Keywords

6.1 Introduction

Plastic goods are exceptionally durable, lightweight, economical, and adaptable in nature, which allows them to replace many conventional materials such as paper, metal, glass, and wood. Polymeric molecules with extended chains, consisting of carbon, silicon, hydrogen, oxygen, and chloride atoms, are harnessed in the production of plastics (Rojas-Parrales et al. 2018). Presently, the most prevalent polymers employed in various applications include polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), and polyethylene terephthalate (PET) (Andler et al. 2022). Overall, plastics have become a key commodity in our society, akin to electricity or the Internet. However, in addition to all of the benefits, these low-cost materials have fostered a “use and toss” consumer culture, resulting in several environmental challenges (Naqash et al. 2020). Despite the increasing recognition, a staggering quantity exceeding 300 million tonnes of plastic waste is produced annually, with the majority being left unrecycled following disposal. Plastic debris has now been included among the roster of global perils, alongside climate change, ocean acidification, and ozone depletion (Gallowaya and Lewisa 2016). Plastic pollution is reported in almost every environment on the planet, regardless of its origin. Water systems including rivers, lakes, and oceans are acting as “plastic collectors” from adjoining terrestrial systems/watersheds (Yang et al. 2020). Plastic marine debris has accumulated in the world’s oceans as a result of improper plastic disposal. Their primary land-based entry points into the marine environment are accidental loss or improper handling of plastics, dumping plastic waste, and poorly managed landfills (Phelan et al. 2020).

Microplastic (MP) pollution, or plastic particles smaller than 5 mm, is particularly noteworthy. These tiny plastic particles emerge from various sources (Kasmuri et al. 2022). Primary MPs are formed deliberately and utilized in personal care items like toothpaste, cosmetics, etc., providing a direct supply of microplastic. Secondary MPs are formed when plastic debris is broken down by several abiotic factors including erosion, weathering, sunlight, and aquatic immersion (Yang et al. 2020). The extensive utilization of polymer-based items and their enduring resistance to physical and biological deterioration over extended periods is evident in the ubiquitous presence of MP across the globe, found in both surface water and sediment within marine ecosystems (Zalasiewicz et al. 2016). Given the substantial influx of plastic waste into marine ecosystems, the prevalence of microplastics in these environments comes as no surprise. In fact, specific regions with elevated concentrations of MPs, known as MP hotspots, have been identified in ten distinct estuaries located in North West England (Onyena et al. 2022). Microplastics were found in more than 90% of samples collected from 25 beaches along Hong Kong’s coastline (Fok and Cheung 2015). Furthermore, numerous “microplastic hotspots” with a maximum microplastic concentration of about 517,000 particles m−2 were found in North West England (Hurley et al. 2018). Additionally, highly noticeable microplastic pollution has been observed in the coastal waters of Africa (Alimi et al. 2021; Oni et al. 2020).

Plastic particles can adsorb organic material and nutrients from their environment while also acting as inert surfaces in nutrient-depleted water bodies. This ability makes MPs essential substrates for the development of microbial biofilms on these synthetic particles (Shen et al. 2019). As a result, MP biofilms may be considered a new microbiological niche in the surroundings, particularly in pelagic waters (Arias-Andres et al. 2018). Earlier, the diversity and function of “plastisphere” microbial communities and further the future of plastics in the marine environment have been discussed. The significant differences, similarities, and influencing variables of MP-associated biofilm communities in freshwater and marine environments have also been outlined in past studies (Harrison et al. 2018). In this chapter, initially we compile the diversity and composition of the MP-associated microbial communities and their molecular and microscopic characterization. Secondly, we discuss the different factors that are affecting the development of MP-associated biofilms. Thirdly, we have summarized the MP degradation in marine environment, highlighting the necessity of more research on biodegradable plastic. Furthermore, the potential role of enzyme driven degradation linked to microplastics has been discussed in detail along with addressing the future challenges. The primary goal of this chapter is to discuss an overview of how microbes form on MPs, as well as the community structure, functions, and environmental functions of these biofilms in marine systems. We emphasize the role of MPs in pelagic systems as a recent, developing microbial niche with uncertain impact on environmental and biological changes, as well as biogeochemical cycling in water resources.

6.2 The Plastisphere: Marine Microbial Colonization

The increasing prevalence of MP contamination in marine systems throughout the world has clearly declared it a severe environmental issue. Numerous bacterial and fungal strains have been shown to be capable of degrading various forms of plastic (Krueger et al. 2015). MP biofilm populations, however, differ dramatically from bacterial communities on natural surfaces such as wood pellets, organic matter, and glass beads (Oberbeckmann et al. 2018; Ogonowski et al. 2018). Even previous innovative research investigations on marine MP-related biofilms indicated that these communities differed greatly from those in the surrounding water (Oberbeckmann and Labrenz 2020). It is anticipated that sessile organisms lead distinct lifestyles compared to free-living microorganisms, leading to variations in microbial community compositions between natural particles and free-living microorganisms. Consequently, unless the existence of a genuine microplastic-associated plastisphere, distinct from those associated with natural particles like wood, cellulose, or glass, is verified, the role of biofilms on microplastics will remain uncertain (Rieck et al. 2015). Certain environmental and biogeographical parameters, like salinity and nutrient content, have a significant influence on the production of MP biofilms in marine environments (Oberbeckmann and Labrenz 2020). The microplastic surfaces themselves have an impact on colonization processes, although a minor one. It has also been argued if some overrepresented and perhaps hydrocarbon clastic marine microplastic biofilms may use plastics as a source of energy because of their ability to decompose extremely complex biopolymers such as lignin and petroleum-based goods (Ogonowski et al. 2018). Oberbeckmann et al. (2016) discovered that, rather than the surface properties of the plastics, the microbial communities that colonized polyethylene terephthalate (PET) after weeks of exposure in the North Sea were predominantly driven by spatial and seasonal parameters (Oberbeckmann et al. 2016). Another study revealed that nutrients such as total phosphorus and total nitrogen, as well as salinity, influenced the average development rate of biofilms on plastic trash, with salinity being the most critical factor impacting bacterial colony diversity (Li et al. 2019). Furthermore, it was discovered that plastic colour influences the community structure and functional diversity of the plastisphere. However, several research found no effect of the sample site, environment, or polymer type on the microbial communities in plastispheres (Wen et al. 2020). Ecologists have been working hard to understand the processes that define microbial communities in plastisphere biofilms as they develop in situ. Understanding the relative role of deterministic and stochastic mechanisms in microplastic bacterial assembly dynamics in natural water settings is critical. Despite the fact that both stochastic and deterministic factors influence community assembly in a variety of environments such as the human body (Venkataraman et al. 2015), soils, and oceans (Hamdan et al. 2012), the mechanisms of plastisphere microbial community assembly remain unknown. The proportional contributions of stochastic and deterministic processes, especially the functions of basic ecological processes such as dispersion, selection, drift, and diversification that influence bacterial distribution patterns across plastisphere niches, are mainly understood.

6.3 Microplastic-Associated Biofilms

Microbes, notably bacteria, diatoms, and algae, may adhere to MPs in the aquatic environment and quickly colonize the surface or even the insides of MPs to create biofilms (Harrison et al. 2014). MPs’ hydrophobic surfaces may provide an appropriate habitat for aquatic microbial populations to grow and build biofilms (Sun et al. 2020). Furthermore, MPs can serve as a separate and novel home for aquatic microbial communities since the taxonomic makeup of microbial communities on MP surfaces differs from that of microbial communities in surrounding water, whether in free living or particle attached forms (Yang et al. 2020; Deng et al. 2021). Furthermore, due to differences in roughness and surface morphology among different types of MP surfaces, the types of microorganisms present in the MP-related biofilm may vary with the type of MP (Miao et al. 2021; Qiang et al. 2021). The formation of microbial coatings on MPs might alter their density and convey them from the surface to deeper areas of water bodies (Cózar et al. 2014; Harrison et al. 2014). Furthermore, MP-associated biofilms have the potential to influence the weathering process and the physiological aspects of MPs (Deng et al. 2021). Furthermore, MP-associated biofilms may influence MP aggregation and, as a result, their subsequent deposition or suspension (Lagarde et al. 2016). MP-associated biofilms can also serve as vectors for pathogenic bacteria in fish, animals, and humans, as well as aid in the transfer of biotoxins generated by certain toxic algae (Harrison et al. 2018). In addition to pathogenic germs, bacteria linked with antibiotic resistance and polycyclic aromatic hydrocarbon degrading bacteria have been identified in MP-related biofilms (Sun et al. 2020) Microplastic-associated biofilms have the potential to influence the environmental effect, dispersion, and degradation of microplastic contaminants (Harrison et al. 2018). UV irradiation, salinity, light availability, pressure, and temperature can all influence the production of microplastic-associated biofilms (Harrison et al. 2018). Furthermore, whether the habitat is freshwater or marine, the microbial community makeup of the microplastic-associated biofilms might vary (McCormick et al. 2014).

6.4 Microscopic and Molecular Characterization

Although the surface layer of plastic provides an optimal situation for microbial colonization, the succession of microbes requires extensive exploration. Utilizing scanning electron microscopy (SEM) alone is insufficient to analyse the makeup and structure of biofilms developed on plastisphere. However, SEM is still a beneficial firsthand experience observing bacteria, other protists, and diatoms for visualizing the structure and spatial patterns of microbial communities on plastic (Zettler et al. 2013). SEM observations have found the microbes hiding in grooves and pits, which indicated that the plastic’s surface had deteriorated. Although SEM provides a thorough examination of the life that exists on the surface of plastic waste, the taxonomic resolution that can be obtained from this method is constrained. With the exception of morphologically distinct protists like diatoms and a few filamentous cyanobacteria, it is very difficult, if not impossible, to distinguish between different groups of microbes at the species level using SEM alone. SEM is not only expensive and labour-intensive but it also faces difficulties with data interpretation and quantification at the moment due to a lack of automation. Epifluorescence microscopy, in conjunction with the use of phylogenetic probes and through fluorescence in situ hybridization (FISH), has the ability to improve our understanding of how microbes interact on the plastic surface by allowing us to examine microbial diversity on the surface with significant taxonomic clarity (DeLong et al. 1989). To study plastisphere communities, FISH procedure with combinative labelling and spectral imaging was formed for investigating a probe set with four newly designed probes and three pre-existing phylogenetic probes. This technique for imaging using light, fluorescence, confocal, or electron microscopy was effective in keeping track of biofilm development. The technique further enabled not only the observation of surface coverage but also the observation of biofilm structures and microbial associations (Schlundt et al. 2020). In general, FISH offers several advantages, such as the capacity to provide precise abundance data rather than relative abundance information for the targeted cells and the ability to reveal both the spatial arrangement of the microbial community and taxonomic insights regarding the composition of the community on the plastic surface. Since the structure and function of biological systems are closely interconnected, investigating the spatial distribution of microbes can offer insights into their physiology and interactions. However, an inherent challenge associated with FISH techniques arises from the fact that phylogenetic probes specifically target and bind to ribosomal RNA (rRNA) within the cell (Valm et al. 2012). The fact that phylogenetic probes target and hybridize with ribosomal RNA (rRNA) in the cell presents a potential problem with FISH techniques. The fluorescent signal may be weak in communities that are not actively expanding, resulting in overlooked taxa. To enhance the fluorescent signal, multiple techniques such as catalysed reporter deposition FISH (CARD-FISH), nested FISH approach, or a next-generation in situ hybridization chain reaction (HCR) using multiple probes are obligated. However, using signal augmentation techniques can complicate a procedure. Another restriction of FISH strategies is that they require the purchase of fluorescently labelled probes, which necessitates molecular knowledge of the communities to be probed. For example, in one study, clone libraries were used in accordance with Sanger sequencing. They used genetic analysis and T-RFLP prior CARD-FISH to evaluate the phylogenetic associations of the microorganisms (Eich et al. 2015). The use of up to three different fluorophores simultaneously using the FISH technique was also constrained. The abundance of taxa which can be identified in an individual sample was consequently decreased. But a novel FISH technique called CLASI-FISH might be able to solve this issue by employing multiple fluorophores in parallel (six or more) (Valm et al. 2012). The amount of taxa which could be labelled in an individual sample is enhanced by labelling a given taxonomic group with a combination of different fluorophores (2–3) during the same period. Depending on the number of fluorophores used, this technique can significantly increase the number of distinguishable microbes, possibly reaching hundreds in a single FISH experiment. Microscopy, spectroscopy, statistical analysis, and digital fluorescence microscopy images can all be used to differentiate the fluorophores. This approach is currently being tested and applied to plastic debris after being effectively implemented to human oral microbes, opening up an exciting new field of study.

Sanger sequencing of rRNA genes, other biomarkers, as well as physiological characterization and ecological experimentation, are all facilitated by the capability to identify microorganisms in the lab. Because more than 99% of the microbes in any surroundings are not suitable for growing using traditional cultivation techniques, the majority of the microbial community is still unexplored (Amann et al. 1995). The development of culture-independent strategies enabled us to investigate the majority of the environment’s uncultivable microbes. As far as molecular techniques are concerned, clone libraries, denaturing gradient gel electrophoresis (DGGE), terminal restriction fragment length polymorphisms (T-RFLP), and high-throughput sequencing have previously been used to identify microbes on plastic surface in aquatic systems. All of these methods rely on the polymerase chain reaction (PCR), which has historically been unable to provide info on absolute species abundances due to PCR bias. Table 6.1 summarizes the main techniques used to analyse the microbial diversity on plastics in aquatic habitats. The 16S rRNA gene could be sequenced either entirely or partially in accordance with Sanger sequencing when it comes to clone libraries. Bacteria can typically be categorized to the species level, thanks to the remarkable taxonomic/phylogenetic assignments made possible by complete 16S rRNA gene sequence data. However, because of the time and labour involved in creating clone libraries, most studies only process below 1000 clones in a specified library (Konopka 2009). Furthermore, the genetic fingerprinting techniques DGGE and T-RFLP utilize the PCR products that have been amplified from environmental DNA (Muyzer and Smalla 1998).

Table 6.1 List of techniques utilized for analysing microbial communities inhabiting plastic surfaces
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Next-generation sequencing has enhanced read depth for 16S rRNA marker genes, overcoming many of the detriments of first-generation sequencing technologies. The sequencing reactions are enormously parallelized by the amplicon sequence analysis, which amplifies and sequences genetic markers like the 16S rRNA gene for multiple ecological samples at once (Sogin et al. 2006). Amplicon sequencing has until now been restricted to 500 bp marker genomic regions, rendering it difficult to categorize unknown reads using phylogenetic inference (Li et al. 2015). The established molecular procedures for characterizing bacterial species on plastic that have been previously discussed rely on sequencing the entire or a portion of the 16S rRNA gene. These methods can only identify taxa in a taxonomic sense. Shotgun metagenomics, however, has the capacity to both extract metabolic diversity and ascertain the taxonomic structure of all microbes present on plastic This molecular strategy aims to accomplish two aspects: initially, it enables for the independent analysis of annotated genomic DNA fragments, and further, it allows for the assembly of fragments into genomic bins for comparative genomic analysis. For two reasons, we assume that a thorough knowledge of the entire microbial community is necessary. For starters, plastic can act as a vector for pathogenic microorganisms (Barnes 2002). Pathogenic Vibrio and harmful algal blooming species have already been discovered on plastic surface (Masó et al. 2003; Kirstein et al. 2016). Therefore, understanding the microbiota is essential for transport to other regions. Second, it is important to look into community members other than bacteria, like fungi, in terms of the deterioration of plastic. Plastics can be degraded by both bacterial and fungal microorganisms in terrestrial environments (Koutny et al. 2006). Fungal organisms will likely be included in the plastic microbial community investigations, which will likely yield unique knowledge into the ecosystems and biological function of the plastisphere.

6.5 Factors Affecting Plastisphere

The microbial development on MPs believed to be a complex procedure impacted by a lot of considerations (Carson et al. 2013). This is significant information because the structure of microbiota on such biofilms differs from other situations and is essential to the functioning of marine habitats (Rummel et al. 2017). Understanding the elements having an impact on biofilm development and the makeup of microorganisms on MP particles is crucial. Using this knowledge, MP pollution can be decreased by identifying potential biodegradable MP species (Lobelle and Cunliffe 2011). Two aspects influence the expansion and makeup of microbial communities on MP particles (Carlén et al. 2001). The substrate’s physicochemical characteristics, which include the hydrophobicity, plastic type, specific surface area, and roughness, are the first determinant. The environment, which also includes nutrients, temperature, salinity, and location, is the second factor (Stoodley et al. 2002).

6.5.1 Physiochemical Factors

Various types of MPs influence the development of biofilms and the colonization of microbial communities, according to numerous studies (Miao et al. 2019). The majority of research studies have used MPs made from polyethylene, polypropylene, polyethylene-terephthalate, polystyrene, and polyvinyl chloride, due to their frequent presence in water environments. Compared to biofilm communities on the surfaces of PP and PE, those on the surface of PS have more bacteria (Pinnell and Turner 2020). Studies revealed that various MP structures (like plastic form, item size, and roughness) or other MP characteristics are what causes different biofilm development (like solid particles, synthetic items, with more polymer quantity and 5 mm in size) (Frère et al. 2018). On the surface of PET, numerous Mycetozoa, Bacteroides, and Cyanobacteria have been observed in addition to algae like Bacillariophyta and brown algae (Pinto et al. 2019; Eich et al. 2015). But the procedure underlying the preferential colonization of common microorganisms on a particular MP surface remains poorly understood.

The microbial development rate and the density of microbes on MPs are positively correlated with roughness, as the rougher surface is considered better for the development and functioning of microbes on MPs (Miao et al. 2021; Hirai et al. 2015). PVC had more operational taxonomic groups than PP because of its rough surfaces and greater surface area, which made bacterial interaction relatively easy (Miao et al. 2019). Plastic surfaces are capable of absorbing both inorganic and organic material from the water environment, creating a layer that affects the adhesion of biofilms to MP surfaces (Rummel et al. 2017). The fact that the layer’s formation is related to how rough the surface of MPs is evident that the layer’s chemical composition had an impact on the plastisphere. During the last stages of biofilm development, hydrophobicity is firmly parallel with micro-community, adhesion, and microbial communities on MP particles (Pompilio et al. 2008). Even so, compared to other physicochemical characteristics of MPs, a small number of studies found that the plastic’s hydrophobicity had a minor impact on the adherence and firmness of bacteria (Heistad et al. 2009; Hook et al. 2012). Due to unknown factors like cell size, MP fragment size, and shape, the correlation of hydrophilicity or hydrophobicity of MP boundary and level of cell adherence is still unclear (Foulon et al. 2016; MacHado et al. 2010).

6.5.2 Environmental Aspects

The variety of bacteria living on the surface correlates unfavourably with nutritional status, whereas the mean expansion rate of MP bacterial growth correlates with it positively (Oberbeckmann et al. 2018). The relationship between nutritional levels based on TN and TP and plastisphere formation has received the most consideration. Scientists have discovered that TN and TP quantity are mandatory for the development of biofilm and serve to the growth of particular microbial communities (Miao et al. 2021). The biosorption and decomposing activities of microbes on MP exterior surface vary with nutritional state. Green algae and autotrophic diatoms are the major species on the MP biofilm communities in lakes with average and little concentration of nutrients, while autotrophic species are taken up by predators like ciliates (heterotrophic), which shift MPs to a nutritious habitat (Miao et al. 2020).

Due to MPs’ prolonged exposure to the surroundings and alteration in seasons, the environmental temperature where MPs are present alters, which obviously impacts the variety of microbes on biofilm (Carson et al. 2013). Both the enzyme production of cells and the physiological behaviour of microorganisms are significantly impacted by temperature (Xu et al. 2019). Summer temperatures caused the biofilm on PET surfaces to be thicker, and the comparative richness of Cytophagales and Ignavibacteriales varied seasonally (Pinnell and Turner 2020). Further, salinity and light conditions also have an impact on the formation of biofilms, in addition to seasonal temperature variations (Oberbeckmann et al. 2016). Environment salinity has a negative relationship with biofilm growth rate and a positive relationship with bacterial community diversity (Oberbeckmann et al. 2018). Salinity affects the MP surface biofilm’s entire bacterial community as well as the distribution of particular bacteria (Guo et al. 2017). The overall process of biofilm formation on the MPs’ surfaces is adaptable (Caruso 2020). To investigate the dynamic procedure of biofilm development in its initial, central, and late phases, hypothetical analyses have been utilized (Jiang et al. 2018). Based on the recent observations, the actual number of microbial population on the surface of MPs has grown consistently with exposure period (Tu et al. 2020).

6.6 Microplastic in Marine Environment

The concern of marine ecosystems being contaminated by MPs has been on the. MPs are pervasive and abundant in the marine environment, with higher concentrations occurring along coastlines. Numerous marine organisms have shown the ability to ingest MPs, which opens the door to the transmission of hydrophobic waterborne contaminants or chemical additives to the marine biota (Andrady 2011). The amount of plastic that contributes towards municipal waste accounts for 10% of all waste produced globally, which has been intensified by the widespread usage of disposable (use and throw) plastics (Ncube et al. 2021). Despite the fact that some plastic garbage is recycled, the bulk is disposed of in landfills, where it may take generations to break down and degrade (Moore 2008). Plastics that are entering the maritime environment via careless dumping have been of particular concern (Gregory 2009). Despite the fact that plastic is globally recognized as a pollutant and that laws are in place to reduce the quantity of plastic trash entering the marine, it has been estimated that up to 10% of plastics manufactured end up in the seas and thereby contaminating marine ecosystem. The irresponsible dumping of waste materials that enter our seas and oceans either directly or indirectly leads to marine litter (Lopez Lozano and Mouat 2009; Ryan et al. 2009). Approximately 80% of the plastics in marine litter come from terrestrial sources (Andrady 2011). Primary MPs used in cosmetics and air-blasting dumped disposal plastics, and plastic leachates from waste sites are some of the common sources of plastic found in marine litter since these types of plastic have a significant potential to reach the marine environment via wastewater systems, rivers and canals, drainage systems (Derraik 2002; Thompson 2006; Moore 2008). Numerous studies have demonstrated that freshwater systems with the unidirectional flow are also responsible for the transfer of plastic waste into the marine ecosystem (Moore et al. 2002; Browne et al. 2010). Moore (2008) calculated the proportion of plastic particles that were present in water samples taken from two rivers in Los Angeles (California, USA) (Moore 2008). The extrapolated statistics showed that over the course of 3 days, only these two rivers will discharge almost two billion microplastic particles into the marine ecosystem.

Several worldwide studies have been conducted in recent years to assess the prevalence of floating MPs and microbeads in various water bodies across the world (Eriksen et al. 2013b; Cózar et al. 2014; Reisser et al. 2015). According to a study by Li et al. (2016), the North Atlantic subtropical gyres are the most affected by floating MPs and microbeads (Li et al. 2016). According to Lebreton et al. (2012), the subtropical gyres have become the hotspots for MPs as a consequence of the plastic waste that came from both terrestrial and marine sources (Lebreton et al. 2012). The South Pacific subtropical gyre was found to possess an approximate average of 26,898 particles per square kilometre, varying in size from 0.355 to over 4.750 mm (Eriksen et al. 2013a). Similarly, the North Pacific subtropical gyre region has been documented as accumulating approximately 22,290 tonnes of floating plastic waste (Law et al. 2010). According to Eriksen et al., the South Pacific subtropical gyre has a high concentration of microplastic debris because of natural processes including wind currents around the coasts of Indonesia and Ecuador (Eriksen et al. 2013a). The Arctic Ocean has been found to contain between 38 and 234 particles/m3 of MPs (Obbard et al. 2014), which is particularly alarming given that the amount was two times more than that previously recorded for the Pacific Gyre (Goldstein et al. 2012). MPs are persistent in the marine environment, and because of their tiny size, they are permeable to a wide variety of marine organisms, including fish, corals, sea urchins, zooplanktons, lobsters, worms, and many more (Browne et al. 2008). After being consumed by these marine species, the non-degradable MPs bioaccumulate up the food chain (Gregory 1996, 2009) until they reach higher tropic levels (Carpenter and Smith Jr 1972). Microplastic waste has been found in fish, crustaceans, turtles, and seabirds all around the world (Derraik 2002; Cole et al. 2011) and thus obstructing the digestive system, inhibiting stomach enzyme release, imbalanced hormone levels, delayed ovulation, and infertility (Azzarello and Van Vleet 1987; McCauley and Bjorndal 1999; Derraik 2002; Wright et al. 2013). Compared to marine habitats, MP pollution in terrestrial and freshwater ecosystems has received less attention (Ncube et al. 2021). About 10% of microplastic ends up in the ocean, and 7–8 million pieces of plastic accidentally wind up there from terrestrial sources (Lacerda et al. 2019). The Mediterranean Sea, which has an average depth of 1500 m, is known as a hotspot for plastic pollution since its MP concentration is four times higher than that of the North Pacific Ocean. This is explained by the Mediterranean Sea’s distinctive semi-enclosed topology and the nearby nations that produce large amounts of plastic waste (Sharma et al. 2021).

6.7 Microbial and Enzymatic Degradation of Microplastic

The degradation of MPs is essential for maintaining the ecological balance. Mechanical and chemical degradation processes are widely used. The rate of degradation depends mostly on polymer structure, and composition, besides the temperature, humidity, and the site of deposition. The exposure of MPs to ultra-violet rays showed variation in rate of degradation (Arpia et al. 2021). The chemical degradation is mediated by hydrolysis, oxidation, and photooxidation. This leads to the release of gases into the environment which can disturb the ecosystem (Da Costa et al. 2018). Biodegradation of MPs is a preferred strategy for minimizing the plastic contamination in the environment (Rojas-Parrales et al. 2018; Wani et al. 2022a). The breakdown of large or small plastic polymers takes place through the aerobic or anaerobic action of microorganisms. In aerobic degradation of MPs, oxygen acts an electron acceptor with water and carbon dioxide as major by-products. In anaerobic mode of MP-biodegradation, bacteria utilize iron, sulphate, manganese, and carbon dioxide as electron acceptors. MP polymers are not transported into the cells because of their large size and water insoluble nature. The degradation of MP polymers is driven by three major steps, i.e., microbial attachment on MP surface, utilization of MP as carbon source, and finally the degradation (Wani et al. 2023). MP polymers are depolymerized by enzymatic action both intracellularly and extracellularly. Exoenzymes degrade plastic polymers into small molecules with water, methane, and carbon dioxide as end products, the process is known as mineralization. The other processes of microbial degradation of MP include biodeterioration, biofragmentation, and assimilation (Bacha et al. 2021). Microorganisms that are capable of plastic polymer degradation have been isolated from different sites. Staphylococcus, Streptococcus, Klebsiella, Pseudomonas, and Micrococcus have been characterized with plastic-degrading potential. The biodegradability is enhanced by the addition of auto-oxidizing agents, which ensures easy degradation of the plastic polymers. Bacillus vallismortis has been reported to have LDPE degradation potential with 75% efficiency after 120 days of incubation (Nourollahi et al. 2019). Bacillus amyloliquefaciens degrades polyethylene when it is preliminary treated with heat. Besides B. safensis and B. mycoides degrade HDPE and LDPE when pretreated with sunlight and mercuric acid. Microorganisms adapt fluctuating environmental conditions through different cellular and genetic pathways. They are known to colonize MP surfaces which changes some of the properties of the polymers like strength, molecular weight, and roughness (Awasthi et al. 2017). The homoatomic and heteroatomic plastic backbone makes biodegradation of MPs a complex process. There is significantly weight loss in MP with the action of microorganisms, but process is slower as compared to the chemical degradation. The direct application of enzymes secreted by microorganisms plays an essential role in speeding up the process of MP degradation. Cutinases, isolated from Thermobifida fusca are well studied for their MP-degradation potential. MPs do not dissolve in water, but some of the water-soluble polymers are degraded easily and changed into ketones, acids, and alcohols. MP-biodegradation is monitored by: (1) changes in surface properties of the polymer, (2) consumption rate of oxygen, (3) evolution of oxygen, and (4) changes in MP’s physical and chemical properties (Zhang et al. 2021). Table 6.2 gives an insight into the ability of microorganisms in degrading the MP-contaminants.

Table 6.2 Microbial-assisted degradation of different plastic polymers
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Different factors play their role in plastic degradation by microorganisms. The difference in the functional groups, hydrophobicity, structure (linear/branched), bond type (ester, ether, urethane, amide), base composition, polymer form (pellet, film, powder), polymer molecular weight, density, toughness, and morphology drives the degradation rate of different polymers. The rate of MP degradation reduces with the decrease in polymer solubility (Weinstein et al. 2016). Plastic polymers are less susceptible to microbial degradation with the decrease in the degree of solubility. Amorphous polymers are vulnerable to enzyme-mediated degradation than the crystalline polymers. Thus, MPs can restrict microbial action in hydrophobic environment by inhibiting water absorption process (Santo et al. 2013).

6.8 Conclusion and Future Prospects

The limitations in standard sampling protocols, extraction methods, detection procedures, and analytical tools bring a lot of hurdles in management, surveillance, and degradation processes. The heterogeneity of MPs is a serious matter of concern in understanding their physiochemical parameters. The lack of established, uniform, and standard policies for the assessment of MPs and NPs is challenging. The collection of inadequate amounts of MP from the contaminated sites makes it difficult to detect the plastic polymers. The available analytical tools are not good enough to detect low concentrations (Veerasingam et al. 2020; Hossain et al. 2022). There is a paramount need for highly efficient and multi-residual tools to solve the challenges confronted in MP analysis and characterization. The low rate of biodegradation driven by bacteria and fungi is also very challenging. Since most of the microorganisms are unculturable, and it is difficult to study the process of bioremediation in laboratory conditions. Metagenomics, an unculturable study method, is one of the recent approaches to detect the novel microorganisms and genes capable of degrading plastic polymers (Handelsman 2004; Wani et al. 2022b, d). Metagenomics in association with other meta-omics approaches can be useful in predicting the efficient plastic-degrading bacteria. The recent genome editing and genetic engineering technologies can be useful in amplifying the specific genes and subsequently cloning and expressing them in a suitable vector and the host (Wani et al. 2022c, e; Mir et al. 2022). In this chapter, we have given a detailed account of the plastisphere along with the different analytical tools used in its characterization and analysis. We have highlighted the applicability of microorganisms in degrading different types of plastic polymers.