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7 Bioremediation of Plastics and Polythene in Marine Water

by microorganisms through the cell membrane and enter the central metabolic

pathway.

The electrons released from the substrate are finally consumed by a terminal elec-

tron acceptor, which in the case of aerobic microbes, is oxygen, and for anaerobic

microorganisms is nitrates and sulfates. For traveling down various metabolic routes

to terminal electron acceptors, electrons gain energy from oxidation via β-oxidation.

Degradation of plastic is generally a surface phenomenon where the oxidative and

hydrolytic enzymes act on to eject out electrons and other simpler sources of carbon,

which can be assimilated by microorganisms into their metabolic pathway, where

they contribute toward the growth.

7.6.5

Mineralization

The conversion of all complex forms of polymer moieties into simpler molecules

such as carbon dioxide, water, and oxygen, etc., constitute mineralization. This is the

final step in the degradation of plastic, and the final product obtained is primarily

the microbial biomass.

7.7

Biotechnology in Plastic Bioremediation

Biotechnology has been a boon to the field of biological and environmental science.

Its use in the field of bioremediation of plastic has led to various outcomes which

have benefitted the environment. The solution based on biotechnology may either be

stand-alone, or they may complement the existing technologies. The term “biodegra-

dation” presumes a nearly stand-alone method, but in nature, both abiotic and biotic

factors contribute equally to complete degradation of the polymer under consid-

eration. Moreover, abiotic degradation processes occur much before the microbial

attack; hence, abiotic factors largely determine how the plastic will be biodegraded.

The main drawback of biodegradation of plastic is that it takes a longer time for an

initial attack on the polymer chain. This can be overcome either by pretreating the

polymer making it more susceptible to microbial attack or genetic modification of

organisms to enhance its inherent capability of biodegradation. The pretreatment

of the polymer may pose various problems, which are huge capital investment, the

involvement of hazardous chemicals, which pose an environmental risk.

Genetic engineering makes it possible to enhance and alter existing properties of

the degradative enzymes, to modify and cluster multiple genes coding for enzymes

into a single organism. These newer genes hence will produce proteins that will not

only be genetically diverse but also be functionally rich and ultimately give us a

pool of novel biocatalysts. For example, biosynthetic genes phbA (for 3-ketothiolase),

phbB (NADPH-dependent acetyl Co-A reductase), and phbC (PHB synthase) have

been cloned to produce PHA (polyhydroxy alkanoic acid) and PHB (poly(3-hydroxy

butyric acid)). These genes are clustered in a single operon and have been expressed

in Escherichia coli and Pseudomonas sp. [36].