7.6 Mechanism of Biodegradation

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layers of the cells, its rate of growth, and the production of suitable metabolites

responsible for degradation. Any variation in the properties mentioned above has

a direct impact on the degradation efficiency. Some examples of biofilm-forming

microbes include Rhodococcus ruber (mushroom-like structures) and Alcanivorax

borkumensis, which pioneers in the degradation of low-density polyethylene by the

formation of thick biofilm [35].

7.6.2

Biodeterioration

The next stage in the process of biodegradation after surface colonization and biofilm

formation is biodeterioration. This stage is augmented by the release of exopolysac-

charides (EPSs), a characteristic feature of biofilm-forming microorganisms. The

EPS released in large quantities have the microorganisms to stick to the surface more

tightly hence forming a more robust biofilm. The EPS, along with enzymes (exo- and

endo-enzymes), have a significant impact on biodeterioration.

7.6.3

Biofragmentation

This stage largely includes the breaking up of polymer chains to oligomers, dimers,

and monomers. The step mostly utilizes enzymes that distort the basic polymer

geometry holding the polymer chain together, easing microbial attack. Microor-

ganisms can directly utilize the resulting oligomer and monomer units as a carbon

source, contributing to the production of biomass.

The initial attack occurs on the terminal moieties causing a sequential reduction

in the molecular weight. The enzymes involved in the fragmentation of polymer

belong to class oxidoreductases and hydrolases.

Hydrolases work by mainly acting on the carboxylic linkages specifically.

Examples of hydrolases include esterases, lipases, and cutinases. The presence of

three amino acids (aspartate, serine, and histidine) residues in the active site is a

characteristic of hydrolases, and these three amino acids aid in the production of

nucleophilic alkoxide group (–O), which attacks the ester bond-forming alcohol

and acyl–enzyme complex.

Oxidoreductases, on the other hand, add oxygen atoms to alcohol and peroxide

groups, which are easier to fragment and consume.

Two factors which affect the oxidation of plastic structure are

1. Length and exposure and type of additive used

2. Type of microbial species involved in the process

7.6.4

Assimilation

Biodegradation of plastic is an electron transfer process. The driving force for this

process is mainly channeled through the oxidation of molecules that were obtained

from chain fragmentation. Major products that are the result of enzymatic attack

are amides, alcohols, and organic acids. These compounds are easily assimilated