136

9 Biodegradation of Plastics by Microorganisms

9.3.6.5

Lignin Modifying Enzymes

Lignin modifying enzymes such as laccases (EC 1.10.3.2), manganese peroxidases

(MnP, EC 1.11.1.133), and lignin peroxidases (Lip, EC 1.11.1.14) are known to

degrade lignin, a complex cross-linked aromatic polymer of phenypropanoid units

[35]. These enzymes are responsible for the biodegradation of PE. In the presence

of iron, laccase, a thermo-stable enzyme isolated from R. ruber C208 can degrade

UV-irradiated PE films both in culture supernatants and in cell free extract. The key

mechanism involved in this process includes the increasing of carbonyl groups and

decreasing of molecular weight within the amorphous component of PE films. Sim-

ilarly, laccase isolated from Trametes versicolor can degrade high-molecular-weight

PE membrane, in the presence of 1-hydroxybenzotriazole, which oxidized

non-phenolic substrates by the enzyme. However, high-molecular-weight PE also

degraded by a combination of MnP from white-rot fungi (Phaerochaete chrysospo-

rium ME-446) and MnP isolated from IZU-154 [36]. This high-molecular-weight PE

also degraded by cell free supernatant from P. chrysosporium MTCC-787 containing

both extracellular LiP and MnP, respectively. The combination of Lip and MnP

enzymes permitted the degradation of 70% of the pre-oxidized high molecular

weight of PE with 15 days of reaction.

9.4

Current Trends and Future Prospects

There is an emerging trend in the use of environmental-friendly bio-based and

fossil-based biodegradable plastics. The proper use of biodegradable plastics

in the form of sustainable waste management approaches should be prac-

ticed worldwide. A recent research suggested that hydrolysis of PET and its

mono-2-hydroxyethyl-terepthalic acid to ethylene glycol and terephthalic acid is

occurred by two enzymes isolated from I. sakaiensis, 201-F6 strain [19]. Research

also illustrated that Pantoea spp. and Enterobacter spp. have the ability to degrade

LDPE [37]. Tan et al. [38] found some microbes convert the organic styrene (an

industrial waste material from plastic processing) into PHA. They also recognized

that P. putida NBUS12 is an efficient and effective styrene degrading bacterium.

Achromobacter xylosoxidans, a recently characterize bacteria, was found to affect

the structure of HDPE. Similarly, a thermophilic bacterium, named, Anoxybacillus

rupiensis Ir3 (JQ912241), was isolated from soil in Iraq, which confirmed a good

capacity and efficiency to utilize aromatic compounds as carbon sources followed by

degradation [39]. Extensive research is therefore required worldwide to improve the

process of degradation of bio-based and fossil-based plastics in order to recognize

their potential eco-friendly applications and waste management plans.

Innovative and eco-friendly biodegradable plastics should be used in the pack-

aging, agriculture, and heath industry which is the simplest strategy to resolve the

plastic-related problem throughout the world. Bio- and fossil-based biodegradable

polymers should be exploited more proficiently and effectively to degrade in

the cells, eco-friendly, or under optimized facilities. At present, however, only

non-biodegradable petroleum products are utilized for the processing of plastics,