Research Progress in Microbial Degradation of Microplastics

The presence of microplastics is increasingly recognized as a major issue in environmental protection across the world, primarily as a result of their long-lasting nature and the potential harm they can inflict on ecosystems.Current methods for degrading microplastics include physical capture, chemical degradation and biological methods.Among them, microbial degradation has received much attention because it is economically feasible and environmentally friendly.This paper reviews the degrading microorganisms, microbial metabolites and microbial degradation mechanisms of three frequently utilized polymers, namely polyethylene, polystyrene and polyethylene terephthalate, and analyses the current problems of microbial degradation of microplastics, in order to provide ideas for the research on the development of microplastic-degrading enzyme preparations.


Introduction
Particles of plastic smaller than five millimeters in size are known as Microplastics(MPs) .Currently, MPs are readily accessible everywhere in the globe, affecting various environments such as rivers, lakes and farmland [1].The environment's buildup of MPs has negatively impacted water, soil bodies and living things [2].Due to the ubiquitous presence of MPs and their irreversible harm, MPs pollution has become a global issue, and the development of techniques to weaken MPs is urgently needed.
Currently, methods for degrading MPs include physical capture, chemical degradation and biological methods [3].Microbial degradation of MPs has attracted widespread attention and research as a solution that achieves good degradation effects without causing secondary pollution [4].Microbial degradation of MPs is a process that utilizes the active enzymes of microorganisms to decompose MPs.This method is environmentally friendly, ecologically friendly, and sustainable [5].The process of microbial degradation of MPs mainly consists of two steps: adsorption and degradation.Firstly, microorganisms adsorb MPs particles through secretion products or surface structures.Secondly, microorganisms secrete enzyme substances to break down MPs into smaller fragments, ultimately degrading them into harmless substances such as H2O and CO2 [6].Although microbial degradation of MPs has potential in addressing plastic pollution, it still faces some challenges.For instance, there is a wide variety of microplastic particles, and different bacteria are needed for the breakdown of different forms of MPs [7].Consequently, further research and development are needed to identify suitable bacterial strains and enzymes for degrading different types of microplastics.An overview of the various types of MPs' degradation processes as well as the breakdown mechanisms of biodegradable MPs are given in this article.A thorough grasp of the accomplishments obtained in the field of MPs microbial degradation research can better evaluate its role in managing MPs pollution and provide guidance for future research and applications.

Fungal degradation of PE
Polythene (PE) is the most extensively utilized plastic as a result of its hydrophobic qualities, intensified multidimensional framework, and large molecular weight.Polyethylene displays multi-purpose, light weight, low cost and easy processing properties.Currently, the fungi reported for degrading PE mainly include Aspergillus sp. and Penicillium sp..It is worth noting that Zalerion maritimum isolated from the ocean by Paco et al. showed a weight loss rate of up to 70% after 28 days of PE degradation [8].This article provides a summary of the reported fungal species and their degradation effects on PE (Table 1 [10].This article provides a summary of the reported fungal species and their degradation effects on PS (Table 2).

Fungal degradation of PET
Polyethylene terephthalate (PET) exists as a flawless, luminescent and fragile polymer amalgamated by the polycondensation of terephthalic acid (TPA) and ethylene glycol (EG) .Fabrics, limited-use juice containers, salad dressings, breath and oxygen fresheners, containers for drinks, meat-related product packaging and cosmetics are all common places to find it driven by its great resistance to temperature against short circuit and mechanical stability.Currently, the fungi reported for degrading PET mainly include Aspergillus sp. and Penicillium sp.. A. niger has been shown to convert calcium to magnesium PET, resulting in a 52.94% decrease in weight [12].This article provides a summary of the reported fungal species and their degradation effects on PET (Table 3). .This article provides a summary of the reported bacterial species and their degradation effects on PE (Table 4).[10].This article provides a summary of some reported bacterial species and their degradation effects on PS (Table 5).[21].This article provides a summary of some reported bacterial species and their degradation effects on PET (Table 6).

MPs-degrading Enzymes
Microbial degradation of MPs involves the role of various enzymes,including Lipases, Esterases, Laccases and Keratinases [23].Currently, the sources of enzymes capable of degrading microplastics are limited and the variety is scarce.

Degradation Mechanism
There are normally four steps to the biodegradation process (Figure 1) [26].(3) EG can be further involved in metabolic pathways as a precursor to the substrates of the TCA cycle.At the same time, EG can be converted to substances such as acetic acid and isocitrate.Different microorganisms may have different degradation abilities, and certain environmental conditions may promote microbial degradation.Shabbir et al. [28] demonstrated that to achieve efficient degradation, MPs' physical attributes must permit microbiological adhesion to their surfaces and the polymer structures (chemical bonds).Moreover, the bioreaction shouldn't be impacted by the level of branching and polymerization.The amorphous and crystalline sections of polymers, as well as layered thickness and crystal size, all affect the rates of enzymatic breakdown.

Conclusion
In conclusion, significant progress has been made in the research on microorganisms and their active metabolites for the degradation of microplastics.However, further research is needed to strengthen the following areas: (1) Selection of highly active strains from nature for the degradation of microplastics and conducting engineering research on the treatment of microplastic pollution.
(2) Leveraging interdisciplinary advantages to conduct research on synthesic enzyme for the difficult-to-degrade microplastics.
(3) Strengthening the research on detection methods during the process of microplastic degradation to improve research efficiency.

Figure 1 .
Figure 1.Microbial Degradation Mechanism of MPs.(1) Microbes adhere to the surface of MPs by secreting extracellular enzymes.(2)Secreted hydrolytic enzymes catalyze oxidative reactions, breaking the long chains of MPs into dimer, monomer and oligomer.(3)Dimer, monomer and oligomer are further absorbed by microbes for biodegradation, converting them into small molecular intermediates such as fatty acids that can be utilized by microbes.(4)Small molecular intermediates like fatty acids undergo further bio-metabolic reactions within microbial cells, ultimately decomposing into CO2, H2O, and inorganic salts.There have been few studies on the enzymatic mechanism of MPs, most of which have focused on PET enzymatic degradation.The PET enzymatic degradation mechanism is divided into three steps (Figure2)[27].

Figure 2 .
Figure 2. Enzymatic degradation mechanism of PET.(1) PETase breaks down PET into MHET and, as a byproduct, traces of BHET and TPA.(2)The secondary enzyme MHETase further transform MHET to EG and TPA.(3) EG can be further involved in metabolic pathways as a precursor to the substrates of the TCA cycle.At the same time, EG can be converted to substances such as acetic acid and isocitrate.Different microorganisms may have different degradation abilities, and certain environmental conditions may promote microbial degradation.Shabbir et al.[28] demonstrated that to achieve efficient degradation, MPs' physical attributes must permit microbiological adhesion to their surfaces and the polymer structures (chemical bonds).Moreover, the bioreaction shouldn't be impacted by the level of branching and polymerization.The amorphous and crystalline sections of polymers, as well as layered thickness and crystal size, all affect the rates of enzymatic breakdown.
2.1.2.Fungal degradation of PSPolystyrene (PS) is an artificial polymer made up of chains of styrene molecules.Packaging containers, cups, laboratory tools and the food industry are just some of the many places that see it put to use based on its massive dimensions, great hydrophobicity and inadequate responsiveness.Nevertheless, owing to its unique properties, PS is notoriously difficult to eradicate from natural systems.Currently, the fungi reported for degrading PS mainly include Mucor sp., Aspergillus sp. and Penicillium sp.Yanto et al. used Pestalotiopsis sp.(NG007) to degrade PS, and PS's weight reduction rate at 74.4% after 30 days

Table 2 .
PS degradation by fungi.

Table 3 .
PET degradation by fungi.
[15]1.Bacterial degradation of PECurrently, the bacteria reported for degrading PE mainly include Rhodococcus sp., Bacillus sp. and Pseudomonas sp.. Rhodococcus sp. was isolated by Nanda et al. from a waste treatment plant, and and found that it could degrade PE with a weight reduction rate of 33% after 21 days[15]

Table 4 .
PE degradation by viruses.Currently, the bacteria reported for degrading PS mainly include Streptomyces, Bacillus sp. and Pseudomonas sp.. Yanto et al. used Pseudomonas aeruginosa BLSP4 to degrade PS, and after 30 days of cultivation, PS saw a weight reduction rate of 63.4%

Table 5 .
PS degradation by viruses.
Currently, the bacteria reported for degrading PET mainly focus on Bacillus sp. and Ideonella sp.. Ideonella sakaiensis 201-F6 was isolated by Yoshida et al. from the microbial consortia "46" and found that it efficiently degraded low crystallinity PET (19%) within 42 days, almost completely.This bacterium is currently the shortest and most efficient in degrading PET

Table 6 .
PET degradation by viruses.

Table 7 .
Table 7 serves as a list of enzymes that catalyse the dehydration of different MPs.Different enzymes of degrading microplastics.