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The Role and Application of Microbial Enzymes in Microplastics’ Bioremediation: Available and Future Perspectives

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Abstract

Microplastic contamination is an increasing environmental concern worldwide, with microplastic particles found in various ecosystems, including soil. Because microplastics are non-biodegradable, they pose severe removal and disposal issues. The employment of biological organisms to break down or degrade toxins, known as bioremediation, has emerged as a remedy for microplastic pollution. Microbial enzymes play an important role in bioremediation by facilitating the breakdown and degradation of microplastics, as many plastic-degrading enzymes have been discovered and purified over the last decades. Thanks to the high-throughput newly investigated "omics" techniques and also efforts on enzyme engineering, this exploration has now been speeded up, but despite these advances, plastic and microplastic pollution issues are still among the unsolved environmental concerns. This chapter emphasizes the importance of microbial enzymes in the bioremediation of microplastics by exploring their diversity, methods of action, and prospective uses in tackling microplastic pollution and how far are we from using this approach to solving this problem. This understanding will pave the way for the creation of efficient and environmentally friendly ways to tackle the growing menace of microplastics.

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... Non-hydrolyzable polymers like PE and PP, however, are more resistant to microbial degradation due to their structural stability and absence of functional groups vulnerable to enzymatic attack (Giyahchi and Moghimi 2023). Some microbial communities with lignin-degrading enzymes may facilitate limited degradation of these polymers, but the process is slow and often incomplete in natural settings (Danso et al. 2019;Patrício Silva 2022). ...
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... The rate of plastic removal depends on the chemical structure of the plastic and the presence of suitable microbes. Until now, various studies have examined the structure of microplastics and the ability of microorganisms and their enzymes to degrade them (Giyahchi and Moghimi, 2023;Ahmed et al., 2022;Cai et al., 2023;Miri et al., 2022;Othman et al., 2021;Tareen et al., 2022;Vaksmaa et al., 2021;Wu et al., 2023;Yuan et al., 2020;. Moreover, ISO and ASTM standards provide various detailed guidelines for analyzing biodegradation in distinct environments (refer to ASTM D6400 & D6868, EN13432, ISO 17088, and ISO, 18606 for composting environment, ISO 17556 and ASTM D 5988 for soil, and ASTM D6691, D7991for marine environments) (Funabashi et al., 2009;Sawada, 1998;Zumstein et al., 2019). ...
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Low-density polyethylene (LDPE) has been used extensively leading to its over-accumulation in the environment. Recent studies have shifted their focus on devising techniques that could accelerate the biodegradation process such as formulation of microbial consortia and the use of nanoparticles. In this study, LDPE biodegradation was carried out using a bacterial consortium composed of the two most efficient LDPE-degrading alkaliphilic bacterial strains isolated from hyperalkaline spring (pH 11) in Zambales, Philippines through enrichment techniques. Phenotypic and molecular analyses revealed that the isolates were phylogenetically-affiliated to Bacillus pseudofirmus and Bacillus agaradhaerens. The supplementation of iron oxide nanoparticles (IONPs) significantly increased the bacterial growth, along with shortened lag phase and longer stationary phase. The bacterial consortium, in the presence and absence of IONPs, were able to reduce the weight of the residual polymer up to 18.3 0.3 % and 13.7  0.5 %, respectively after 60 days of incubation. Bacterial adhesion to hydrocarbon test demonstrated higher hydrophobicity of the consortium with IONPs which was corroborated by an increased protein content of the cells adhered on the films. End product analysis by Fourier transform infrared (FTIR) and scanning electron microscopy (SEM) revealed chemical bond shifting and pronounced disruption of surface texture in the presence of IONPs, respectively, thereby confirming the biodegradation process. The combination of the two isolates supplemented with IONPs exhibited maximum degradation of LDPE as revealed by various analyses. This study highlights the significance of bacteria-nanoparticle interactions, and the formulation of alkaliphilic bacterial consortium grown in the presence of nanoparticles in accelerating the rate of LDPE film degradation.
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Microplastics pretreatment of prior to biodegradation is an efficient approach for their bioremediation. We isolated Achromobacter denitrificans from compost and used it for biodegradation of thermo-oxidative pretreated polyvinyl chloride (PVC) and low-density polyethylene (LDPE). About 12.3 % and 6.5 % weight loss, and 326.4 and 112.32 mg L-1 extracellular protein were observed in bacterial flasks with PVC and LDPE, respectively. The pH in treated PVC reached to 5.12 and the thermal stability increased by 29 °C. The chemical modification in LDPE was demonstrated through oxidation of antioxidants (Phenol group), formation of new groups (Aldehyde group), and chain fracture in the main backbone by Fourier transform infrared spectroscopy. Formation of peaks at the range of 1700-1850 cm-1 in LDPE attributed to formation of carbonyl groups as the degradation result. Scanning electron microscopy confirmed LDPE and PVC degradation by surface alterations. Consequently, thermo-oxidative pretreatment can be considered as a suitable strategy for improving microplastics biodegradation.
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The worldwide rapidly increasing amount of plastic waste is considered a major concern in the environmental crisis today. Recycling post-consumer plastics is believed to be a viable approach to mitigate the global plastic waste challenge. Plastic recycling is considered attractive due to the reason that it could help achieve waste valorization while meeting the goals of environmental quality standards. To this end, the current research efforts are heavily focused towards developing innovative technologies for new enzyme discoveries for effective plastic waste management. Recent research has shown biocatalytic depolymerization mediated by enzymes as an efficient and sustainable alternative for plastic treatment and recycling. Using this approach, researchers have discovered different plastic-degrading enzymes that have been derived from microbial sources. Further, the concept of protein engineering has been implemented to modify and optimize plastic-degrading enzymes. In this brief opinion, we describe some of the recent trends and notable advances in mining novel plastic-degrading enzymes through cutting-edge omics-based techniques in order to improve the enzyme catalytic efficiency.
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Nowadays, a growing number of microplastics are released into the environment due to the extensive use and inappropriate management of plastic products. With the increasing body of evidence about the pollution and hazards of microplastics, microplastics have drawn major attention from governments and the scientific community. As a kind of emerging and persistent environmental pollutants, microplastics have recently been detected on a variety of substrates in the world. Therefore, this paper reviews the recent progress in identifying the sources of microplastics in soil, water, and atmosphere and describing the transport and fate of microplastics in the terrestrial, aquatic and atmospheric ecosystems for revealing the circulation of microplastics in the ecosystem. In addition, considering the persistence of microplastics, this study elucidates the interactions of microplastics with other pollutants in the environment (i.e., organic pollutants, heavy metals) with emphasis on toxicity and accumulation, providing a novel insight into the ecological risks of microplastics in the environment. The negative impacts of microplastics on organisms and environmental health are also reviewed to reveal the environmental hazards of microplastics. The knowledge gaps and key research priorities of microplastics are identified to better understand and mitigate the environmental risks of microplastics.
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Microplastics contamination is becoming a major concern worldwide. More than 1 million seabirds and 100,000 sea animals have died due to plastic contamination. In addition, plastic particles have been found in juvenile turtles. Statistical data on plastic pollution indicate that this is a serious issue. Due to their small size, microplastics have a large surface area and have more ability to absorb into biological cells. The hydrophobic surface of microplastics attracts co-contaminants such as heavy metals, pharmaceutical toxicants, flame retardants, and other plasticizers, which can then enter biological organisms. Microplastics are usually recalcitrant in the environment, causing microplastics to be transported along the food chain, with humans as the final consumer. Research has been conducted to evaluate the best way to treat and remediate microplastic pollution. Research on microplastic degradation is focused on biological and non-biological approaches. To date, microorganisms such as algae, fungi, and bacteria have attracted the attention of scientists as a tool for microplastic treatment. The degradation of microplastics is closely related to the enzymatic reactions produced by the microorganisms. Here we review microplastics degradation through enzymes from the microorganism’s perspective. We present the enzymes that have been isolated from microorganisms for specific microplastics; the mechanisms of microplastics degradation by various enzymes; and the types of microplastics for which degradation mechanisms remain unclear.
Chapter
Plastic pollution has become a serious issue on Earth. Although efficient industrial recycling processes exist, a significant fraction of plastic waste still ends up in nature, where it can endure for centuries. Slow mechanical and chemical decay lead to the formation of micro- and nanoplastics, which are washed from land into rivers and finally end up in the oceans. As such particles cannot be efficiently removed from the environment, biological degradation mechanisms are highly desirable. Several enzymes have been described that are capable of degrading certain plastic materials such as polyethylene terephthalate (PET). Such enzymes have a huge potential for future biotechnology applications. However, they require model systems that can be efficiently adapted to very specific conditions. Here, we present detailed instructions, how to convert the model diatom Phaeodactylum into a solar-fueled microbial cell factory for PETase expression, resulting in a whole cell catalyst for PET degradation at moderate temperatures under saltwater conditions.
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The Front Cover shows the historical progression of human use of enzymes. In the distance, representing the distant past, ancient Egypt that was already manufacturing beer using (unknowingly) microbes. Closer to the present, the discovery and description of microorganisms, followed relatively quickly by the identification and investigation of enzymes. Finally, in the present, a very resourceful enzyme (represented by the Si−C bond forming cyt C mutant; PDB: 6cun) is throwing increasingly complex molecules (an organosilicon compound and islatravir) at the reader to represent the powerful chemical tools that biocatalysts have become. In their Review, C. M. Heckmann and F. Paradisi highlight the developments across several fields that were necessary to create the modern field of biocatalysis. More information can be found in the Review by C. M. Heckmann and F. Paradisi.
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Microplastics could act as a carrier for pesticides in the water environment and pose a potential risk. This study mainly investigated the effects of reaction time, microplastics dosages, pH, and NaCl salinity on the adsorption characteristics of three pesticides (Imidacloprid, Buprofezin, Difenoconazole) on polyethylene (PE) microplastics in aqueous solution. The results showed that high pH and low NaCl salinity were conducive to the adsorption. The adsorption data were well fitted by the Freundlich isotherm model and the pseudo-first-order kinetics, indicating that it was mainly controlled by physical function. The adsorption capacity of three pesticides on PE microplastics followed the order of Difenoconazole > Buprofezin > Imidacloprid. The thermodynamic study indicated the adsorption of all pesticides as spontaneous and exothermic processes, and the elevated temperature was favorable to the adsorption. SEM-EDS and FTIR results verified that pesticides were adsorbed on the microplastics but the adsorption process was mainly controlled by intermolecular Van Der Waals Force and the microporous filling mechanism. Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulation results indicated that surface adsorption was the exclusive mechanism for the adsorption of pesticides on microplastics, and the final adsorption configurations revealed that there were complex interactions between the pesticide molecules and the C, H atoms in PE molecules. The results of this study illustrated that PE microplastics are potential carriers for pesticides in the water environment.