Microplastics and nanoplastics in the environment: Macroscopic transport and effects on creatures

Highlights

• Discuss the interface transport of microplastics/nanoplastics (MPs/NPs) among three major environment compartments.

• Analyze the effects of MPs/NPs to creatures that need to span aquatic, terrestrial and atmospheric environment.

• Completely review the transport pathway and toxicity of MPs/NPs in individual environments.

Abstract

Industrial progress has brought us an important polymer material, i.e. plastic. Because of mass production and use, and improper management and disposal, plastic pollution has become one of the most pivotal environmental issues in the world today. However, the current researches on microplastics/nanoplastics are mainly focused on individual aquatic, terrestrial and atmospheric environments, ignoring the fact that the natural environment is a whole. In this regard, the transport of microplastics/nanoplastics among the three environment compartments, including reciprocal contributions and inherent connections, and the impact of microplastics/nanoplastics on organisms living in multiple environments are research problems that we pay special attention to. Furthermore, this paper comprehensively reviews the transport and distribution of microplastics/nanoplastics in individual compartments and the toxicity of organisms, either alone or in combination with other pollutants. The properties of microplastics/nanoplastics, environment condition and the growth habit of organisms are critical to the transport, distribution and toxicity of microplastics/nanoplastics. These knowledge gaps need to be addressed urgently to improve cognition of the degree of plastic pollution and enhance our ability to deal with pollution. Meanwhile, it is hoped that the paper can provide a relatively complete theoretical knowledge system and multiple "leads" for future innovative ideas in this field.

Introduction

The invention of plastic has opened a new era, which has brought great convenience to our production and life due to its stable chemical properties, good insulation, light weight and durability characteristics (Gu et al., 2020). According to statistics, the global production of plastics is close to 360 million tons (Mt) in 2018 (Fig. 1A), and output is expected to double in the next 20 years. But, only 6–26% of these plastics are recycled, which means that up to 94% of the plastics probably be landfilled or enter the environment through various routes (Alimi et al., 2018, Nizzetto et al., 2016, Barnes et al., 2009). In this regard, the ocean may be an immense reservoir for plastics. PlasticsEurope estimates that 6–12 Mt of plastic enters the ocean every year, then there will be over 250 Mt of plastic accumulated in the ocean by 2025 (Wright and Kelly, 2017, Jambeck et al., 2015).

The most critical point is that the natural degradation of plastics is extremely slow. After physical, chemical and (or) biological action, large pieces of plastic will be broken into microplastics (MPs) (1 µm–5 mm) or nanoplastics (NPs) (<1000 nm) (Jahnke et al. 2017). In recent decades, as a new type of pollutant, MPs have attracted widespread attention in the academic world. Researchers have discovered the presence of MPs around the world, for example, 4601–5732 m of deep sea sediments in the western Pacific (240 items per kg dry weight of sediment) (Zhang et al., 2020a), and coral reefs in the Xisha Islands of the South China Sea (0.2–45.2 items/L) (Ding et al., 2019), even the snow in the mountains (Materić et al., 2020) and the north and south poles (Tekman et al., 2020, Peeken et al., 2018, Dawson et al., 2018) have been contaminated with different degrees of MPs/NPs. From the production and post-use phase of plastics, MPs/NPs are released into the environment at every stage. How many MPs/NPs are in the actual environment? How are MPs/NPs transported in the environment and what are their exchange volume? Although there are related reports (Kawecki and Nowack, 2019, Mai et al., 2019), small-scale simulation experiments do not fully reflect environmental conditions owing to the diversity of nature (type, shape, density, etc.) of MPs/NPs, the complexity of the environment and other factors, small-scale simulation experiments do not fully reflect environmental conditions, therefore these are still knotty problems.

Furthermore, some related researches on the impact of MPs/NPs on aquatic organisms are developing rapidly (Dauvergne, 2018, Avio et al., 2017, Koelmans et al., 2017, Eerkes-Medrano et al., 2015, Koelmans et al., 2014, Auta et al., 2017), and the interest in soil and atmospheric organisms is also increasing (Duis and Coors, 2016, Qi et al., 2018, Guo et al., 2020a, Roman et al., 2019). These researches, which mostly focus on species that are common in a certain environment (like fish, invertebrates, bivalves), have reported a wide variety of neutral and deleterious effects in various species. In general, MPs/NPs will limit the growth and development of organisms, increase mortality, disrupt endocrine metabolism, and even change gene expression, whereas like some filter-feeding animals, even if they digest a lot of MPs/NPs, which will not affect their survival (Huuskonen et al., 2020, Kalčíková et al., 2020, Bringer et al., 2020, Mohsen et al., 2020, Pedersen et al., 2020, Nanninga et al., 2020, Sökmen et al., 2020, Elizalde-Velázquez et al., 2020). Thus, the toxicity studies of MPs/NPs are still in the immature stage, scholars are not clear about the reasons for this difference, also face many technical challenges. So far, there is no uniform standard method for sampling, detection and risk assessment of MPs/NPs in various countries, which makes it impossible to compare the research results and only give a general guess. It is unclear how much impact the entire ecological environment has (Rillig and Lehmann, 2020).

Herein, we summarize the existing research on the transport and toxicology of MPs/NPs in aquatic, terrestrial, and atmospheric environments. Through this review, we emphasize: 1) the mutual transfer and circulation of MPs/NPs in aquatic, terrestrial and atmospheric environment; 2) the interaction between MPs/NPs and biology, MPs/NPs and environment, biology and environment, which will ultimately impress the ecotoxicology of MPs/NPs; 3) the deficiencies of current research and knowledge framework. This review summarizes the research results of MPs/NPs from the entire biosphere, which not only provides a more complete and updated knowledge system for scientists in this field, but also provides a solid foundation for future research directions.