Plastic waste generated on land enters the marine environment through riverine runoffs by leaching from open dumpsites or sewage effluents, washing off from beaches, spilling during transport or accident, or dumping of plastic wastes into the sea, resulting in plastic debris being found in all major ocean basins across the world ^[1][17]. Table 1 shows the countries in which the maximum amount of mismanaged plastic waste reaches the ocean ^[2][18]. Based on the mismanaged plastic waste in ocean per capita, it can be concluded that Asian countries need to adopt more efficient strategies to reduce the amount of plastic waste reaching the ocean.

The numerous effects from ingestion of macroplastics, microplastics, and nanoplastics and entanglement due to macroplastics have been well-documented in several mammal, fish, turtles, and bird species ^[3][19]. The ingestion of plastics leads to suffocation or blocking of the digestive tract and ultimately death ^[4][20]. Microplastics, due to their small size and increased surface area, can enter into tissues and cells and react with other chemicals in the environment (Figure 12). Experimental evidence demonstrates that exposure to microplastics and nanoplastics in oysters hinders feeding and has negative effects on fecundity and offspring quality ^[5][6][16,21].

In recent times, it has become increasingly evident that trophic transfer and bioaccumulation of plastic and other associated chemical pollutants is taking place through the food web. A well-studied example includes the filter feeders, such as mussels, wherein the microplastics and other pollutants are accumulated and are then transferred to benthic predators and humans through the consumption of wild or farmed shellfish ^[7][8][22,23]. Studies on mussels have established that microplastics can enter from the gut into the circulatory system, and they were found in mussel haemolymph. The presence of microplastic particles in the gastrointestinal tracts of seals and cetaceans indicates the trophic transfer taking place from the prey fish to top predators ^[9][10][24,25]. Moreover, studies carried out on fish, seabirds, and mussels indicate the potential of plastics to cause bioaccumulation of environmental pollutants ^[11][12][13][26,27,28].

Microplastic contamination is present not only in the marine environment, but there is increasing evidence that microplastics are also abundantly present in the terrestrial environment through the physical or chemical decomposition of larger plastic materials ^[14][15][29,30]. Plastic products abundantly used in daily life can generate microplastic particles due to fragmentation, aging, and deterioration. These microplastics accumulate in soil either by fragmentation of discarded plastic items or by the leaching of buried wastes present in the landfills ^[1][17].

Disposed plastic waste, when exposed to the natural environment, could be ingested by terrestrial biota because they mistake it as food ^[16][31]. However, very few studies have been conducted to study the occurrence of microplastics in the terrestrial biota. A survey of the presence of anthropogenic plastic waste of the size range of 0.5–5 mm in terrestrial birds in China was conducted which found that 62.6% of total particles found in birds’ digestive tract were plastics ^[17][32]. This research indicates that there is an urgent need to study the prevalence and effects of microplastics in terrestrial birds. Soil organisms such as earthworms moved 73.5% of microplastics from the soil surface down into the bulk soil by burrow formation ^[15][30]. Moreover, pollutants such as metals and organic contaminants carried by microplastics could affect the soil and groundwater quality due to the leaching of these chemicals through desorption into the soil ^[18][33].

The innumerable applications of plastics in day-to-day life has resulted in the presence of plastic wastes in large amounts in urban water systems ^[19][34]. Municipal wastewater treatment plants are another source for the presence of microplastics in the terrestrial environment. The wastewater collected from households, hospitals, and industries containing microplastics end up in sewage sludge ^[20][21][35,36]. The usage of sludge in agricultural lands as fertilizer has resulted in deposition of microplastics in the soil. A study conducted by ^[22][37] assessed that in European countries, through sewage sludge application, 125–180 tons of microplastics per million residents have been released to the terrestrial environment. The challenges faced in extraction and identification of microplastic particles from soil or sludge could be the reason for the limited availability of data on the presence of microplastics in the terrestrial environment ^[23][38] though wastewater treatment plants which used primary clarification, wherein microplastics are removed based on their densities either by sedimentation or floatation before the wastewater is subjected to biological treatment, have proven to be more efficient in the removal of microplastics from wastewater ^[24][39].

The presence of microplastics is also found in large quantities in drinking water. The effluents from wastewater treatment plants and stormwater runoff from urban and agricultural lands are the major sources of microplastics in drinking water ^[25][40]. Na et al. ^[26][41] investigated whether drinking water treatment plants, which use different processes such as coagulation, sand filtration, and disinfection using UV and UV/H₂O₂, can remove microplastics from drinking water. Through their studies, they concluded that the type of coagulant used and the pH and organic matter naturally present in water affect the efficiency of microplastics coagulation. Moreover, after sand filtration, at least 16% of microplastics which were <10 μm in size were further fragmented by UV/H₂O₂, causing the leaching of chemicals in water and resulting in increased toxicity of the water sample.

The extensive use of plastics for several decades and their release into the environment have resulted in wide range of associated problems. All plastics are synthetic polymers which are synthesized by combining monomer chemicals, some of which are toxic and carcinogenic in nature, and these monomer residues in plastic products can be hazardous to humans ^[27][42]. Moreover, the presence of additives such as fillers and plasticizers, coloring agents, flame retardants, and other substances pose health risks for living beings and also reduce the reuse and recycling potential of plastics ^[12][27]. Plasticizers such as phthalates are widely used to make soft PVC (polyvinyl chloride), and products containing phthalates include clothing, packaging materials, toys, flooring, and other items used daily. Since the phthalates are not chemically bound to plastics, they can leach out from the plastics into the surrounding environment, causing health hazards, as they can influence the endogenous production of several hormones, hampering reproduction and development. Similarly, bisphenol A used in polycarbonate baby bottles, water bottles, and protective coating inside metal food containers has been proven to be “environmental oestrogen” ^[28][43].

The presence of primary and secondary microplastics has been proven in aquatic life, and their entry into the food chain through human consumption leads to their biological accumulation. The authors of ^[29][30][44,45] designed a study to check the presence of microplastics in blood using Py-GC/MS. Their experiments demonstrated the presence of PMMA (Poly (methyl methacrylate), PP (Polypropylene), PS (Polystyrene), PE (Polyethylene) and PET (Polyethylene Terephthalate)) plastic particles in 77% of donors in a quantifiable amount. PET, PS, and PE were found in a maximum amount in blood samples in about 2.4, 4.8, and 7.1 μg/mL, respectively. The plastic particles concentrations could be due to exposure of humans to personal care products (in toothpaste, face scrubs, and lip gloss), use of nanoparticles in drug delivery, inhalation through air, or consumption of food and water containing microplastics. Furthermore, ^[31][46] analyzed the presence of microplastics in the feces of patients with inflammatory bowel disease (IBD) and healthy people. They detected the presence of microplastics in the feces of patients with IBD at much higher concentrations than compared to the feces of healthy individuals. Fifteen types of microplastics were detected in feces, of which PET and polyamide were predominantly present in the form of sheets and fibers.

According to the researchers, there is a positive link between the presence of microplastics in feces and IBD. Either the exposure to microplastics causes the disease, or the patients of IBD retain microplastics in feces because of the disease. Though microplastics can have a negative impact on human health, more experimental evidence is required to prove the harmful effects of these microplastics in the human body. Moreover, Deng et al. proved the presence of fluorescent and pristine polystyrene microplastics of 5 μm and 20 μm in mice liver, kidney, and gut tissue using fluorescence spectroscopy and histological analyses ^[32][47]. Moreover, through biochemical biomarkers and metabolomic profiles studies, they concluded that exposure to microplastics affects metabolic pathways and leads to oxidative stress and neurotoxic responses. Scarcity of proper analytical tools for isolation, detection, quantification, and characterization of microplastic particles which are <10 μm has resulted in difficulty in estimating the risk of microplastics to human health. However, it has been reported that particulate particles in air which are smaller than 2.5 μm in size and arise from diesel exhaust have the capacity to enter cells and induce the formation of reactive oxygen species (ROS) and inflammation and are linked to increased chances of death due to cardiovascular or respiratory diseases or lung cancer. By comparing these results with microplastics of smaller size, a parallel can be drawn to estimate the risk of microplastics on human health ^[33][10]. Thus, effective measures need to be taken to reduce the production of microplastics, and ecofriendly methods need to be used for the biodegradation of plastics, as the physical and chemical methods used for degradation of plastics have severely harmful effects on environment. Microorganisms have the ability to degrade plastics, but various strategies need to be adapted to increase the potential of these microorganisms to degrade plastics. In this rentry, the researchersview paper, we have discussed the various biotechnological interventions which can be used to improve the biodegradation of plastics by microorganisms and result in a reduction in the production of microplastics.

Based on frequency of use, plastics are divided into seven broad groups. (1) Polyethylene Terephthalate (PET or PETE), (2) High-Density Polyethylene (HDPE), (3) Polyvinyl Chloride (PVC or Vinyl), (4) Low-Density Polyethylene (LDPE), (5) Polypropylene (PP), (6) Polystyrene (PS or Styrofoam), (7) Other. Out of them, many are degradable. The particular biodegradable plastics are (acrylonitrile butadiene styrene, acrylic, acetyl cellulose, cellulose triacetate, alkyd, cellophane, epoxy resin, polyamide, polyacrylonitrile, and poly(butylene adipate-co-teraphthalate)). Therefore, the above plastics are used in several sectors. Numerous studies carried out in the past few years have reported about the capability of several microorganisms in the degradation of different synthetic plastics. It is estimated that more than 50% of the plastic waste ends up in the landfill and 19% of the waste is managed by incineration. Only about 9% of plastic waste is recycled and, the remaining 22% of mismanaged plastic waste ends up in terrestrial and aquatic environments. The majority of the bacterial species which have the ability to degrade plastic are Gram-negative bacilli, and, among them, Pseudomonas spp. has the highest capability in the biodegradation of plastics ^{[34][35][36][37][38]}[48,49,50,51,52]. For polyethylene degradation, Pseudomonas spp. was isolated from three different sources. Among them, the Pseudomonas spp. isolated from sewage sludge dump was most efficient in the biodegradation of natural and synthetic polyethylene.

Compared to the other species, Pseduomonas spp. formed the most viscous and flocculent biofilms on the surface within a period of three weeks ^[36][50]. Apart from Pseudomonas spp., several other bacterial species capable of carrying out the biodegradation of plastics include Ideonella, Klebsiella, Streptomyces, Mycobacterium, Flavobacterium, Rhodococcus, Escherichia, and Azotobacter. Along with bacteria, fungi are also involved in the biodegradation of plastics, as they hasten the biodegradation process by sharing the metabolic intermediates ^[39][53]. Penicillum and Aspergillus spp. are reported to have the capability to degrade polyethylene by the formation of biofilms which decrease the hydrophobicity of the surface ^[39][40][53,54]. Aspergillus spp. is reported to have the ability to degrade LDPE, whereas Penicillum spp. can degrade both LDPE and HDPE ^[41][55]. Among the Aspergillus spp., Aspergillus niger has the highest potency in degrading polyethylene (38%), followed by Aspergillus flavus (31%), in a period of 60 days ^[42][56].

Biodegradation of plastics takes place when microorganisms such as bacteria and fungi, through their enzymatic action, convert them into metabolic products such as methane, carbon dioxide, water, etc. ^[43][57]. Biological deterioration of plastic pollutants depends on several factors, including surface area, molecular weight, hydrophilicity or hydrophobicity, crystallinity, functional groups, chemical structure, etc. Increases in molecular weight leads to decreases in degradation rate, as the solubility also decreases, making the plastics less susceptible to microbial attack because it becomes difficult for microbes to assimilate plastics through the cell membrane ^[44][58]. Similarly, crystallinity is another important factor affecting biodegradability, with amorphous-domains-containing polymers being more vulnerable to enzymes produced by microorganisms ^[45][59]. Moreover, the hydrophobic nature of plastics also inhibits the susceptibility of them to microbial attack by hindering water absorption, though this problem can be solved by biofilm formation on plastics ^[46][60]. The process of biodegradation of plastic is depicted in Figure 23.