Microbial Degradation Mechanism of Microplastics

Xueqin Wang

Chongqing College of Architecture and Technology, China

Keywords:

Microplastics, Microorganism, Microplastic Degradation Mechanism, Insect.

Abstract:

Microplastics refer to plastic particles with less than a diameter of 5mm and are a widely distributed, persistent

pollutant with environmental toxicity and biotoxicity. Microbial degradation of microplastics is an

environmentally friendly treatment technique without secondary pollution. On the basis of exploring the

degradation mechanism of microplastics by animals and microbial degradation mechanism of microplastics,

this paper analyzes the problems in the existence of microplastics degradation research, and puts forward the

research of microplastics degradation in the future, so as to provide a basis for the deep degradation research

of microplastics.

1 INTRODUCTION

Microbial degradation of microplastics mainly is the

use of microbial feeding, metabolism and other

processes to convert plastics and other organic waste

into water and carbon dioxide and other end products

into the geochemical cycle. Microbial degradation of

plastics is an environmentally friendly, no secondary

pollution, and relatively economical treatment

technology. At the same time, microbial genes are

heritable and adaptable, microbial genome is small,

and there are a large number of suitable molecular

operation means and tools can be used, which on the

one hand is conducive to the study of metabolic

mechanism, also to improve the efficiency of

microbial degradation or metabolic path, achieve the

actual production application demand. Therefore, the

biodegradation of microplastics has been widely

concerned by scholars. At present, on the

biodegradation of microplastics, one part studies the

rodent degradation effect of animals (mainly insects

and soil animals) on microplastics, and the other part

focuses on the study of microbial degradation of

microplastics, including some bacteria, fungi and

actinomycetes.

Wang Xueqin (1984-), female, associate professor,

master

2 MECHANISM OF

MICROPLASTICS

DEGRADATION BY ANIMAL

The degradation of plastic by insects or soil animals

mainly depends on the role of intestinal microbial

populations and enzymes with biodegradable plastic

in their intestines (Giacomucci, 2019). Different

insects have different ability to degrade plastics, and

different insects degrade different plastics. However,

the degradation efficiency of microplastics isolated

from the intestines of these animals in vitro is much

lower than that in the gut of (Yang, 2015), which may

mean that the degradation of plastics in insect (or soil

animal) intestines is the result of a combination of

biological-chemical reaction mechanisms.

The plastic degradation process by animals can be

divided into the following four steps:

1) Animals use mouthparts to chew plastic film

and grind in the foregut. This is mainly a physical

process.

2) Gut microorganisms in insect’s or soil animal’s

gut adhere to and erode plastic fragments. Bacillus,

Corynebacterium, microptococcus and streptococcus

erode the plastic and change its physical and chemical

properties in preparation for further degradation.

3) Plastic fragments are bioenzymatic or

enzymatically hydrolysed into oligomer fragments.

392

Wang, X.

Microbial Degradation Mechanism of Microplastics.

DOI: 10.5220/0012021500003633

In Proceedings of the 4th International Conference on Biotechnology and Biomedicine (ICBB 2022), pages 392-395

ISBN: 978-989-758-637-8

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

Key enzymes such as lipase and mitochondrial

carnitine acetyltransferase in insects or soil animals

cause-C-C-breakage of plastic and eventually

degrade plastic membranes.

4) After being degraded into oligomers by the

above three processes, plastics are further degraded

and metabolized, and they are eventually absorbed or

excreted by gut microbes or insect bodies.

Figure 1 summarizes the mechanism of plastic

degradation by microorganisms and key enzymes in

the gut of insects or soil animals.

Figure 1: Mechanism of plastic degradation in insect’s or soil animal’s gut.

As can be seen from Figure 1, the degradation

mechanism of plastic by insects or soil animals is

quite complicated. The degradation process of plastic

is the result of the chewing and crushing of plastic by

insects or soil animals, and the combined action of

enzymes, special secretions and microorganisms on

plastic particles in the insect gut.

3 MECHANISM OF

MICROBIOLOGICAL

DEGRADATION BY

MICROPLASTICS

In addition to animal gut microorganisms can

degrade plastic, microorganisms in the natural

environment or some special environment have the

ability to degrade plastic, and some bacteria can use

plastic as the sole carbon source.

In the process of plastic degradation,

microorganisms often secrete extracellular enzymes

to promote the degradation of plastic, which is the

most core active material in the process of plastic

degradation (Albertsson, 1998). For example,

trichomonas testosterone Comamonas testosteroni is

capable of producing different enzymes on the broken

link of ester bonds and the open ring of the benzene

ring, monooxygenase in microbial cells oxidizes PE

to hydroxyl group, which is further oxidized by

alcohol dehydrogenase, which can be further oxidized

to ester group, which further hydrolysis into

carboxylate low molecule products catalyzed by

esterase, and finally the product enters the central

metabolic cycle in the cell (Wasserbauer, 1990). PET

enzymes in the strain first hydrolyze PET into

ethylene glycol terebenzoate monomer (MHET) and

terephthalic acid (TPA) (Yoshida, 2016).

Subsequently, the enzyme speculated as lipoprotein

MHET enzyme hydrolyzes MHET into terephthalic

Step 1

Insects eat plastic

The degradation process of plastic in the insect gut

Microbial Degradation Mechanism of Microplastics

393

acid and ethylene glycol (EG). TPA is metabolically

oxidized by 1,2-dioxygenase, 1,2-dioxygenase, 1,2-

dihydroxy-3,5-cyclohexadiene-1,4-diethyl acetate

dehydrogenase after being transported to a specific

unit. The benzene ring on the end product, primary

catechin acid (PCA), is opened by PCA3,4-

dioxygenase.Figure 2 summarizes the metabolic

pathwayfor microbial degradation of PE and PET

Figure 2: Putative metabolic pathway of PE and PET by microbes.

To sum up, the microbial degradation of

microplastics is mainly divided into four steps.

1) Bio-deterioration. The microbial community

and abiotic factors work together to intercept the

polymer (plastic) into fragments.

2) Depolymerization. Microorganisms secrete

enzymes and free radicals. Transform the polymer

into an oligomer, dimer, or monomer. This process is

mainly a process in which hyperomers are cleaved or

oxidized to produce low molecular intermediates.

3) Assimilation. The depolymerized molecules

are recognized by the receptors on the microbial

surface to pass through the plasma membrane and

enter the microbial cell. This process is when low

Alcohol

Ketone

Ester

Esterase

Carboxylic acid

Alcohol

TPA, EG

MHET

PCA

PET

Pca34

ICBB 2022 - International Conference on Biotechnology and Biomedicine

394

molecular products are taken up by microorganisms

and assimilated into their own cellular material.

4) Mineralization. The molecules are

depolymerized to small molecule compounds such as

CO2, N2, CH4 and H2O. The depolymerization

material is degraded by microorganisms into

methane, water and so on under aerobic conditions.

4 CONCLUSION

So far, although it has been confirmed that a variety

of plastics can be degraded by microorganisms, and

the microorganisms or biological enzymes that

degrade plastics have been found, but the current

research on the microbial degradation of plastics still

has the following problems.

1) At present, it is found that the degradation

efficiency of plastics by microorganisms is very slow.

So the means of optimizing microbial degradation

should be further explored, and the microbial

degradation pathway of high-efficiency plastics

should be deeply studied.

2) At present, there are few kinds of strains with

the ability to degrade plastics, so it is urgent to find

the microbial and enzyme systems that effectively

degrade plastics and enrich the resource pool of

degraded strains.

3) The microbial degradation mechanism of

microplastics is not clear, and is only a preliminary

understanding of the degradation process. The

research on the enzymatic, chemical and energy

metabolism of the microplastic degradation process

should be strengthened, the key enzymes and their

mechanisms should be determined, and the gene

modification research of the key enzymes for

degradation should improve their activity and yield.

4) Some microorganisms in the gut of animals

have the ability to degrade microplastics in animals,

but not in the in vitro environment. In future studies,

the degradation level can be improved by the

modification of known degrading bacteria or the

optimization of experimental conditions.

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