Microplastics can break the dynamic balance of normal redox by affecting the activity of antioxidant enzymes in vivo, resulting in the oxidative damage of biological macromolecules and interfering with normal life activities.

Microplastics cause oxidative stress in organisms, which can lead to cell damage and cell membrane structure damage, resulting in a decrease in cell membrane permeability, affecting the normal physiological process of cells (e.g., reducing cell growth rate and fecundity). Isocitrate dehydrogenase (IDH) is essential to maintain the redox balance of cells. Microplastics can inhibit the activity of IDH, leading to oxidative stress and muscle injury in organisms [18]. Photoautotrophic organisms (e.g., algae and cyanobacteria) are the basis of aquatic food networks, which are crucial to aquatic ecosystems. Algae exposed to nanoparticles has been observed to inhibit the photosynthesis of algae and begin to produce reactive oxygen species (ROS). The intake and accumulation of microplastics increase the ROS content in algae cells, leading to oxidative stress in algae cells [19].

Inhibition of acetylcholinesterase (AchE) was found in various organisms exposed to microplastics. It can also be thought of as hindering neuronal function and may decrease neuronal network function [20]. AchE dissolves acetylcholine (ACH) into choline and acetic acid, which are crucial to the normal function of the nervous system [21]. AchE is often used as a biomarker in experiments to determine whether the experimental target causes neurotoxicity. For example, after black crucian carp larvae ate polystyrene microplastics [(24.7 $\pm$ 0.2) nm], their brain tissue showed swelling and low activity [22]. Oliveira et al. [23] studied the impact of polyethylene (PE) microplastic particles on marine organisms, the larvae of Pomatoschistus microps were exposed to a solution of 184 $\mu$g/L PE microplastics (1–5 $\mu$m), the results indicate that the particles inhibited the activity of AchE, leading to the blocking of neurotransmitter transfer in fish and ultimately affecting nerve function.

In modern industrial production, plastic synthesis and processing add organic and inorganic components to plastic to expand the efficiency of the final plastic product. Chemicals such as phthalates, polybrominated diphenyl ethers, and bisphenol A are added to plastics as plasticizers, flame retardants, stabilizers, and fungicides. These substances can thus be used as endocrine disruptors, causing serious endocrine-disrupting effects in development and reproduction [24]. Phthalates and bisphenol A can destroy the thyroid function of amphibians and affect larval development [25]. Moreover, phthalates can cause endocrine disruption in fish by affecting the signaling pathway that targets the nuclear hormone receptor and possibly destroying their endocrine regulation [26, 27].

After biological intake, microplastics can enter the immune system through their accumulation and transfer in tissues and organs and interfere with immune response. Microplastics can interfere with the natural defense mechanism of fish and can be a stress source. Therefore, the entry of microplastics into fish can lead to reduced phagocytic activity of immune cells, reduced cell viability, and destruction of the lysosome membrane [28]. Microplastic exposure may also cause inflammation and affects the activation of pro-inflammatory cytokines. Microplastics can affect the release of cytokines or changes in inflammatory response genes in in vitro cell line models [29].

Adsorption of organic pollutants by microplastics is a notable interaction. Xu et al. [31] found that microplastics could be used as the carrier of sulfate in a water environment. Li et al. [32] found that polyamide particles, which are frequently observed in water environments, can be used as carriers of antibiotics. Rochman et al. [33] found complex metal mixtures on plastic fragments composed of various plastic types. These studies show that microplastics have an adsorption effect on organic pollutants in the environment. Moreover, the type, particle size, and aging degree of microplastics affect the adsorption of organic pollutants. Different types of microplastics have different adsorption capacities for organic pollutants. Wang et al. [34] studied the adsorption effect of PE and nylon fiber on hydrophobic (phenylpropene) and hydrophilic (phenol) organic pollutants; the results showed that the adsorption capacity of PE was approximately 1–2 orders of magnitude higher than that of nylon fiber, indicating the importance of surface functional groups (with or without hydrophilic groups) for plastics. The adsorption process of a polymer is closely related to its particle size. Zhang et al. [35] studied the adsorption of 3,6-dibromocarbazole and 1,3,5,6,8-tetrabromocarbazole on polypropylene microplastics with different particle sizes in simulated seawater; they found that the adsorption capacity of microplastics increased with the decrease in particle size. Napper et al. [4] found that rough particles of pollutants were easier to adsorb than smooth particles; they used adsorption experiments of PE on binary mixtures of organochlorine pesticide bis-p-chlorophenyltrichloroethane (C-DDT) and H-phenylpropene. The aging degree of microplastics influences their adsorption capacity. Alimi et al. [36] found that aged microplastics adsorb more PCBs than unaged microplastics. In addition, there are a variety of adsorption mechanisms in the process of microplastic adsorption of organic pollutants, and the degree of influence of various adsorption mechanisms is jointly determined by the properties of plastic, water environment and organic pollutants.

Microplastics in the environment easily migrate, diffuse, and redistribute globally because of their small particle size, stable chemical properties, and hydrophobicity. Microplastics play a notable role in the transportation of pollutants, especially hydrophobic microplastics [37]. JAVIER et al. [38] studied the adsorption of polychlorinated biphenyls (PCBs) by microplastics; the results showed that the adsorption process was reliable and repeatable for the samples of double distilled water and treated urban sewage, indicating that there were different interactions between organic pollutants and plastics. Wang et al. [34] demonstrated that a high abundance of plastic fibers can lead to a higher pollutant transfer effect than marine sediments. In the study of the interaction between microplastics and freshwater microalgae by Lagarde et al. [39], microplastics colonized and aggregated rapidly by microalgae, which may lead to changes in the buoyancy of polymers. Therefore, the amount of sedimentation of polymers and microplastics differs due to the chemical properties of microplastics, resulting in different transport routes and uneven distribution. The transport routes of microplastics to the water environment include surface runoff, river inflow, sewage discharge, and shipping et cetera. In recent years, the amount of microplastics in rivers has increased year by year, and the prevention and control of microplastics have also been extended from the ocean to the inland. Surface runoff and river inflow are the main ways for the migration of inland pollutants. Meanwhile, rainfall is the most direct factor to form surface runoff. Therefore, rainfall is a major environmental factor for microplastic pollution in inland waters. Xia et al. [40] proved the close relationship between rainfall and microplastic abundance in the rainfall on East Lake: the abundance of microplastics changed significantly and was significantly affected by rainfall.

Microplastics contain toxic substances. Lambert et al. [41] found that the ecological toxicity of microplastics may be related to their physical and chemical properties, including particle size, particle shape, crystal, polymer, and additive components. Microplastics can cause disorders of the intestinal flora, destroying the ratio of probiotics and pathogenic bacteria [42]. Fish intake of microplastic debris at a certain concentration of organic pollutants may damage the endocrine system function of adult fish [43]. Studies have shown that microplastic organic pollutants can be deoxygenated in the intestine of fish [44]. Obviously, microplastics do have toxic effects on organisms.

After microplastics adsorb organic pollutants, the two interact. Studies, for example, Giacomo et al. [45] have shown that microplastics can adsorb polycyclic aromatic hydrocarbons, highlighting the high bioavailability of these chemicals after intake and the toxicological effects of multiple molecular and cellular pathways on microplastics. Studies have also found that when microplastics are exposed to low concentrations (e.g., 2 ug/L) of 17$\alpha$-ethylenediethylene glycol (EE2), the inhibitory effect of EE2 on zebrafish movement is reduced because of the decrease in free dissolved EE2 concentration. However, when the concentration of EE2 is increased to 20 ug/L, EE2 and microplastics show a strong destructive effect on zebrafish [46]. The findings of these studies show that synergistic or antagonistic effects occur when microplastics adsorb persistent organic pollutants.

Data show that microplastics exist in air, drinking water, and food, which are essential for human survival. Microplastics floating in the air enter the human respiratory system through breathing. The intake of food and drinking water containing microplastics transports microplastics into the human digestive system [47, 48, 49]. Results showed that the number of microplastics entering the human body through salt, drinking water, and breathing was (0–7.3) $\times$ 10${}^4$, (0–4.7) $\times$ 10${}^3$, and (0–3.0) $\times$ 10${}^7$ per person per year, respectively [50]. Although the impact of microplastics on human health remains unclear, researchers have generally posited that microplastics have a potential negative impact on human health [51]; for example, microplastics can penetrate the lungs. Furthermore, frequent contact may cause adverse symptoms in the respiratory system and increase the carcinogenic rate [52].

In agriculture, the extensive use of chemical fertilizers and plastic films containing microplastics has caused the microplastic content in agricultural soil to exceed that in the marine environment [53]. However, the study of plant ecotoxicological effects of microplastics remains in the preliminary stage [54]. Only a few investigations have demonstrated the effects of microplastics on plants such as mung bean [55], wheat [56, 57], and soybean [58]. Studies have shown that di (2 – ethyl hexyl) phthalate (DEHP) in PVC agricultural film can be precipitated in large quantities under weak acidic conditions [59], resulting in potential toxicological effects on plants. Liu et al. [60] found that plasticizers in plastics (phthalate esters) can inhibit the germination of wheat seeds and cause programmed cell death of wheat seeds at high concentrations (1500–1800 mg$\cdot$L${}^{-1}$). In addition, the surface of weathered microplastics in the environment is rough and has a large specific surface area and negative charge [61, 62], which can adsorb heavy metals and organic pollutants, becoming the environmental carrier of pollutants and having a certain impact on plants. Therefore, the combined effect of microplastics and heavy metals or pesticides in agricultural systems requires further research. In addition, Giorgetti et al. [63] reported that 50 nm polystyrene nanoplastics could be internalized in onion root division cells, causing oxidative stress, cytotoxicity (e.g., mitotic abnormality), and gene toxicity. Therefore, studying the types and particle sizes of microplastics is essential to revealing the plant toxicity of microplastics.

The amount of plastic fibers found deposited in the air of France is 2–355 / (d$\cdot$m${}^2$), with an average of 0.9 plastic fibers per m${}^3$ outdoor air [64, 65]. Microplastics in the above air can enter the marine environment. At present, although indoor toxicological experiments have demonstrated that microplastics can produce various toxic effects on organisms, the concentration of microplastics used in those experiments is much higher than that in the real environment. Therefore, whether microplastics harm the ecosystem in low concentrations in the real environment remains unknown. Notably, many studies have shown that microplastics can be enriched along the food chain through aquatic products and salt.

Plastic degradation in soils is slow because of the lack of sun exposure and mechanical wear [66]. These microplastics can continue to accumulate in the soil and form a sink of microplastics in the soil. Because microplastics are surrounded by soil particles or combined with soil aggregates, they may affect soil bulk density, permeability, water holding capacity, and water stability agglomeration in soil [67]. Anderson et al. [68] studied the effects of four types of microplastics (e.g., PP and PET) on bulk density, water holding capacity, and water stability agglomeration of sandy soil and found that only PET had a negative correlation between soil bulk density and water stability agglomeration, and other types of microplastics had no effect. However, Zhang et al. [69] demonstrated a different result: PET had a negative impact on soil water holding capacity and had no effect on soil bulk density [70]. This difference may be due to the use of different soil physicochemical properties in the experiment. In addition, microplastics may change soil porosity through permeability or affect soil permeability and further affect water evaporation in soil. Due to the lack of research on this aspect, further exploration is needed to fully verify the above theories.