Introduction

Nanoplastics contamination in the ecosystem has recently aroused global concern (Sharma et al. 2023). These plastic fragments, typically smaller than 1μm, are prevalent in aquatic, terrestrial, and atmospheric environments (da Costa et al. 2016; Klaine et al. 2012; Song et al. 2022). They originate from the physical weathering, chemical erosion, and biodegradation of larger plastic products, as well as from wastewater treatment processes and the use of personal care products (Carr et al. 2016; Desforges et al. 2014; Duis and Coors 2016). The widespread use of polyethylene films in agriculture and other industrial activities further contributes to the release of nanoplastics into the environment (Liebezeit and Liebezeit 2014; Zhang et al. 2012). Their ubiquitous presence classifies them as emerging environmental pollutants with unique impacts due to their nano-scale properties.

Accurate detection of nanoplastics is crucial for understanding their environmental impact. Current detection methods face challenges related to selectivity, size, and quantity (Schwaferts et al. 2019; Zarfl 2019). Most research has focused on laboratory methods, but recent advances have improved the detection of nanoplastics in real environmental samples, often requiring a combination of techniques (Oliveira and Almeida 2019). Techniques like asymmetric flow field-flow fractionation (AF4), membrane filtration, mass spectrometry, and optical-based methods are commonly used (Habumugisha et al. 2022; Jimenez-Lamana et al. 2020; Battistini et al. 2021). The development of reliable detection methods is essential for assessing nanoplastics’ effects on ecosystems.

Nanoplastics can enter various environmental matrices through different pathways. For instance, thermal cutting of polyethylene foam in industrial activities releases plastic fragments ranging from 20 to 220 nm into the air (Zhang et al. 2012). Moreover, 3D printing technology, widely used for rapid prototyping and small-scale model building, has been reported to release ultrafine particles in the range of 11.5–116 nm into the air (Stephens et al. 2013). The use of sewage sludge as fertilizer and even domestic activities like cloth drying can also contribute to the presence of nanoplastics in the environment (Liebezeit and Liebezeit 2014).

The impact of nanoplastics on the nitrogen cycle, a critical biogeochemical process, has been documented. Nanoplastics can disrupt microbial functions, leading to oxidative stress and enzyme inhibition, affecting nitrogen cycling (Ma et al. 2022; Sun et al. 2018). The surface charge of nanoplastics plays a significant role in their toxicity, with positively charged nanoplastics showing higher toxicity (Liu et al. 2022a, b). Understanding how nanoplastics affect key functional bacteria involved in nitrogen cycling is crucial for assessing their broader environmental impact (Miao et al. 2019; Yang et al. 2020). This review is different from earlier review article in nanoplastics (Table 1).

Table 1 Differences between this article and other review articles on nanoplastics
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Here, we review the occurrence, detection methods, and environmental effects of nanoplastics, particularly on nitrogen cycling (Fig. 1). We examine the presence of nanoplastics in various environments; we summarize and evaluate the latest nanoplastics detection methods, discussing their advantages and limitations; and we review and discuss the impact of nanoplastics on the nitrogen cycling process.

Fig. 1
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Source, occurrence, fate, and effect of nanoplastics in the environment and their detection. TOC: total organic carbon

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Nanoplastics occurrence in the environment

From 1950 to 2015, the plastic production has rapidly increased from 2.3 to 448 million tons. It is predicted that the plastic production could be doubled by 2050. Approximately, 8 million tons of plastic trash are ended in the ocean each year from coastal countries (Fig. 2). The environment is being increasingly contaminated by a substantial influx of nanoplastics due to improper handling and the escalating utilization of plastic products. Plastic products could transform into nano-scale plastics by photodegradation, biodegradation, mechanical abrasion, weathering, and thermal oxidation deterioration in the environments. The occurrence and fate of the nanoplastics could be varied in different environmental matrix. Earlier studies reported the occurrence of nanoplastics polyethylene terephthalate and polypropylene in the surface snow and snow pits in the Austrian Alps (Atugoda et al. 2022). In Australia, North America, and Europe, a significant amount of microplastics is released into agricultural and farm soil with a conversion rate of 100%, resulting in an annual addition of nanoplastics ranging from 2.3 to 63 t ha−1 into the agro-soil system (Chen et al. 2023). Ter Halle et al. (2017) reported the existence of nanoplastics in the North Sea, where polyethylene, polyvinylchloride, and polyethylene terephthalate were found.

Fig. 2
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a Proportion of plastic demand across various industries. b Market share of different types of plastics. c Primary sources of nanoplastics in environmental pollution. c Sources of nanoplastics in the environment. These sources contribute plastics to the environment, which, through various physical, chemical, and biological processes, ultimately form nanoplastics with particle sizes smaller than 1 μm, impacting the ecological environment (Osman et al. 2023; Han et al. 2024).

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However, there is still lacking of standard detection methods for nanoplastics from environmental samples, so research about the accurate concentration of nanoplastics in the ecosystem is rare. Many of the earlier work investigated the environmental effect of nanoplastics by laboratory-scale experiments, where engineered nanoplastics were used as model. The characteristics of nanoplastics, its biological and ecological impacts, could be more complicated and ambiguous as compared with microplastics. It is also declared by earlier work, nanoplastics could be the least known but most hazardous marine litter. The detection technologies of nanoplastics from environmental samples should be developed and improved, which could benefit the study of nanoplastics effect to the eco–system.

Methods for nanoplastics analysis

Nanoplastics detection is still facing technical challenges. Proper pretreatment is necessary for the detection of nanoplastics from environmental samples, including (i) sampling, (ii) isolation and extraction, (iii) separation and preconcentration, (iv) size determination, (v) identification, and (vi) quantification (Asamoah et al. 2021; Strungaru et al. 2019; Velimirovic et al. 2021). Quantitative analysis methods of the nanoplastics from environmental samples were reviewed in this work, including mass spectrometry-based methods, optical principle-based methods, and TOC-based methods (Fig. 3). The detailed information of different nanoplastics detection methods was also reviewed (Table 2). The type of nanoplastics, size range, specific detection methods, and their limits of detection were compared.

Fig. 3
figure 3

Detecting methods for nanoplastics. Triton X-15 is an oil-soluble emulsifier and solutizer. TOC (Total Organic Carbon) is the total organic carbon. The chemical formulas include chlorosulfonic acid (ClSO2OH), dichloromethane (CH2Cl2), ferric ion (Fe3+), hydrogen peroxide (H2O2), ferrous ion (Fe2+), and potassium persulfate (K2S2O8).

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Table 2 Methods for nanoplastics detection
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Mass spectrometry

The concentration of nanoplastics in environmental samples is usually low, and many conventional assays are unable to detect them. The excellent detection capabilities of inductively coupled plasma mass spectrometry (ICP-MS) in single particle mode (SP-ICP-MS) have made it a viable candidate for the analysis of nanoplastics (Marigliano et al. 2021). For metal nanoparticles, ICP-MS is a commonly used detection method that provides data on the elemental composition, size (spherical equivalent diameter [nm]) and size range, density (particles/mL), and concentration (mg/L) of the particles (Velimirovic et al. 2021). To solve the difficulties of carbon detection by ICP-MS, Jimenez-Lamana et al. (2020) developed a method to conjugate nanoplastics with nanoparticles (NPs) containing functionalized metals (Au) so that they can be detected in single particle (SP) mode (SP-ICP-MS). The selectivity of the method is achieved by coupling negatively charged carboxyl groups on the surface of the nanoplastics to the positively charged gelatin attached on custom synthesized Au nanoparticles. The adsorbed gold generates an SP-ICP-MS signal that allows the counting of individual nanoplastic particles for their precise quantification (error < 5%). The minimum size that could be detected and quantified depends on the functionalization degree and surface labeling availability. The lowest size detectable by this strategy is reported to be 135 nm for fully functionalized nanoplastics.

Marigliano et al. (2021) exploited the properties of nanoplastics that can easily adsorb compounds and explored three different labeled metal probes: metal ions, hydrophobic organometallic compounds, and inorganic nanoparticles. It is found that the strategy of labeling by positively charged gold nanoparticles showed good results in both size determination and quantification. However, whether this method could accurately determine nanoplastics under interference from natural organic matter and other natural colloids remains unclear. Although the previous studies have demonstrated the feasibility of ICP-MS for the detection of nanoplastics, various interferences still need to be excluded for the detection of environmental samples. For real environmental water samples, Jimenez-Lamana et al. (2020) pretreated with acid digestion using a mixture of 5 mM HNO3 and 40 mM HF, followed by effective separation of NPs from various matrices by in situ labeling with gold nanoparticles. The pretreatment has improved the gold labeling efficiency, and the nanoplastics recovery rate also increased (72.9–92.8%).

Pyrolysis gas chromatography–mass spectrometry (Py-GC–MS) thermally degrades polymer mixtures into their monomers and additives at high temperatures (> 500 °C) in an inert environment. The ability to selectively identify and quantify various polymers in environmental samples using a fully automated system is one of the key benefits of Py-GC–MS (Velimirovic et al. 2021). Zhou et al. (2019) first proposed a Triton X–45 (TX-45)-based cloud point extraction (CPE) to preconcentrate trace amounts of nanoplastics in environmental waters. After heat treatment of the preconcentrated samples at 190 °C for 3 h, the obtained CPE extracts could be detected by GC–MS and the nanoplastics can be quantified. This method showed good reproducibility and high sensitivity for Polystyrene nanoplastics at 66.2 nm and PMMA nanoplastics at 86.2 nm, as an example. Zhou et al. (2021) and Li et al. (2022a, b) proposed different sample pretreatment methods to quantify nanoplastics by Py-GC–MS, protein corona-mediated extraction and agglomeration with alkylated ferroferric oxide followed by micropore membrane filtration. The Py-GC/MS-based detection could be used for environmental water samples and cucumber plant samples (Xu et al. 2022a, b; Li et al. 2021).

Thermodesorption–pyrolysis GC/MS (TD-PTR-MS) is a new technique to the identification and quantification of nanoplastics with size less than 20 μm and in low concentration. The creation of hydronium ions from water vapor in an ionizer is the basis of this technology. The analytes are interacting with previously produced hydronium ions in a drift chamber. The proton transfer process produces ions that are separated depending on their mass-to-charge ratio in a mass analyzer before identification (Reichel et al. 2020).

Matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF-MS) has recently emerged as a potent method for identifying minute plastic debris in complicated environmental matrix. Due to its mild ionization and broad mass detection range, this approach is already well established for (bio)polymer analysis. Recently, for the first time, thermal fragmentation and MALDI-TOF MS were used to identify and quantify polystyrene and polyethylene terephthalate nanoplastics. Based on the fingerprint peaks in both the low and high m/z bands, data were gathered. This process is quick and easy to use and shows good performance for both identifying and quantifying tiny plastic (Lin et al. 2020; Wu et al. 2020). Similar with Py-GC–MS, further investigation in combine with other process should be considered in the context of determining particle size (Chou et al. 2022; Habumugisha et al. 2022; Sullivan et al. 2020; Velimirovic et al. 2021; Yakovenko et al. 2020).

To summarize briefly, mass spectrometry-based nanoplastic detection method has great advantages in obtaining nanoplastics concentration. The mass spectrometry-based technology pretreatment process requires destroying the nanoplastics, thus the original structure and size of nanoplastics from the environmental samples were lose. The mass spectrometry-based technology has difficulty to repeat the detecting, further study should focus on improve the accuracy and repeatability of this series of detecting methods.

Optical instruments

Asymmetrical flow field-flow fractionation (AF4) has been regarded as one of the most promising methods in characterizing colloidal particles during the last decade. The main principles of this technology are based on fluid dynamics in microscale channels and Fick’s law, analytes are separated according to their diffusion properties (i.e., diffusion coefficients) (Gigault et al. 2017). Gigault et al. (2017) established four particle size subdivision methods to achieve high-resolution separation at different particle size levels: 10–100 nm, 100–200 nm, 200–450 nm, and 450–800 nm in diameter. Correia and Loeschner (2018) further demonstrated the feasibility of separating nanoplastics by AF4 by coupling this technique with a multi-angle light scattering technique (AF4-MALS). The detection method of AF4-MALS is limited for only polyethylene and polystyrene in environmental samples. Battistini et al. (2021) further proposed asymmetric flow field flow-field fractionation (AF4) coupled with multi-angle light scattering (MALS) and ultraviolet diode array (UV-DAD) detectors. By this technology, plausible peak resolution and selectivity were obtained by using 20, 60, 100, and 200 nm polystyrene nanoplastic mixture solutions for separation determination. This process has great potential for simultaneous detecting nm polystyrene nanoplastic mixtures of different sizes and concentrations and can be used for different materials nanoplastics.

The principle of Raman spectroscopy detection is based on the Raman scattering effect; the scattering spectra with different frequencies from the incident light could obtain information on the vibration and rotation of molecules (Chang et al. 2022; Schwaferts et al. 2020). Schwaferts et al. (2020) achieved particle separation/characterization by the combination of field flow fractionation (FFF), ultraviolet, and multi-angle light scattering with subsequent chemical identification by online Raman microspectroscopy (RM). However, this method has a minimum detection concentration of 1 mg/L, which greatly limits its application in the detection of environmental samples. Although subsequent attempts were made by Caldwell et al. (2021) to create surface-enhanced Raman scattering spectroscopy substrates by using spherical gold nanoparticles with diameters of 14 nm and 46 nm to improve the scattering signal obtained during Raman spectroscopy measurements, the experimentally obtained detection limit was as low as 32 mg/L, which is still difficult to meet the direct determination of environmental samples. By exploiting the coffee ring effect, Chang et al. (2022) develop a novel nanowell-enhanced Raman spectroscopy (NWERS) substrate composed of self-assembled SiO2 sputtered with silver films (SiO2 PC@Ag). Individual polystyrene nanoplastic particles less than 200 nm were directly visible on the NWERS substrate and could achieve Limit of detection (LOD) of 5 ppm in bottled water, tap water, and river water. A combined method involving membrane filtration and surface-enhanced Raman spectroscopy (SERS) was proposed by Yang et al. (2022a, b). A bifunctional Ag nanowire membrane was employed to enrich nanoplastics and enhance their Raman spectra in situ, which avoid sample transfer and loss. Polystyrene nanoplastics at concentrations of 10–1–10–7 g/L and sizes of 50–1000 nm were successfully detected by Raman maps, and the feasibility of the method was verified in environmental water samples. Hu et al. (2022) proposed the use of Ag nanoparticles as adsorption nuclei for nanoplastics in environmental samples, KI was added to Ag nanoparticles as coagulant and detergent to remove surface impurities, and subsequently the Ag nanoparticles were detected by Raman spectroscopy. Four different sizes (50, 100, 200, and 500 nm) of polystyrene (PS) nanoplastics were used to evaluate this proposed method, which has exhibited high sensitivity (detection limit of 6.25 μg/mL for 100 nm polystyrene nanoplastics), interference resistance, good reproducibility, and quantitative analysis. Finally, the environmental water sample was analyzed to verify the feasibility of this process. High recovery rate (87.5–110%) was obtained for nanoplastics with different sizes and concentrations.

Total organic carbon

Total organic carbon (TOC)-based detection methods become famous in the determination of nanoplastics in recent years. The nanoplastics are composed of organic carbon which allows them to be detected by TOC detection. While the sample collected from the environment could also contain complicated organic matters and interfere the detection of the nanoplastics, thus, the TOC-based nanoplastics detection technique focuses more on the removal of organic matter from the sample.

Li et al. (2022a, b) proposed membrane filtration and chemical pretreatment as pretreatment process for environmental samples, to achieve nanoplastics determination by TOC. Nanoplastics are a wide range of carbon-containing particles that are mostly made up of the six elements C, H, O, N, S, and Cl. This is quite similar with the dissolve organic matters such as protein, polysaccharides, humic acid, fulvic acid, lipid, and so forth. The composition and characteristics of nanoplastics are also similar with particulate black carbon (PBC). Particulate black carbon is a complex carbonaceous continuum produced by incomplete combustion of fossil fuels. Although many techniques have been developed, TOC method is one of the most widely used to quantify particulate black carbon after coexisting inorganic carbon and organic non–black carbon matter being removed (Li et al. 2022a, b). This implied that with proper pretreatment to remove the non-nanoplastic organic compounds from environmental samples, the nanoplastics also could be detected by the TOC-based detection.

As reported by earlier work, two parallel water samples were filtered with carbon free glass fiber membranes (pore size: 0.3 μm). One of the filter membranes with collected particulate matter was pretreated with potassium persulfate oxidation and Fenton digestion, and the total concentration of microplastics and nanoplastics (MNP) and particulate black carbon (PBC) was quantified as TOCMNP&PBC using a TOC analyzer. Another filter membrane was pre-treated with sulfonation and Fenton digestion to quantify particulate black carbon as TOCPBC, and TOCPBC was subtracted from TOCMNP&PBC to calculate the TOC of microplastics and nanoplastics. The feasibility of this method was verified by the determination of various microplastics and nanoplastics with representative plastic types and sizes (0.5–100 μm) in tap water, river water and seawater samples with low detection limits (7μgC L−1) and high spiked recoveries (83.7–114%). The researcher also rejected small size nanoplastics with a 20 nm pore size alumina membrane, achieving a high retention of 97% for 0.11 μm PMMA nanoparticles and 0.07 μm polystyrene nanoparticles. By designing sequence filtration step, the concentration of nanoplastics with different size can be determined by this method. This method is suitable to be used for the detecting of nanoplastics from environmental samples; the filtration under low pore size membrane might be a problem for many researchers. This process could only detect the concentration and rough size of the nanoplastics, which should be improved in the further study.

Effect of nanoplastics on the nitrogen cycle

Nitrogen is essential for the biosynthesis of important cellular components like proteins and nucleic acids. Approximately, 50% of the world’s population relies on synthetic fertilizers to produce food (Gruber and Galloway 2008; Kuypers et al. 2018). The use of fertilizer has almost doubled the nitrogen input to terrestrial and marine ecosystems. Microbial nitrogen transformation mechanisms play a crucial role in the global nitrogen cycle in the natural environment (Fig. 4). As an emerging pollutant, nanoplastics could affect the function of organisms associated with the nitrogen cycle (Gruber and Galloway 2008; Kuypers et al. 2018). The effect of nanoplastics to the nitrogen cycling in the environment was comprehensively reviewed (Table 3).

Fig. 4
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Nitrogen cycling in natural systems

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Table 3 Research reports on the effect of nanoplastics to the nitrogen cycling process
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Artificial wetlands, as an important environmental remediation facility, are also an important nano-plastic sink in the environment. The contamination of nanoplastics to the wetland has aroused wide concern in recent decade (Fig. 5). Yang et al. (2020) reported that the exposure to nanoplastics has decreased the total nitrogen removal in artificial wetlands by 29.5–40.6%. Polystyrene nanoplastics could penetrate cell membranes, disrupting membrane integrity and the balance of reactive oxygen species. In addition, Polystyrene nanoplastics has inhibited the microbial activities in vivo, including enzyme (ammonia monooxygenase, nitrate reductase and nitrite reductase) activities and electron transport system activities (ETSA). These adverse effects, accompanied by a decrease in the relative abundance of nitrifying bacteria, e.g., Nitrosomonas and Nitrospira, and denitrifying bacteria, e.g., Thauera and Zoogloea, and directly contributed to the deterioration of nitrogen removal.

Fig. 5
figure 5

Negative effects of nanoplastics to microorganism and plants. The upper figure represents a schematic diagram of a wastewater treatment plant, with accompanying text summarizing the adverse effects of nanoplastics on microorganisms. The bottom figure depicts a schematic of an artificial wetland, accompanied by text summarizing the negative impact of nanoplastics on plants. ABC transporter stands for ATP-binding cassette transporter, which is involved in the transport of various molecules across cellular membranes

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Moreover, exposure to nanoplastics also could lead to a decrease in leaf and root activity and thus affected the nitrogen uptake rate by plants. Subsequently, Xu et al. (2022a, b) constructing 12 nanoplastic exposed wetland systems over a period of 300 days, at different sizes (3 mm–60 nm) and concentrations (10–1000 µg/L). Results showed that the accumulation of nanoplastics was a decisive disturbance to microbial communities. Ma et al. (2021) evaluated the effects and toxicity mechanisms of the nanoplastics to the wetland system by metagenomic analysis. The results were in consistent with earlier work in terms of enzyme activities (Xu et al. 2022a, b) and found that electron transport system activity (ETSA) was also inhibited by nanoplastics. Furthermore, metagenome-based results implied that the polystyrene nanoplastics have inhibited the relative abundance of genes involved in nitrogen transformation, including the biochemical metabolic processes of nitrification and denitrification (electron production, electron transport, and electron consumption processes). It also showed that polystyrene nanoplastics can affect nitrogen transformation by reducing the abundance of electron donor and ATP-producing genes involved in carbon metabolism (glycolysis and tricarboxylic acid cycle metabolism).

The effects of nanoplastics on macrophytes in artificial wetlands were also reported by earlier study. The results indicated that polystyrene nanoplastics significantly affected the superoxide dismutase, catalase, peroxidase activity, and the malondialdehyde content in all phases of the plants. Results shown that 10 mg/L polystyrene nanoplastics have inhibited the biosynthesis of major photosynthetic pigments. The metagenomic analysis and taxonomic annotation revealed a negative effect of polystyrene nanoplastics on major functional bacteria involved in the nitrogen cycling (Ma et al. 2022). Functional annotation analysis revealed that some indispensable metabolic pathways (oxidative phosphorylation, pyruvate metabolism, ABC transporter, two-component system, biofilm formation, etc.) were also inhibited. In addition, nanoplastics inhibited the overall pattern of genes related to nitrogen metabolism and part of the glucose degradation related functional genes (Ma et al. 2022). Liu et al. (2022a, b) used the typical submerged macrophyte Utricularia vulgaris (U. vulgaris) as a model plant to conduct nanoplastic exposure experiments. It is reported that 291 out of 548 metabolites route were changed, with 25–34% of the metabolites route having up-regulated and 32–40% were down-regulated. The tricarboxylic acid cycle and amino acid metabolic pathways were disrupted. The reason that nanoplastics affected the gene regulation requires further investigation.

The nanoplastics contamination in high concentration wastewater treatment system has also been reported by earlier work. Song et al. (2022) investigated the effect of continuous exposure of nanoplastics (R1:0, R2:10, R3:100, R4:1000 μg/L) on nitrification and denitrification process in a sequencing batch reactor wastewater treatment system for more than 200 days. The results showed that although nanoplastic exposure shows no significant inhibition to total nitrogen removal, the ammonia oxidation rate and denitrification rate were significantly reduced as compared with control group (R1: 0 μg/L). Moreover, the maximum reaction rate (Vmax) of N2O reduction was improved after long-term exposure to high concentration of nanoplastics. The effect of nanoplastics on nitrifying and denitrifying bacteria was different, with the percentage of denitrifying bacteria in the top 20 genera increasing from 31.76 to 63.42%. The dominant genera in R4 and R3 were Thauera, Azoarcus and Defluviicoccus, indicating that they were tolerant to nanoplastics. The functional prediction results showed a significant enhancement of membrane transport function and a significant decrease of lipid metabolism function in R4. This may be due to the adsorption of nanoplastics on the bacteria cell surface and stimulate function change.

The novel nitrogen removal process affecting nanoplastics was also investigated by researchers. Xu et al. (2022a, b) examined the effects of continuous exposure to Polystyrene NPs (1–2 mg/L for 225 d) on the denitrification performance in an anaerobic ammonia oxidation (Anammox) system. It was reported that exposed to low concentrations of Polystyrene NPs had no significant effect on the Anammox denitrification performance (Thamdrup 2012; Xu et al. 2022a, b). Significant inhibition occurred when polystyrene nanoplastics accumulated to 118 mg polystyrene nanoplastics/g VSS. The nitrogen removal performance of the reactor was stable during the first 100 days, and dramatically decreased at the 200–225 days. This inhibition effect could be attributed to the bacteria cell membrane damage by nanoplastics, which resulted in a reduction of ammonia oxidation rate and mass transfer rate.

Sun et al. (2018) investigated the effect of plastic debris on the inorganic nitrogen conversion efficiency of marine bacteria. The polystyrene nanoplastics on the marine bacterium Halomonas alkaliphila growth inhibition, chemical composition, inorganic nitrogen conversion efficiency, and reactive oxygen species (ROS) production were investigated. The results showed that nanoplastics at high concentration could inhibit the H. alkaliphila growth rate, chemical composition, and ammonia conversion efficiency. In addition to this, nanoplastics were found to induce oxidative stress in bacteria, resulting in elevated levels of reactive oxygen species. Moreover, positively charged amine-modified nanoplastics induced stronger oxidative stress (Saygin and Baysal 2021; Silva de Oliveira et al. 2020; Sun et al. 2018). This result is consistent with earlier work (Song et al. 2022; Tang et al. 2022; Zhou et al. 2022).

Miao et al. (2019) investigated the effects of polystyrene beads (polystyrene, ranging from 100 nm to 9 mm in diameter) on biofilms (Fig. 6). The results showed that the effect of 100 nm polystyrene nanoplastics on biofilms was highly dependent on the surface modification of Polystyrene particles, with positively charged polystyrene nanoplastics (amide modified) being the most toxic to biofilms. Moreover, the excess production of reactive oxygen species (ROS) indicates the oxidative stress induced by polystyrene nanoplastics. To counteract this change, the increase in total antioxidant capacity (T-AOC) indicates that the antioxidant activity of biofilms was enhanced, which is in consistent with earlier work (Sun et al. (2018). High concentrations of polystyrene beads (100 nm, 100 mg/L) significantly reduced the chlorophyll-a content and functional enzyme activities of b-glucosidase and leucine aminopeptidase. This suggests that nanoplastics could have negative impact on nitrogen cycling in freshwater biofilms. Subsequently, Miao et al. (2022) combined high-throughput gene chip (GeoChip 5.0) technology to expose the effect of nanoplastics to the freshwater biofilms. The results showed that the abundance of genes involved in total nitrogen cycling was increased under polystyrene nanoplastics exposure. The most significant increase in the abundance of nitrogen fixation genes (24.4%) was experienced under 1 mg/L polystyrene nanoplastics treatment. The inner mechanism requires further investigation.

Fig. 6
figure 6

Impact of nanoplastics on nitrogen cycling in ecosystems, with emphasis on biofilm microorganisms, algae, plant roots and leaves, and soil. ROS refers to reactive oxygen species

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Nitrogen (N) cycling is a key predictor of ecological stability and management in terrestrial ecosystems. The effects of nanoplastics on N cycling in soil ecosystems were also been reviewed. Nanoplastics dispersed in the soil may be consumed by animals such as the soil oligochaete Enchytraeus crypticus (Fig. 6). It was shown that the composition of the gut microbial community of soil animals was largely altered after culturing with plastic debris. The relative abundance of Rhizobacteriaceae, Flavobacteriaceae, and Heterosporaceae, which contribute to nitrogen cycling and organic matter decomposition, was significantly reduced (Zhu et al. 2018). The presence of nanoplastics also affects plant growth, seed germination and gene expression, and induce cytotoxicity by exacerbating the production of reactive oxygen species (Maity and Pramanick 2020). Furthermore, in studies on the effects of nanoplastics on wheat, metabolomic analyses revealed that all polystyrene NPs treatments altered the metabolic profile of leaves mainly by regulating energy and amino acid metabolism pathways (Lian et al. 2022, 2020). The presence of different types and sizes of plastic polymers in the soil also differentially affects soil nitrogen transformation (Iqbal et al. 2020). Zou et al. (2022) used cabbage (Brassica campestris ssp.) as a model plant to study the effects of nano-polystyrene on fertilizer nitrogen (N) translocation, gaseous N loss and soil microbial community. The results revealed that exposure to nanoplastics resulted in more N uptake by plant roots, and the above-ground parts of the plants have less nitrogen. The presence of polystyrene nanoplastics had a significant effect on soil mineral N (Guo et al. 2023; Zou et al. 2022). The addition of polystyrene nanoparticles reduced nitrous oxide and ammonia emissions in the soil by 27% and 37%, respectively, and led to a decreasing abundance of ammonia oxidation genes but had a very different effect on denitrification genes. Metagenomic sequencing data showed that nano-polystyrene had no significant effect on N cycling pathways, while it significantly altered the composition of bacterial and fungal communities (Zou et al. 2022).

To summarize, the studies on the effects of nanoplastics exposure to the nitrogen cycle process were mainly done at the laboratory scale. Nanoplastics exposure has negative effect to the microbes, biofilm, plants, and so forth. Thus, the corresponding wastewater treatment process, natural nitrogen cycling process in aquatic system, and soil matrix were inhibited. Besides which, the effect of nanoplastics to the real ecosystem is still not clear yet and rarely reported.

Conclusion

This study reviewed nanoplastics occurrence in ecosystems, the cutting edge nanoplastics detection methods, and nanoplastics effect to the nitrogen cycling. The results indicated that due to the limitation of nanoplastics detection methods, the occurrence and fate of nanoplastics in the environment was rarely reported and requires further investigation. The TOC-based methods could be developed as a cost-effective process for environmental matrix nanoplastics detection in the near future. The nanoplastics effect to the nitrogen cycling has been investigated in different system, such as wetland system, wastewater treatment process, soil system, and so forth. Microbial analysis and metagenomic analysis were conducted for the illustrating of nanoplastics impact to the nitrogen cycling. It is implied that nanoplastics could adsorbed on the cell membrane and disrupt the membrane integrity and thus affected the microbial function. Nanoplastics also could induce oxidative stress to the microbe cell and reducing the key enzyme activity. While most of the research of nanoplastics effects was based on laboratory scale experiment, whether the nanoplastics dose is similar to its real value in the environment is not clear yet. This work enhances the understanding of the cutting edge nanoplastics detection methods and its effect to the nitrogen cycling. The detecting methods for nanoplastic in ecosystem should be further improved and standardized, by which, the effect and mechanism of nanoplastics to the ecosystem can be clearly clarified.