**2. Ecological impact**

Over the years, several studies across the globe have reported MPs in different environmental compartments including rivers, lakes, estuaries, oceans, harbours, groundwater and in the atmosphere, as well as in Antarctica. Once they enter the environment, their residence time lasts for decades due to their low degradation rates, resulting in long-lasting impacts [15–19].

In natural environments, MPs are exposed to a variety of degradation processes through different environmental conditions including weathering, biodegradation, oxidation, mechanical forces and phytodegradation. Phytodegradation of MPs is reported to produce greenhouse gases (GHGs), mostly, methane and ethylene, thus, contributing to climate change. The emission of GHGs by the degradation of MPs is relatively low, however, with continuous degradation, the same amount of MPs may release more GHGs over time [20–23].

In the atmosphere, MPs can be transported with winds around the earth. Airborne MPs may influence earth's climate by scattering and absorbing solar and terrestrial radiation, leading to atmospheric warming or cooling depending on particle size, shape and composition. However, the radiative effects of airborne MPs on climate are less understood [22, 24].

In marine environments, the widespread of MPs affects the light transmission, thereby influencing the efficiency of phytoplankton photosynthesis, which impacts both their growth and role in balancing the marine environment. Studies have shown that the photosynthetic rate of phytoplankton (*Dunaliella tertiolecta*) is reduced by 45% after being exposed to MPs. Additionally, MPs may influence the circulation of organic matter and nutrients, which affects the carbon stock of ocean [25].

In terrestrial ecosystems, MPs can cause significant environmental changes with potential consequences on soil function, plant growth, soil biota and microbial communities; ultimately, MPs have the potential to impact the biodiversity. When dispersing in the soil matrices, MPs form aggregates and cause alteration in the physical properties of the soil, including water holding capacity, soil bulk density and soil structures. For example, MPs can create channels for water movement in the soil, thus, accelerating the evaporation of soil water. This further leads to destruction in the soil structure, which may result in desiccation cracking on the soil surface. The impact of MPs on the soil is not limited to the physical properties, MPs can also affect soil chemistry, for instance, by altering the levels of dissolved organic carbon,

*Why Microplastics Are Exceptional Contaminants? DOI: http://dx.doi.org/10.5772/intechopen.109173*

#### **Figure 1.**

*Ecological impacts of MPs. Source: Shen et al. [20].*

phosphorus, and nitrogen. This leads to changes in the nutrient cycling processes in the soil. There is also a growing body of evidence suggesting that MPs can affect soilplant interactions, which in turn impacts plant growth. Several studies have reported significant changes in plant biomass, leaf and root traits and tissue elemental composition [26–36]. In short, MPs have profound effects on the ecosystem at all levels (**Figure 1**).

### **3. Toxicity**

The toxicity of MPs comes from (i) their chemical constituents, which include both the polymers (polyaromatic hydrocarbon) and the chemical additives; (ii) the environmental pollutants adsorb onto their surfaces; (iii) pathogens colonized onto their surfaces.

During plastic processing and manufacturing, a variety of chemicals are added to enhance/adjust their properties and to make them into materials fit for intended purposes. Most of these chemicals are toxic and harmful to the environment, such as dyes, phthalates, flame retardants, pigments and stabilizers. Some of these additives tend not to be strongly bound within the matrix of the polymer and they can potentially desorb and be leached out into the host environment [37–39].

On the other hand, due to their small size and greater surface area, MPs have a tendency to adsorb wide range of contaminants from the surrounding media. Pollutants such as persistent organic pollutants (POPs), metals, pesticides and pharmaceuticals are readily bound to MPs. In natural environments, and depending on the prevailing environmental conditions, MPs may act as a sponge removing and/or concentrating these contaminants. It is reported that the concentrations of contaminants on the surface of MPs may reach up to 100-fold higher than the concentrations reported in the surrounding media. Once MPs are ingested, these concentrated contaminants can be released inside organisms. Arguably, the virgin MPs will release plastic additives, while the aged MPs will most likely release adsorbed pollutants. Most of these chemicals are reported to be toxic; for instance, POPs are known to be carcinogenic, while metals are known as endocrine disruptors. Additionally, in aquatic environments, MPs are susceptible to biofouling different pathogens/microbial organisms including fungi, bacteria and algae colonize MPs' surfaces and form biofilms. Therefore, MPs act as carriers or micro-vector for transporting a complex mixture of contaminants (**Figure 2**). The leaching of additives from plastic combined with the chemicals adsorbed to plastic renders MPs a 'cocktail' of toxic contaminants. When particles containing adsorbed chemicals are ingested by an organism, pollutants can be released [9–11, 38, 41–47].

The toxicological effects of the uptake of MPs by several aquatic biotas are reported in a variety of exposure studies, including both physical and bio-chemical changes. For instance, MPs were observed to cause oxidative stress, immune destruction and alterations in the level of enzyme activity, tissue morphologies, kidney functions, gene expression and the total protein and glucose. Further, MPs may inhibit weight gain and growth. This, in addition to physical changes, such as abnormally impaired movement coordination, increased respiration and abnormal swimming patterns [48–53].

**Figure 2.** *Interaction of MPs with co-existing pollutants. Source: Wang et al. [40].*
