**6. The role of microbes in removal of micro-plastic from marine ecosystem**

The environmental problems caused by microplastics in the marine ecosystem are continuously growing [82]. The most common microplastics, also known as synthetic polymers, that are found in the marine ecosystem include PE, PA, PP, PS, PES, AC, PU, HDPP, LDPP, PI, PMMA, PFE, PVC and PVDC [4]. Nevertheless, many conventional plastics, such as PE, PP, PS, PVC, and PET, are not biodegradable, and their increasing accumulation in the ecosystem has posed a danger to the environment [83]. To contend with this man-made challenge, contemporary wastewater treatment facilities need to necessitate fresh technologies [84]. Modern technology provides methods for limiting the availability of microplastics in an aquatic environment. However, such technologies seem to be either inadequate or prohibitively expensive, in addition to being timeconsuming in both circumstances. Despite the fact that several microplastic products are considered structural pollutants that do not readily biodegradable or deteriorate at an extremely slow rate, microbial degradation is still a prevalent remediation technique because it is inexpensive and environmentally friendly nature [84, 85].

Microbial degradation can be achieved by using single or connected bio-cultures, including bacteria, algae, and fungi, which have been demonstrated to consume these polymeric materials and generate them into environmentally sustainable carbon compounds. In essence, no microbial techniques can eliminate microplastics from the ecosystem entirely and in an acceptable amount of time [84]. According to research, saturated synthetic polymer chains do not favor microbe degradation, whereas biodegradable polymers incorporate heteroatoms inside the hydrocarbon chains and hence degrade quickly when exposed to favorable weather conditions [86].

The removal rate of microplastic is determined by its creation and the circumstances under which it is exposed, which can range from abiotic factors (wind, waves, heat, and humidity) to microorganism assimilation, such as bacteria, algae, and fungi [87]. As a result, polymer degradation can be categorized as either abiotic or biotic [88]. Abiotic degradation refers to decomposition characterized by factors in the environment, such as temperature, UV irradiation, wind, and waves. Biotic degradation, on the other hand, is defined as the degradation process triggered by the actions of microorganisms that transform and ingest the polymer, modifying its qualities [89].

#### **6.1 Mechanism of biodegradation of microplastics by microbes**

The adherence of the microorganism to the surface of the polymer, preceded by the colonization of the external surface, growth of the microbial, use of the polymer as a source of carbon and energy, and final degradation of the polymer is the primary mechanism for microbial degradation [88, 90]. Microorganisms can stick to the surface of a polymer if it is hydrophilic. Once anchored to the surface, the organism can grow by utilizing the polymer as a source of carbon and energy. Polymer biodegradation happens by hydrolysis after colonization; first, the enzyme catalyzes the substrate material and then facilitates the hydrolysis reaction. Polymers degrade into small molecular weight oligomers, dimers, and monomers before finally mineralization to CO2 and H2O [83]. The surface composition can quantify the scope of colonization on the polymer, as hydrophilic areas are much more conveniently colonized by microbes. This is a restriction since the polymer's water-repellent surface contradicts the porous structure of the microorganisms (**Figure 5**) [89].

#### **6.2 Biotic degradation**

Microplastic biodegrades as a consequence of degradation by microbes in the marine environment. However, because of their size, macroplastics (larger plastic debris) do not make the optimum source of nutrients for biotic degrading agents;

*The Risks of Microplastic Pollution in the Aquatic Ecosystem DOI: http://dx.doi.org/10.5772/intechopen.108717*

#### **Figure 5.** *Mechanism of microbial degradation of microplastic [91].*

either the enzymes secreted by the microbes are insufficient to denature the macroplastics, or they contain not easily and quickly biodegradable for biological cell uptake [29]. Synthetic polymer plastics must first be changed into carbon molecules prior to being mineralized by microbial pathogens during the degradation reaction. Plastics' (polymers') organic molecules' size is bigger than the particle sizes of a microorganism's cellular membrane. As a result, they must be metabolized into tiny pieces before being assimilated and biodegraded within microbial cells. As a result, finer particles of plastic created as the result of environmental factors degradation are of sufficient size to be broken down even more by microbial cells [92]. Bacteria, fungi, and algae are the most common microorganisms found in marine ecosystems.

Microbial enzymes are responsible for biotic degradation. Chemical compounds are converted into simplified chemical compounds, metabolized, and deposited in primary-level cycles, such as carbon, nitrogen, and sulfur through microbial degradation. Carbon dioxide, methane, and microbial extracellular matrix components are among the by-products of this system [93, 94]. Microbial character traits, such as microbe form, propagation, developmental stage (temperature, pH, availability of oxygen, essential minerals, etc.), and enzymatic categories (intracellular and/or extracellular enzymes contributing to exo or endo polymer cleaving). Surface conditions (size, water-soluble, and hydrophilicity properties), first-order frameworks (chemical composition, molecular mass, and molecular dissemination), and relatively high structures (thermodynamic stability, melting temperature, fracture toughness, crystalline structure, and degree of crystallinity) are among the chemical and physical properties of polymers [83].

#### *6.2.1 Biodegradation by bacteria*

Many bacteria genera that are commonly found in the marine environment like Bacillus species (e.g., Bacillus subtilis and Bacillus cereus, and Bacillus megaterium), Brevibacillus, Streptomyces, Amycolatopsis, Clostridium, Methanosarcina barkei, Schlegelella, Pseudomonas aeruginosa, Azotobacter spp., Alcanivorax, Hyphomonas, and Cycloclasticus species, Rhodococcus ruber, Serratia marcescens, Staphylococcus aureus, and Streptococcus pyogenes, and other bacterial strains also lead to the microbial degradation of plastics [29, 95–102]. The Bacillus species were discovered to secrete extracellular hydrolytic enzymes, such as lipase, xylanase, keratinase, chitinase, and protease, which resulted in the biodegradation of microplastics [103]. Methanosarcina barkei bacteria strain can degrade the most commonly used plastic polymer, PVC. They can stick to the surface of PVC surfaces and discharge exopolymeric compounds to produce a biofilm, preceded by the discharge of enzymes to breakdown the plastic through enzymatic hydrolysis of the synthetic polymer bonds which resulted in the biodegradation of PVC [104, 105]. Likewise, Rhodococcus ruber will also degrade PE by producing an enzyme laccase, which ultimately resulted in PE degradation [106]. Azotobacter spp., which releases hydroquinone peroxidase, could also degrade PS. PET can also be degraded by Alcanivorax, Hyphomonas, and Cycloclasticus species, which could also alter the physiochemical properties through the use of ester bond hydrolysis [107].

#### *6.2.2 Biodegradation by fungi*

Many fungal genera, such as Acremonium, Zalerion maritimum, o Curvularia sp., Cladosporium, Debaryomyces, Emericellopsis, Eupenicillium, Fusarium, Mucor, Paecilomyces, Pullularia, Rhodosporidium, Verticillium, Aspergillus sp., Aureobasidium, Chaetomium, Cryptococcus, Fusarium, Rhizopus arrhizus, Trichoderma, Penicillium sp., Thermoascus, Tritirachium album, Humicola insolens, Rhodotorula aurantiaca, and Kluyveromyces sp. [83, 108–112] also contribute to the microbial degradation of plastics. It has been demonstrated that Aspergillus clavatus can biodegrade LDPE [113]. Zalerion maritimum, the ocean's dominant fungal species, could also degrade PE [114]. The main mechanism of plastic degradation by fungi, such as bacteria, involves fungi adhering to the polymer surface, in which they grow to create a biofilm and produce enzymes that degrade the carbon-carbon bonds occurring in the plastic. The above enzymes have the potential to accelerate the oxidation process as well as degrade plastic into tiny pieces (e.g., oligomers, dimers, and monomers). For example, fungi found in marine habitats, such as Penicillium citrinum and Fusarium oxysporum, breakdown PET, and Trichoderma harzianum release manganese peroxidase, lignin peroxidase, and laccase that breakdown PE and PU [114].

#### *6.2.3 Biodegradation by algae*

Algae are frequently used throughout tested microorganisms for investigating the harmful effects of microplastics. However, various algae, both photorespiration and heterotrophic, have been extensively researched for their key responsibilities in the microbial degradation of microplastics [84, 85]. They are capable of removing both inorganic and organic contaminants from a diverse range of environments by soaking up, removing impurities, or metabolizing them into healthy and safe levels [115, 116]. *The Risks of Microplastic Pollution in the Aquatic Ecosystem DOI: http://dx.doi.org/10.5772/intechopen.108717*

They colonize the outer layer of microplastics by secreting extracellular polymeric compounds, and this colonization could well result in effectual deterioration. The existence of polymeric materials, as well as plastic wastes, encourages the generation of extracellular polymeric compounds [117]. Several algal species are effective at microbial degradation of microplastics. These include Phormidium lucidum, Oscillatoria subbrevis, Scenedesmus dimorphus, diatom Navicula pupula, Chlorella, Spirogyra, Nostoc, Spirulina sp., Anabaena spiroides, and Navicula pupula [118–120]. Bioactive compounds produced by some algae have been found to biodegrade microplastics. Phormidium lucidum and Oscillatoria subbrevis, for example, can break down easily PE and LDPE [121]. Discostella spp., Navicula spp., Amphora spp., and Fragilaria spp. algal biofilms have been discovered to deplete LDPE, PP, and PET in the marine ecosystem [122]. After forming a biofilm on the plastic surface, algae use the carbon available on the plastic as a feed ingredient, softening and lessening the plastic. Furthermore, species can produce extracellular polymeric compounds and enzymes, such as PETase, which degrade PET [123]. Plastic degradation by algae remains in its early stages and requires more research.

### **7. Conclusion and recommendation**

#### **7.1 Conclusion**

Plastic pollution in the marine ecosystem is a growing concern due to the negative effects it has on aquatic habitats. Microplastic pollution has become a serious global issue that has a detrimental effect on the food chain in the marine ecosystem. The main sources of microplastic pollution in the marine ecosystem have been identified to result from general littering, plastic waste mismanagement, fishing gears, synthetic textiles, marine coatings, personal care products, plastic pellets, city dust, and release of wastewater from sewage treatment plants. This is the outcome of indiscriminate waste dumping, which is either directly or indirectly transmitted to our seas and oceans. Because microplastics are the same size as prey and are mistaken for food, they pose a threat to many marine organisms. When swallowed, it has a negative impact on marine organisms, facilitating the transmission of artificial chemicals or hydrophobic watery toxins to aquatic life. Microplastic pollution has contaminated various drinking sources, salt water, and other regularly consumed foods. Chemical toxication, indigestibility, choking of marine ecosystems, and a pathway for microbial propagation are all negative effects of microplastic contamination on the marine environment. Furthermore, the effects of microplastic pollution vary from the molecular level of an organism to its physiological mechanisms and include bad organism health and poor economic services. These threats increase the risk to aquatic fish's and human survival. Significant awareness about the harmful effects of microplastics has prompted some regions of the world, including the United Kingdom, the United States, and Canada, to take action. These initiatives have focused almost entirely on prohibiting the use of microbeads in various items, such as personal care and skincare products.

#### **7.2 Recommendation**

Microplastics have been found to be consumed by a variety of marine organisms in laboratory and field research. More research is needed to determine whether

microplastic consumption alone causes unfavorable health impacts, such as mortality, morbidity, and reproductive success, or whether such a contaminant can be consistently transferred up the food chain in the marine ecosystem. Toxic chemical transfer to biota via microplastic intake is a major concern. However, just a few studies have reported on toxicity investigations, including microplastic vectors. More quantitative research should be conducted to investigate the toxins [toxic chemicals] transfer of microplastics to marine species, as well as any possible dangers of transfer from consumable marine organisms to people.

The most pressing need in this subject is to raise public understanding about the inert impacts of microplastics. This would encourage numerous inventions aimed at reducing the use and consumption of plastic and its byproducts. The most essential way to reduce plastic entry into the ecosystem is to gather and reuse plastic particles. To avert future threats, the best answer is to discontinue production and seek alternatives to plastic items.
