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Mechanism of plastic biodegradation

Oxidative degradation is the principal mechanism for the degradation of plastics. These mechanisms reduce the molecular weight of the material. In this mechanism of plastic biodegradation, microorganisms stick with polymers and eventually colonize on the surface. This enzyme-based hydrolysis of plastics occur in two steps: firstly, the enzymes attach to the polymer substrate followed by hydrolytic division (Figure 2). The extracellular and intracellular enzymes that are produced by the microbes convert the polymers into monomer, dimer, and oligomer. Degradation

Mechanisms of microbial mediated plastic biodegradation

Figure 2. Mechanisms of microbial mediated plastic biodegradation.

products of polymers like oligomers, dimers, and monomers are much low in molecular weight and are eventually converted to CO, and H20 by mineralization (Tokiwa et al. 2009). The by-products produced during conversion enter into the microbial cell and can be utilized as the energy source (Shimao 2001). Under aerobic conditions, bactexia use oxygen as electron acceptor and form other organic compounds along with C02 and H20 as last products (Priyanka and Archana 2012). Under anaerobic conditions, nitrate (NOf), sulfate (SO|~), metals such as iron (Fe3) and manganese (Mn4), or even CO, play the role of electrons acceptor from the degraded contaminant (National Research Council 1993). The byproducts of anaerobic respiration are nitrogen gas (N,), hydrogen sulfide (H2S), metals (in reduced form), and methane (CH4), which largely depends on electron accepting species.

Microbes involved in plastic biodegradation

Microorganisms are bestowed with enzymes which play a vital role in destruction of environmental contaminants. They are so tiny that they can reach and contact contaminants easily and efficiently. Optimum quantities of nutrients are required for their work efficiency and metabolism. Bacteria and fungi often produce extracellular enzymes which easily degrade the various sorts of bio and fossil-based plastics (Shah et al. 2014). Bacteria and fungi degrade these plastic polymers into CO, and H,0 through various metabolic and enzymatic mechanisms. The microbial species and strains affect the catalytic activity of enzymes and finally degradation process. Various types of enzymes are known to degrade many polymer types. Microbial enzymes accelerate the biodegradation rate of plastics very effectively without causing any harm to the environment. Enzymes are present in living cell of every' organism and hence in all microbes. Different types of microbes produce different kinds of enzymes in different quantities. For instance, BrevibaciUus spp., and Bacillus spp., produce proteases enzyme responsible for degradation of polyethylene (PE) (Sivan 2011). Fungi are known to degrade the lignin frequently as they produce laccases enzymes which catalyze the aromatic and non-aromatic compounds (Mayer and Staples 2002). Lignin and manganesedependent peroxidases (LiP and MnP, respectively) and laccases are major lignin olytic enzymes (Hofrichter et al. 2001). Lignin-degrading fungi and manganese peroxidase, partially purified from the strain of Phanerochaetechryso sporium, also help in the degr adation of high-molecular weight PE under nitrogen and carbon limited conditions (Shimao 2001). The list of plastic degradation by various microbes and their enzymes has been given in Table 2.

Table 2. Microbes and enzymes responsible for degradation of specific polymer groups.

Polymer

Degrading enzymes

Microbes involved

Polyethylene

terephthalate

(PET)

PET hydrolase and tannase (cutinase, lipases, carboxylesterase)

Actinobactenae.g Thermobifida, Thermomonospora, Ideonellasakaiensis, Saccharomonospora

Cutinase and Lipase

Pseudomonas pseudoalcaligenes, Pseudomonas pelagia

Fungal cutinase

Fusarium, Humicola

PET esterase

Desulfovibhofmctosivorans

Polyurethanes

(PUR)

Lipase

Pseudomonas chlororaphis, Pseudomonas putida, Candida rugosa

Esterase

Comamonas acidovorans, Aspergillus flaws

Cutmase

Thermobifida

Polyethylene

(PE)

Unidentified

Pseudomonas, Ralstonia and Stenotrophomonas Staphylococcus, Streptomyces, Bacillus

Laccase

Aspergillus, Cladosporium, Penicillium

Polyamide (PA)-Nylon

Hydrolases

Pseudomonas, Acliromatobacter

Dehydrogenase

Bacillus cereus, Bacillus sphaericus, Vibrio furnisii and Brevundimonasvesicularis

Peroxidase

White rot fungi

Polystyrene

(PS)

Styrene monooxygenase, Styrene oxide isomerase, Phenylacetaldehyde dehydrogenase

Brown-rot fungi - Gloeophyllumtrabeum

White rot fungi - Pleurotusostreatus, Phanerochaetechrysosporium, Trametes versicolor

Pseudomonas, Xanthobacter, Rhodococcus, Corynebacterium

Source: Danso et al. (2019).

 
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