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Chemical and biochemical pathways and processes of bioremediation

Some pollutants, such as heavy metals, caimot be degraded by means of any physical, chemical or biological processes that involve microorganisms. However, microorganisms can change the bioavailability of such pollutants and their potential biotoxicity by altering the valence state of specific elements by oxidation or reduction (Rockne and Reddy 2003, Adams et al. 2015, Roane et al. 2015).

In an oxidative environment, the pollutants are oxidized by external electron acceptors such as oxygen or sulfate (Roane et al. 2015). When the reaction is reductive, the electrophilic halogen or nitro groups on the pollutant are reduced by those microorganism groups that consume sugars, fatty acids, or even hydrogen (Adams et al. 2015). The halo- or nitro-group on the pollutant acts as the external electron acceptor (Figure 6).

Overall progression of degradation organic products. Top figure

Figure 6. Overall progression of degradation organic products. Top figure: contextualization of organic pollutants; Bottom figure: contextualization of electrophilic pollutants. Source: modified from Rockne and Reddy (2003) and Adams

et al. (2015).

Hence, it can be seen that the degradation of any organic molecule requires the production and efficient utilization of enzymes (Rockne and Reddy 2003).

Another important skill of some species of microorganisms is the capacity of making biofilms, being also usefiil in the process of bioremediation and considered a promising technology. Biofihns are the microbial communities that produce extracellular matrix. Biofilm-linked processes are intrinsically related to biotransfonnation of contaminants in groundwater and soil, as well as in engineered reactors. In most biofilms formation, the microorganisms represent minor mass (less than 10% of the dr}' mass), while the (mucilaginous) matrix is the major substance and may account for over 90% (Satpathy et al. 2016).

Bioemulsifiers are exopolymeric products that help the microorganism communities in the biofilm development. Emulsifiers are found in various natural resources and are synthesized mainly by bacteiia, but also by fungi and yeast (Alizadeh-Sani et al. 2018). Such materials aid the cells in survival, as well as protect themselves from hostile and extreme situations, predators and especially from the loss of water from the cell. Bacterial adhesion occurs in mobile and stagnant phases. Biofilm formation is a complex process of surface attached community transition from numerous free-floating cells (Karlapudi et al. 2018). Bioemulsifiers have double lipophilic and hydrophilic properties and they are higher in molecular weight when compared to biosurfactants, since they are complex mixtures of products such as heteropolysaccharides, lipopolysaccharides, lipoproteins, and proteins (Alizadeh-Sani et al. 2018).

In turn, biosurfactants and bioemulsifiers are amphiphilic molecular chains and are produced as extracellular or as a part of the cell membrane by bacteria (an amphiphilic molecule is a molecule that has hydrophilic (polar) and hydrophobic (non-polar) characteristics). On the other hand, biofihns are bacterial communities that protect the bacterial cells from some hostile conditions (Karlapudi et al. 2018).

Biosurfactants emulsify the molecular chains, intensify the water solubility and make the molecular chains more reachable for the microorganisms (Elkhawaga 2018). Because of their potential advantages, such kind of product is largely used in several industrial segments such as chemistry, aliment production, agriculture, pharmaceutics, as well as cosmetics (Pacwa-Plociniczak et al. 2011).

On the other hand, the fungi, with their enzymes, have the ability to degrade a wide variety of environmentally persistent pollutants and transform industrial and agro-industrial wastes into products. Specifically, mushrooms can produce extracellular peroxidases, ligninase (lignin peroxidase, manganese-dependent peroxidase, and laccase), cellulases. pectinases, xylanases, and oxidases (Nyanhongo et al. 2007). These are able to oxidize recalcitrant pollutants in vitro. These enzymes are typically induced by their substrates. The uptake of pollutants/xenobiotics by mushrooms involves a combination of two processes: (i) bioaccumulation, i.e., active metabolism- dependent processes, which include both transport into the cell and partitioning into intracellular components, and (ii) biosoiption, i.e., the binding of pollutants to the biomass without requiring metabolic energy.

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