell walls have extracellular polymeric substances (EPS) having effects on acid-base properties (Guine et al. 2006) and can bind heavy metals. These substances bind metals through some mechanisms like proton exchange or micro-precipitation of the metals or electrostatic attraction (Comte et al. 2008, Fang et al. 2010) (Figure 1). Saccharomyces cererisiae and Cimnighamellaelegans were identified as bioadsorbent and can remove Zn, Cd and other heavy metals through ion-exchange mechanisms. In order to find out the mechanisms of the EPS, this activity had been studied using the bacterial cells with EPS and without EPS (Fang et al. 2011) but the exact mechanisms in the genetic level were not understood properly. Because of that, the metabolic pathway of these heavy metals and their kinetics in the bacterial cells are not clear. This inability restricts their application in the fields and indicates that further developments are needed in scientific studies to predict their behavior (Gan et al. 2009, Haritash et al. 2009, Onvvubuya et al. 2009, Carter et al. 2006, Kinya and Kimberly 1996).

5.2 Bioaccumulation

The biosorption process does not require any energy as the cell wall has a high affinity towards the heavy metals and the process continues until the equilibrium is reached between sorbate and sorbent (Das et al. 2008). But other bioabsorbtion processes require metabolic energy to accumulate the heavy metals within the cells of the microbes. The bioaccumulatiou process includes both processes, i.e., active process and passive process of removing heavy metals. The potential of fungi as a biocatalyst is more than the other microbes as they are eukaryotes and they can efficiently transform a more toxic compound to less toxic compound (Pinedo-Rivilla et al. 2009). The fungi

Mechanisms of bioadsorption

Figure 1. Mechanisms of bioadsorption.

like Klebsiellaoxytoca, Allescheriella sp., Stachybotrys $p., Phlebia sp., Pleurotusptiinionarius, Born osphaeriarhodina have the potential to bind heavy metals. The contaminated soils with Pb (II) can be transformed to a remediated soil with the help of the biosorption process of Aspergillus parasitica and Cephalosporiumaphidicola (Timali et al. 2006, Akar et al. 2007). Hymenoscyphusericae and Neocosmosporavasinfecta have the mechanism to biotransfonn Hg (II) state to a less toxic, less harmful state (Kelly et al. 2006). The contaminants are mostly hydrophobic in namre and microbes take up these metal contaminants by forming a complex with biosurfactant secreted by them, having a strong ability to form ionic bonds with metals due to low interfacial tension (Thavasi 2011).

5.3 Transformation in redox state

The bioremediatiou reactions also involve the aerobic and anaerobic reactions in the soil controlled by the microbes. Aerobic reactions include oxygen atoms into reactions by different enzymes like monooxygenases, dioxygenases, hydroxylases, oxidative dehalogenases, etc. Some enzymes generate reactive oxygen such as ligninases or peroxidases. In the case of anaerobic reactions, heavy metals are used as a terminal electron accepter to supply energy for the microbes. This technique changes the redox state of the metals and sometimes reduces then mobility in the contaminated soil and it is called immobilization.

The technique sometimes allows ex situ application of the chemicals in that contaminated site to initiate the reactions. It is called solidification (Evanko and Dzombak 1997). The heavy metals can be leached, precipitated, chelated or methylated but never can be destroyed. This transformation process is very important to change then redox state or to change the organic form to inorganic form so that the metals can be made less toxic, water-soluble and precipitated (Garbisu et al. 2001).

The microorganisms help the heavy metal to be oxidized in the oxidative environment, and nitrates and sulfates act as terminal electron accepters. In anaerobic conditions, the organic pollutants get oxidized by the microbes and at that time heavy metals act as terminal electron accepters (Lovley et al. 1988). The higher availability of the metals will accelerate the oxidation of the organic pollutants (Lovley et al. 1996, Sponnami and Widdel 2000). The processes where metals act as terminal electron accepters are called dissimilatory metal reduction (Lovely et al. 2002). Geobacccter species reduces uranium from U+6 to U+J leaving it in an insoluble state (Lovley et al. 1991).

5.4 Molecular mechanisms

The genes control all biological activities. The genes involved in the bioremediation process can be identified and inserted into other microbes by the genetic engineering process to make the bioremediation process more efficient. It redesigns the microbes and increases its potential to work more accurately and specifically. Scientists are interested to find out genes that can produce different coenzymes and sideropliores for binding specific metals even in the adverse condition and dilute concentration (Penny et al. 2010). Deinococcus geothemalis is known to reduce the Hg at high temperatures showing the expression of gene mer operon from E. coli (Brim et al. 2003). Cupriavidus nietallidiiraris (strain MSR33), a Hg resistant microorganism, is made capable of synthesizing organomercuriallyase protein (MerB) and mercuric reductase (MerA) for Hg biodegradation after inserting pTP6 plasmids within the genes (merB and merG) (Rojas et al. 2011). The gene JM109 in E. coli is genetically modified to express GST-PMT and Hg+: transport system simultaneously, which increases the capability of Hg accumulation in very low concentration (Chen et al. 1997).

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