Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. They are used across almost every sector, including to produce packaging, building and construction, textiles, consumer products, transportation, electrical and electronics and industrial machinery. However, what comes along with the mass production of plastic products, is plastic waste. According to data, the world produces about 400 million tons of plastic waste a year. These plastic wastes in the environment may gradually decompose into microplastics (MPs) due to synergistic environmental and biological stresses. Generally, MPs are defined as plastic fragments of <5 mm, whereas nano plastics are plastic particles of <100 nm or 1 μm in size. MPs are resistant to chemical and biological degradation and can easily spread to various environments via wind and waves owing to their small size, lightweight, high durability, and high stability.
Recently, there have been more studies on the effects of MPs on aquatic organisms of different trophic levels. Traces of MPs have been detected in various aquatic organisms including zooplanktons, mussels, fish, waterbirds, and cetaceans. The interactions between MPs and wildlife can cause harm in many ways, such as entanglement, nutrition, deprivation, and damage or obstruction to the gut. For years, a comprehensive pattern of MP accumulation in wildlife is yet to be elucidated. Furthermore, though e trophic transfer of MPs from prey to predator has been demonstrated in laboratory settings, evidence regarding the bioaccumulation and biomagnification of MPs within the food web in nature has been seldom observed. It was believed that predators are considered to be at a higher bioaccumulation risk of MP than lower trophic level organisms owing to their high demands for food and energy and potential trophic transfer of MPs. However, a study on the Amazonian fish food web revealed no differences among the guilds of carnivores, herbivores, and omnivores.
In order to investigate the transfer of MPs in various wildlife taxa such as subtropical freshwater and terrestrial food webs, Rongliang Qiu and his team conducted a study from a small village in South China which used to be an e-waste recycling site. In the sampling area (about 1 km2), multiple taxa of wildlife were collected. All the samples were stored at −20 °C until further analysis. The stomach contents and intestinal tissues of individual snails, fishes, snakes, birds, and voles were isolated to analyze the abundance of MPs. Samples were incubated in both potassium hydroxide and sodium chloride solutions, filtered through stainless steel filters (20 μm) and washed with ethanol. Once the ethanol evaporated, the abundance and size of MPs were measured using an Agilent 8700 laser infrared imaging spectrometer. It is worth mentioning that polyamide was not included in this study as it is not possible to distinguish artificial polyamides from natural ones.
From the analysis, the median abundances of MPs in aquatic invertebrates (snails, insects, shrimps, and crabs) and terrestrial invertebrates (insects and snails) were in the range of 22.6−57.9 and 37.1−182 particles/individual, respectively. The median abundances of MPs in fishes, snakes, birds, and voles were 38.1−130, 132, 150−1250, and 171 particles/individual, respectively. Notably, birds, snakes, and voles exhibited a similar range of MP abundances and weights, which were higher than those of invertebrates and fishes. In terms of the MP size, plastic particles >500 μm in size were not detected in the samples. The reasons can be the active rejection of large plastic particles or the crushing of large pieces of plastics into smaller pieces in the gastrointestinal tract. Approximately 80% of MPs were in the size range of 20−50 μm while MPs in the size range of 100−500 μm were generally <10% of the total. There were 33 types of polymer types of MPs identified in the samples. Among them, ACR, chlorinated polyethylene (CPE), PE, PET, PU, and SI were the main polymer types, which were detected in >50% of the samples.
Figure 1: Compositions of polymer types in organisms.
From these data, the authors were able to draw several conclusions. First of all, the MPs of finer size ranges were more abundant, which agrees with previous studies. This also led the authors to suggest that a standardized analytical method for identifying MPs with a broad size range is urgently needed due to the unfeasible detection of particles <50 μm in size. Another discovery is that in general, birds, voles, and snakes exhibited higher abundances of MPs per individual than fishes and invertebrates. By calculation, the log-transformed abundances of MPs in particles per individual were positively correlated with the log-transformed body weight of the wildlife samples (p < 0.01). This finding validated the assumption that more weight indicates greater food consumption, longer gastrointestinal tract retention, and higher MP exposure compared with less weight. Finally, the log-transformed abundances of MPs in terrestrial and aquatic food webs were plotted against δ13C and δ15N, separately, to assess whether stable isotopes can serve as indicators of MP pollution. While δ15N displays no correlation, the abundances of MPs per individual were positively but not significantly correlated with δ13C values in terrestrial and aquatic organisms (p > 0.05). Higher δ13C values indicate more proportions of aquatic food sources and lower exposure risks of MPs for terrestrial predators.
Figure 2: Relationships with lg-transformed body weights of wildlife species and lg-transformed abundances of microplastics in particles/individual and particles per gram.
Figure 3: Relationships between δ13C and lg-transformed abundances of microplastics expressed as particles/individual and particles per gram in terrestrial organisms.
It is hard to argue whether MP is an adsorbent or a source of organic pollutants in the gastrointestinal tract of organisms. However, from the analyzed bird species, the authors concluded that the low MP abundance detected in birds compared with the amount of food ingested indicates that MPs constitute a negligible factor in the bioaccumulation of chemical pollutants.
The finding of this work has been published on Environmental Science & Technology: Zheng, X.; Wu, X.; Zheng, Q.; Mai, B. X.; Qiu, R. Transfer of Microplastics in Terrestrial and Aquatic Food Webs: The Impact of E-Waste Debris and Ecological Traits. Environ. Sci. Technol. 2022, 57, 1300–1308. https://doi.org/10.1021/acs.est.2c06473
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