Journal of Hazardous Materials 392 (2020) 122273
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Ecotoxicological eﬀects of microplastics and cadmium on the earthworm Eisenia foetida
Yanfei Zhoua,c, Xiaoning Liua, Jun Wanga,b,* a
Key Laboratory of Aquatic Botany and Watershed Ecology, Wuhan Botanical Garden, Chinese Academy of Sciences, Wuhan, 430074, China College of Marine Sciences, South China Agricultural University, Guangzhou, 510642, China c University of Chinese Academy of Sciences, Beijing, 100049, China b
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Editor: Deyi Hou
As microplastics (MPs) have become ubiquitous in both aquatic and terrestrial environments, there has been a growing concern about these new anthropogenic stressors. However, comparatively little is known about the negative eﬀects of MPs, co-contamination of MPs and heavy metals on terrestrial organisms. The objective of this study was performed to understand the adverse eﬀects of exposure to MPs and co-exposure to MPs and cadmium (Cd) on the earthworm Eisenia foetida (E. foetida). Results showed that exposure to MPs only or to a combination of MPs + Cd decreased growth rate and increased the mortality (> 300 mg kg−1) after exposure for 42 d, with MPs + Cd (> 3000 mg kg−1) posing higher negative inﬂuence on the growth of E. foetida. Exposure to MPs might induce oxidative damage in E. foetida, and the presence of Cd accelerates the adverse eﬀects of MPs. Furthermore, the MPs particles can be retained within E. foetida, with values of 4.3–67.2 particles·g-1 earthworm, and can increase the accumulation of Cd in earthworm from 9.7%–161.3%. Collectively, the results of this study demonstrate that combined exposure to MPs and Cd poses higher negative eﬀects on E. foetida, and that MPs have the potential to increase the bioaccessibility of heavy metal ions in the soil environment.
Keywords: Microplastics Eisenia foetida Heavy metals Combined exposure Oxidative stress
⁎ Corresponding author at: Key Laboratory of Aquatic Botany and Watershed Ecology, Wuhan Botanical Garden, Chinese Academy of Sciences, Wuhan, 430074, China. E-mail address: [email protected]
https://doi.org/10.1016/j.jhazmat.2020.122273 Received 2 June 2019; Received in revised form 29 January 2020; Accepted 10 February 2020 Available online 11 February 2020 0304-3894/ © 2020 Elsevier B.V. All rights reserved.
Journal of Hazardous Materials 392 (2020) 122273
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environments. They detected oxidative damage in the tissues of zebraﬁsh; in addition, the accumulation of Cd was clearly enhanced after zebraﬁsh were exposed to both MPs and Cd for three weeks. Likewise, MPs in terrestrial environments can act as well as vectors for increasing metal exposure to soil organisms (Hodson et al., 2017). It is reasonable to assume that MPs combining with none-essential elementary metals pose higher associated risks. Cadmium is one of the most toxic heavy metals and is commonly encountered in soil; its typical background concentrations are in the range of 2–20 mg kg−1 (Khan et al., 2017). MPs in soil are highly likely to form combined pollutants with heavy metals, and thus, it is important to consider the synergic pollution of MPs and heavy metals when assessing the risk of MPs to soil fauna. Accordingly, the speciﬁc objectives of the current study were to ascertain: (1) whether adverse eﬀects on Eisenia foetida (E. foetida) would be induced by polypropylene microplastic (hereinafter denoted as MPs) and MPs + Cd; (2) whether MPs particles could be retained within E. foetida; and (3) whether the accumulation of Cd in E. foetida would be enhanced by the presence of MPs.
Microplastics (MPs) pollution has been gradually increasing as a global environment issue (Alimba and Faggio, 2019; Peeken et al., 2018; Rillig et al., 2019). MPs are usually deﬁned as plastic debris with diameters smaller than 5 mm. This new anthropogenic ubiquituous stressor occurs in aquatic and terrestrial environments(Alimba and Faggio, 2019; Peeken et al., 2018, Rillig2019; da Costa Araujo et al., 2020; Machado et al., 2018). They may be speciﬁcally produced within a micrometre size range (e.g., microbeads and abrasive scrubbers in cosmetic and personal products) (Anderson et al., 2016; Hodson et al., 2017) or derived from the breakdown of large plastic products (Song et al., 2017; Weinstein et al., 2016). MPs can be deposited on environment via discharging of sewage sludge, degradation of large plastic items, stormwater runoﬀ ;, atmospheric fallout, and other sources (Blasing and Amelung, 2018; Dris et al., 2016). Until recently, MPs were mainly found widely distributed in oceans, freshwater, beaches, and sediments (Peeken et al., 2018; Eo et al., 2018; Hendrickson et al., 2018; Wang et al., 2017). Although less widely reported, MPs also occur in terrestrial environments and have been detected both in industrial and remote areas at concentrations as high as 67 g·kg−1 (Rillig et al., 2019; Machado et al., 2018; Fuller and Gautam, 2016; Scheurer and Bigalke, 2018; Zheng et al., 2019). The negative biological eﬀects of MPs are thought to be induced by two mechanisms (Rillig et al., 2017). First, small-sized MPs are accidently ingested, causing direct damages to organisms, and may eventually be accumulated along food webs (Diepens and Koelmans, 2018; Chen et al., 2019). Secondly, MPs provide high speciﬁc surface area upon which, other environment pollutants may adsorb; thus, the MPs act as vectors transferring these contaminants (Holmes et al., 2014; Liu et al., 2019; Tang et al., 2018). Some implications of MPs ingestion have been demonstrated in previous studies, such as increase of mortality and oxidative stress (Lu et al., 2018; Huerta Lwanga et al., 2017a; Yu et al., 2018) and decrease of energy reserves and growth rate (Huerta Lwanga et al., 2017a; Wright et al., 2013). For example, in a terrestrial environment, (Huerta Lwanga et al., 2017a) reported that the mortality and growth rate of Lumbricus terrestris (L. terrestris) were negatively impacted after exposure to MPs. In another study, Yu et al. (2018) showed that oxidative stress was induced in the liver of Eriocheir sinensis by ingestion of MPs, when the concentration of MPs exceeded 4 mg L−1. It’s highly likely that these harmful impacts are related to the retention of MPs within organisms (Hurley et al., 2017). As shown in a previous study, MPs particles could be retained inside of lugworm collected alone French − Belgian − Dutch coastline, an average of 1.2 ± 2.8 particles·g−1 tissue was detected (Van Cauwenberghe et al., 2015). In addition, Watt et al. (Watts et al., 2014) observed that polystyrene (PS) microspheres (8−10 μm) were mainly retained in the foregut of crab after exposure for 20 d. In consideration of the vital role played by the earthworm in soil processes and ecosystem services (Bakir et al., 2014), further research on the impacts of ingestion of MPs and the synergic pollution of MPs and other contaminants in terrestrial environment are clearly necessary. In addition to the demonstrated direct eﬀects of ingested MPs on organisms, MPs might also act as vectors for transferring other contaminants (Holmes et al., 2014; Tang et al., 2018; Imran et al., 2019). These adsorbed pollutants would be desorbed within the organism postingestion, due to the existence of gut surfactants, such as linear alkylbenzene sulfonate and humic acid (Bakir et al., 2014), and this process would presumably lead to higher toxic eﬀects (Diepens and Koelmans, 2018; Lu et al., 2018). Currently, most studies have considered the deleterious eﬀects resulting from co-exposure to MPs and organic pollutants (Bakir et al., 2014; Besseling et al., 2013; Rainieri et al., 2018). Yet, the toxic eﬀects induced by simultaneous exposure to MPs and heavy metals are comparatively underreported, especially for terrestrial organisms. Recently, Lu et al. (2018) provided the evidence of toxic eﬀects from combined exposure of MPs and Cd in aquatic
2. Materials and methods 2.1. Preparation of artiﬁcial soil, microplastics, and test organisms Artiﬁcial soils were prepared to conduct a laboratory scale exposure experiment. Soils were mixed mainly according to OECD guidelines with moderate modiﬁcation (OECD, 2004; Verdu et al., 2018). The speciﬁc components were as follow: 20 % kaolinite clay (Aladdin, China), 70 % quartz sand (Greagent-Reagent, China), 10 % sphagnum peat (Jiﬀy substrates, purchased from a local gardening store). Analytical reagent CaCO3 was used to adjust the initial pH of the soil. Organic carbon content was 5%, total organic nitrogen content was 0.0978 %, initial pH was 6.7, and the cation exchange capacity (CEC) was 14.6 cmol·kg−1. Organic carbon was determined by the Walkley-Black method (Walkley and Black, 1934). Total nitrogen content was measured according to the Kjeldahl method (Page et al., 1982), and pH was determined using a pH-meter (Mettler Toledo FE20) with a ratio of water and soil of 2.5:1. The method for analysing CEC follows Hendershot et al. (Hendershot and Duquette, 1986). Analytical grade CdCl2 (99.9 % purity), obtained from Sigma (USA), was used as the Cd source in this experiment. Prepared CdCl2 solution was sprayed into the soil at a concentration of 8 mg kg-1 Cd2+ dry soil (denoted as mg·kg-1 hereinafter, the actual measured Cd concentration was 8.4 mg kg-1). We selected this Cd exposure dose in order to mimic the ambient concentration of Cd in many farmland soils (Khan et al., 2017; Li et al., 2009; Zhang et al., 2018). Polypropylene (PP) was considered as a model MPs in this study, as it is one of the most commonly produced synthetic polymers and is prevalent in the environment (Peeken et al., 2018; Scheurer and Bigalke, 2018). PP pellets (C3H6)n purchased from Sigma-Aldrich (density 0.90 g·cm−3 at 25℃) were mechanically ground to a diameter of < 150 μm. The average number of particles of MPs was estimated as (1.3 ± 0.05) × 105 particles·g-1 (dry weight). Ground MPs particles were characterized by DXR 2 Raman (Thermo Fisher Scientiﬁc, USA), and a typical Raman spectrum was shown in Fig. 1a. Optic images of ground MPs particles were captured by a stereomicroscope (Nikon SMZ25) with a high-resolution camera (Fig. 1a). The typical surface morphology of MPs particles was obtained by using a Quanta 250 scanning electron microscope (SEM) (Fig. 1b). Four MPs concentration levels were utilized in this experiment: 300, 3000, 6000, and 9000 mg kg-1, dry soil (i.e., 0.03 %, 0.3 %, 0.6 %, 0.9 % MPs, dry weight, dw). The E. foetida individuals used in this experiment were obtained from an earthworm breeding company in Jiangsu, China. After a week of acclimation in artiﬁcial soil, 16 healthy adult E. foetida with obvious clitellum and body weight between 0.3 and 0.4 g were picked out for each experimental beaker. Each E. foetida was gently washed with 2
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Fig. 1. (a) Roman spectrum of the PP microplastics pellets (a). (b) Typical surface morphology of PP microplastics before incorporating into soil.
50 °C for 24 h to remove organic matters. After digestion was completed, micro-ﬁltrated ultrapure water was applied to extract the MPs, and the supernatants were then collected. This extraction process was repeated several times until there were no obvious particles found. Extracted MPs on ﬁlter membranes were placed in a petri dish and kept at 40 °C to constant weight. Before isolating MPs from earthworm, every E. foetida were thoroughly washed by saturated NaCl to remove soil and MPs particles sticking on skins. Subsequently, KOH:NaClO solution (30 mL 50 %) was added and then digested at 50 °C for 48 h Enders et al. (2017). Finally, digestion solutions were ﬁltered (0.45 μm pore size, 47 mm diameter) to collect MPs particles from E. foetida. To ensure that all extracted particles were MPs particles, a DXR2 Raman microscope (Thermo Fisher Scientiﬁc, USA) was used to identify polymeric compositions. Each suspected MPs particle was manually located with a 10× lens of DXR2 Raman. All obtained Raman spectra were compared with the libraries in the OMNIC database (Thermo Fisher Scientiﬁc, USA) with at least 80 % matching score for spectra.
micro-ﬁltrated ultrapure water, and then kept in a petri dish covered with moisturized ﬁlter papers for 48 h. The body weight of each individual was recorded before being transferred into experimental beakers. 2.2. Exposure experiment The pretreated E. foetida were randomly transferred into glass beakers, each beaker containing 16 E. foetida and 500 g soil. Four experimental groups were set up: CK, control group (without MPs and Cd); T1, MPs + Cd group (T1C1 with 300 mg kg−1 MPs+8 mg·kg-1 Cd; T1C2 with 3000 mg kg−1 MPs+8 mg·kg-1 Cd; T1C3 with 6000 mg kg−1 MPs+8 mg·kg-1 Cd; T1C4 with 9000 mg kg−1 MPs+8 mg·kg-1 Cd); T2, MPs group (T2C1 with 300 mg kg−1 MPs; T2C2 with 3000 mg kg−1 MPs; T2C3 with 6000 mg kg−1 MPs; T2C4 with 9000 mg kg−1 MPs); T3, Cd group (8 mg kg-1 Cd alone). Each treatment was prepared in triplicate. To prevent the introduction of other pollutants into the experimental groups, we did not feed the E. foetida during this experiment; however, 2 g dried (105 ℃) horse manure per 100 g soil was incorporated into each beaker during soil preparation (Verdu et al., 2018). The beakers were placed in a culture chamber in the dark with a temperature of 21 °C ± 0.5 °C, and soil moisture was measured (measured with a mobile soil-moisture sensor) every two days. If water evaporated, micro-ﬁltrated ultrapure water was added into the beakers to keep soil moisture constant at 25 % during the trial. The exposure experiment was carried out for 6 weeks (42 d); soils and E. foetida were collected at the second (14 d), fourth (28 d), and sixth week (42 d). Cast collection was performed as follows: at each sampling time each E. foetida in its beaker was picked out and carefully washed, then kept in a glass petri dish covered with moisturized ﬁler paper for 48 h. The moisturized ﬁlter paper in each petri dish was changed every 5 h, in case of repeated ingestion of casts, and then casts were collected and dried to constant weight. At each sampling time (14, 28, and 42 d), ﬁve E. foetida (starved for 48 h) were randomly selected for recording body weight. For measuring lipid peroxides (LPO) levels and glutathione (GSH) content and for extracting MPs, three E. foetida were picked out and thoroughly washed with cold normal saline (0.86 %), then immediately killed by freezing. The rest of the E. foetida specimens were returned to their individual beakers.
2.4. Determination of Cd content in earthworms At the end of 14 d, 28 d, and 42 d, E. foetida was taken from every experiment beakers of the MPs + Cd group and Cd group for determining the accumulation of Cd. All E. foetida were starved for 48 h to induce the excretion of casts, and then thoroughly washed with cold normal saline (0.86 %). Each animal sample was transferred into digestion tubes, then a 3:1 mixture solution of HNO3 and H2O2 was added into the digestion tube with a 1:10 (m/v) ratio of earthworm weight (g) and mixture solution (mL) (Lu et al., 2018; Zhu et al., 2016). Subsequently, E. foetida was digested at 105 °C for 3 h; afterward, cold digestion solutions were ﬁltered and diluted, and the diluted solutions were kept at 4 °C until analysis. Inductively Coupled Plasma-Mass Spectrometry (ICP-MS, Thermo Scientiﬁc X Series 2) was used to measure the content of Cd. The Cd content in the whole body of E. foetida was represented as mg·kg−1 wet weight (wt.). All labware was soaked with HCl (10 % v/v) solution for 24 h, and thoroughly rinsed with microﬁltered ultrapure water prior of use (Holmes et al., 2014; Lu et al., 2018). 2.5. Calculation of mortality and growth rate
2.3. Extraction of MPs
The mortality was calculated according to the percentage of dead earthworms in each treatment at the end of 14 d, 28 d, and 42 d. Growth rate is an important indicator of stress and extensively applied as an index for relevant environmental monitoring and risk assessment (Liu et al., 2009; Huerta Lwanga et al., 2017b). In this study, the growth
MPs in artiﬁcial soil and casts were extracted according to the method suggested by Enders et al. (2017). KOH:NaClO solution (30 mL 30 %) was added into each soil sample (20 g soil/cast), then digested at 3
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each treatment, the growth rate on day 42 was signiﬁcantly lower (P < 0.05) than that on day 14 and day 28 (P < 0.05, supplementary Table 2), and body mass dwindled (except in CK) when compared with the body weight on day 28 (Table 1). Higher mortality was induced by exposure to high MPs concentrations (> 300 mg kg−1) after exposure for 42 d, but the existence of Cd did not obviously raise the mortality (Fig. 2b). As illustrated in Fig. 2b, the mortality in the MPs + Cd group and the MPs group increased from 12 % to 38.4 % and from 12.7%–36.7%, respectively. No signiﬁcant diﬀerence of mortality was found after 28 d among each treatment (Fig. 2b). The mortality signiﬁcantly raised with increase in exposure time (P < 0.05, supplementary table 2, except in T2C1 treatment), and mortality on day 42 was signiﬁcantly higher than that on day 28 and day 14 (P < 0.05, supplementary Table 2).
rate was calculated as follow: (Morg ,1 − Morg ,2 )
K gr (mg ·worm−1·day−1) =
Where Morg,1 and Morg,2 refer to initial body weights (g) and ﬁnal body weights (g) of E. foetida, respectively, and t refers to the exposure days (d). This calculation method was performed in accordance with Huerta Lwanga et al. (2017a). 2.6. Biochemical parameters As suggested in previous studies, both MPs and heavy metals can cause oxidative distress and histopathological in animal organisms (Lu et al., 2018; Li et al., 2018; Rodriguez-Seijo et al., 2017). The production of LPO in organisms can reﬂect the degree of intracellular damage. LPO levels usually be measured as an indicator of lipid peroxidation and of oxidative stress in organisms and has been extensively applied to ascertain the oxidative damage in studies of earthworm toxicology (Lu et al., 2018; Li et al., 2018; Xing et al., 2018; Zaltauskaite and Sodiene, 2014). GSH, a low molecular mass thiol compound and no-enzymatic antioxidant, is widely distributed in organisms. This most common biological thiol was found positively correlated with metal stress and directly involved in the cellular metabolism of genotoxic agents (Adamis et al., 2007; Dou et al., 2019; Per et al., 2017). GSH is known to play a critical role in the detoxiﬁcation mechanism against oxidation stress (Jeong et al., 2017; Mekahlia et al., 2016). Moreover, GSH can reduce the availability of Cd in the cytoplasm through chelating Cd, and signiﬁcantly enhance organisms defence mechanisms against oxidative stress (Adamis et al., 2007; Dou et al., 2019). It also has been considered to play a critical role in coping with reactive oxygen species (ROS) stress with the function of detoxiﬁcation and antioxidant action (Srikanth et al., 2013; Zaidi and Shin, 2016). Therefore, in this study, lipid peroxides (LPO) levels were detected to assess the extent of oxidative injury in E. foetida, and GSH contents were measured to evaluate the detoxiﬁcation and antioxidant action in E. foetida. During the experimental period, LPO and GSH level were measured after exposure for 14 d, 28 d, and 42 d. The whole E. foetida were homogenized on ice in 50 mM phosphate buﬀer (including 0.1 mM EDTA, pH = 7.0). Homogenates were centrifuged at 5000rmp for 15 min at 4 °C. The supernatants were collected for analyzing LPO levels and GSH contents. Assay kits (Jiancheng Bioeng Inst., China) were used to assess LPO levels and GSH contents. All experimental operations were performed following the manufacturer's protocols.
3.2. The adverse eﬀ ;ects of MPs and MPs + Cd on E. foetida Exposure to high MPs concentrations resulted in the increase of LPO levels in E. foetida in both MPs and MPs + Cd groups (Fig. 3a). The combined exposure to Cd and MPs induced higher LPO levels than that MPs alone induced after exposure 14 d (Fig. 3a). For example, on day 42, LPO levels in T1C1, T1C2, T1C3 and T1C4 treatments were higher 114.8 %, 254 %, 61.7 %, and 55.3 % than that in T2C1, T2C2, T2C3 and T2C4 treatments, respectively. Similar results were observed between MPs + Cd and MPss group after exposure 28 d (except between T1C1 and T2C1, Fig. 3a). The LPO levels were greatly increased after combined exposure to MPs + Cd compared with exposure MPs or Cd alone. For example, on day 28, the LPO levels observed in T1C4 (combined exposure) treatment were 44.1 % and 191.9 % higher than that in T2C4 (MPs alone) and T3 (Cd alone) treatment, respectively (Fig. 3a). Exposure to high MPs concentrations increased the GSH content in E. foetida both in MPs and MPs + Cd group (Fig.3b). Signiﬁcantly higher GSH content was obtained after co-exposure to MPs + Cd (P < 0.05). For instance, on day 42, the GHS content in T1C3 and in T1C4 were signiﬁcantly higher than that in T2C3 and T2C4, and higher up to 41.3 % and 35.6 %, respectively (Fig. 3b). Similar results were also detected on day 14 and day 28. The GSH contents in high MPs concentration treatments (> 300 mg kg−1) groups tended to increase with the duration of exposure (Fig. 3b). Overall, GSH content was increased in an MPs dose-dependent manner, and the existence of Cd augmented GSH content (Fig. 3b). 3.3. Microplastics ingestion MPs particles retained within E. foetida signiﬁcantly increased with the increase of MPs concentration in soil (Fig. 4a). After exposure for 42 d, a total of 4.3, 42.3, 53.1, and 67.2 particles·g−1 earthworm was extracted from treatments with MPs concentration of 300, 3000, 6000, and 9000 mg kg-1, respectively (Fig. 4a). Additionally, in a high MPs pollution environment more MPs particles could be egested by E. foetida (Fig. 4b. For example, a total of 48,500 mg kg-1 of MPs particles in cast was recorded at high-dose MPs treatment (9000 mg kg-1). What’s more, MPs concentration in the egestion is higher (up to 5.4-fold, approximately) than that in the soil, and signiﬁcantly diﬀerences were showed between treatments (except between T2C1 and T2C2 treatment). The percentage of MPs in cast on day 14 and 28 ranged from 2.8%–21.7% and 3.1%–25.6%, respectively (Fig. 4b).
2.7. Statistical methods Univariate analysis was performed by using SPSS 22. Data sets used for one-way ANOVA were tested for normality (Shapiro-Wilk) and homogeneity of variances (Levene’s test); logarithmic transformation and rank cases (Blom) were performed in advance, if assumptions of parametric tests could not be fulﬁlled. A nonparametric test (KruskalWallis) was applied to analyse variance. One-way ANOVA was used to determine MP-related data signiﬁcant diﬀerences among groups (posthoc test with Tukey multicomparison). For assessing relevance, a signiﬁcance level of 0.05 was set. 3. Results
3.4. Eﬀect of MPs on accumulation of Cd in E. foetida 3.1. Eﬀ ;ects of MPs and MPs + Cd on E. foetida growth rate, mortality Our results showed that the accumulation of Cd in E. foetida signiﬁcantly increased with exposure to MPs + Cd compared with exposure to Cd alone. After 28 d of combined exposure, Cd accumulation in E. foetida increased signiﬁcantly in relation to the MPs content in the soil (Fig. 5a). In T1C1, T1C2, T1C3, and T1C4 treatments, the Cd content in E. foetida was found to be 3.4, 5, 6.7, and 8.1 mg kg−1, respectively,
Co-exposure to MPs + Cd or singly exposure to MPs (> 3000 mg kg−1) both decreased growth rate, and the presence of Cd signiﬁcantly enhanced the negative impacts imposed by MPs (Fig. 2a). For example, on day 28, the growth rate of individuals in the T1C4 treatment was 81.6 % lower than that in T2C4 treatment (Fig. 2a). In 4
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Fig. 2. Growth rate (a) and mortality (b) of E. foetida exposed to MPs and MPs + Cd (four MPs concentrations: C1, C2, C3, and C4 refer to 300, 3000, 6000 and 9000 mg kg−1, respectively. Cd concentration: 8 mg kg−1;T1C1∼4, MPs + Cd; T2C1∼4, MP alone; T3, Cd alone; CK: without MPs and Cd). Values represent means ± SD (n = 5), diﬀ ;erent letters indicate signiﬁcant diﬀ ;erences (p < 0.05). Table 1 Fresh weight (g, Mean ± SD) of E. foetida. After exposure of 14 d, 28 d, and 42 d in various Treatmentsa. Experimental group
Duration of exposure (days) 14 d 28 d 48 d 0
CK Cd MPs
MPs + Cd
Control T3 T2C1 T2C2 T2C3 T2C4 T1C1 T1C2 T1C3 T1C4
0.33 0.32 0.31 0.31 0.33 0.34 0.32 0.31 0.32 0.31
14 ± ± ± ± ± ± ± ± ± ±
0.02c 0.01c 0b 0.01b 0.02a 0.02c 0.01b 0.01b 0.01a 0.02b
0.44 0.41 0.42 0.42 0.35 0.36 0.39 0.37 0.33 0.33
± ± ± ± ± ± ± ± ± ±
0.02bc 0.01bc 0.04a 0.02a 0.03bc 0.04bc 0.02b 0.01a 0.01a 0.02a
0.5 ± 0.02ab 0.45 ± 0.02a 0.47 ± 0.06a 0.47 ± 0.07a 0.47 ± 0.04a 0.46 ± 0.03a 0.41 ± 0.02a 0.39 ± 0.02a 0.41 ± 0.05a 0.42 ± 0.01a
0.51 0.43 0.46 0.45 0.42 0.39 0.39 0.36 0.35 0.33
± ± ± ± ± ± ± ± ± ±
0.01a 0.01ab 0.06a 0.07a 0.03ab 0.02bc 0.02b 0.02a 0.04a 0.01a
MPs concentrations: C1, C2, C3, and C4 refer to 300, 3000, 6000 and 9000 mg kg−1, respectively. Cd concentration: 8 mg kg−1;T1C1∼4, MPs + Cd; T2C1∼4, MP alone; T3, Cd alone; CK: without MPs and Cd. a Diﬀerent letters indicate signiﬁcant diﬀerences amongst treatments; a > b > c.
Fig. 3. The LPO levels (nmol·mg protein−1) (a) and GSH content (mg GSH· g protein−1) (b) in E. foetida exposed to MPs and MPs + Cd, (four MPs concentrations: C1, C2, C3, and C4 refer to 300, 3000, 6000 and 9000 mg kg−1, respectively. Cd concentration: 8 mg kg−1;T1C1∼4, MPs + Cd; T2C1∼4, MP alone; T3, Cd alone; CK: without MPs and Cd). Values represent means ± SD (n = 3), diﬀ ;erent letters indicate signiﬁcant diﬀ ;erences (p < 0.05).
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Fig. 4. (a) MP particles retained within E. foetida, values represent means ± SD (n = 3), diﬀ ;erent letters indicate signiﬁcant diﬀ ;erences (p < 0.05) (four MPs concentrations: C1, C2, C3, and C4 refer to 300, 3000, 6000 and 9000 mg kg−1, respectively. Cd concentration: 8 mg kg−1;T1C1∼4, MPs + Cd; T2C1∼4, MP alone). (b) MP concentration and the percentage of MP in the cast of E. foetida (airstrike means signiﬁcant diﬀerences, p < 0.05). (c) The typical surface morphology of the PP microplastic particle in soil without E. foetida. (d) The typical surface morphology of the MP particle retained within E. foetida.
after exposure for 42 d (Fig. 5a). These concentrations are greater by 9.7 %, 61.3 %, 116.1 % and 161.3 % than the result of the T3 treatment (Cd alone), respectively (Fig. 5a).
4.1. Eﬀ ;ects of MPs and MPs + Cd on E. foetida growth rate, mortality The growth rate in our study was markedly lower at high doses of MPs (> 300 mg kg−1) after 28 d of exposure, which is in accordance with earlier reports for earthworm (Huerta Lwanga et al., 2017a; Besseling et al., 2013; Huerta Lwanga et al., 2017b). For example,
Fig. 5. (a) Cd accumulation in E. foetida after singly exposed to Cd and jointly exposed to MPs + Cd. (four MPs concentrations: C1, C2, C3, and C4 refer to 300, 3000, 6000 and 9000 mg kg−1, respectively. Cd concentration: 8 mg kg−1;T1C1∼4, MPs + Cd; T2C1∼4, MP alone; T3, Cd alone; CK: without MPs and Cd). Values represent means ± SD (n = 3), diﬀ ;erent letters indicate signiﬁcant diﬀ ;erences (p < 0.05). (b) The correlation between concentrations of Cd and MP particles in E. foetida. (after 42 d of combined exposure to MP and Cd). 6
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(Huerta Lwanga et al., 2017a) exposed L. terrestris to diﬀerent doses of polyethylene (PE) microplastics, and found that growth rate signiﬁcantly declined after exposure to > 28 % MPs in the top soil lay. On day 42, the growth rate in high MPs concentration treatments (T2C3, T2C4, T1C3, and T1C4) was sharply decreased to -10.5 (in T2C4 treatment) and -14 mg·worm−1·d−1 (in T1C4 treatment). (Huerta Lwanga et al., 2017a, b) also observed negative growth rate (-0.5 mg·worm−1·d−1) and body weight decrease after exposure to 60 % MPs in the top soil layer (similar to 6000 mg kg-1 of our study), which is in agreement with our results. Up to a point, the response mechanism of ingestion of MPs within terrestrial and aquatic organisms may be alike (Diepens and Koelmans, 2018; Chen et al., 2019; Huerta Lwanga et al., 2017a); direct physical abrasion and blockage caused by MPs inside organisms may dilute and limit the bioavailability of nutrients (Besseling et al., 2013). As indicated in previous studies, MPs have great potential to accumulate metal contaminants on their surfaces from the ambient environment due to strong hydrophobicity and higher speciﬁc surface area (Wang et al., 2017; Imran et al., 2019; Dobaradaran et al., 2018; Vedolin et al., 2018), and pollutants can easily be released inside of organisms after ingestion (Bakir et al., 2014). For example, the bioconcentration and bioaccumulation of mercury were greatly increased in Dicentrarchus labrax by combined exposure to MPs (Barboza et al., 2018a). Thus, the Cd concentration in E. foetida and bioaccessibility of Cd might have been enhanced by MPs, an explanation for the lower growth rate recorded in MPs + Cd group in the current study. The sharp decrease of growth rate in the control group suggests that the growth of earthworms was presumably inﬂuenced by insuﬃcient nutrients in experimental soils (Liu et al., 2009). However, overall, we observed that higher concentrations of MPs and combined exposure to MPs and Cd in soil caused even lower growth rates. Exposure duration and MPs concentration may interact in E. foetida mortality, as suggested in previous studies (Huerta Lwanga et al., 2017a, b). Low MPs doses and short exposure duration were believed to have no negative eﬀects on the survival and energy reserves of experimental organisms (Huerta Lwanga et al., 2017b; Rodriguez-Seijo et al., 2017; Kokalj et al., 2018). For example, Rodriguez-Seijo et al. (2017) have demonstrated that no signiﬁcant eﬀect on mortality of earthworm (Eisenia andrei) was induced after exposure for 28 d to a low dose of PE microplastics (< 1000 mg kg−1 MPs soildw). A similar result was recorded in this study (Fig. 1b, supplementary Table 3). However, with increasing concentration of MPs in soil and exposure duration, the damage impacts (abrasion and blockage) inside the gut of earthworms may be boosted, and directly lead to weight loss and possibly mortality (Huerta Lwanga et al., 2017a). (Huerta Lwanga et al., 2017b) reported that the mortality of earthworms was signiﬁcantly higher on day 60 than on day 14 (≥28 % w/w in top litter layer), which is in accordance with our results. As detected in a previous study, no signiﬁcant mortality of E. foetida occurred after exposure to Cd (1∼40 mg kg−1) for 14 weeks (Zaltauskaite and Sodiene, 2014). The low content of Cd and relatively short exposure duration might be the reason why higher mortality was not observed after combined exposure to MPs + Cd.
high doses of MPs in this study. The upward trend of LPO levels in highdose MPs treatments after 14 d and the signiﬁcant increase of LPO levels in MPs + Cd (> 300 mg kg−1) suggest that abrasion and toxic impact might be induced by MPs and Cd, and MPs may accelerate the oxidative damages provoked by Cd. Consequently, E. foetida might suﬀer from sustained oxidative stress, intracellular damage and histopathological damage (Lu et al., 2018; Rodriguez-Seijo et al., 2017; Zaltauskaite and Sodiene, 2014; Barboza et al., 2018a). As most plastic additives (such as brominated ﬂame retardants and antioxidants) were mechanically incorporated into polymer without formation of chemical bounds, they can leach from plastics over time after ingestion (Koelmans et al., 2014; Sun et al., 2019). This might be an explanation for that LPO levels in E. foetida had a tendency to increase with exposure duration in high MPs concentration treatments (except T2C2 with 3000 mg kg−1 MPs on day 42) (Fig. 3a). The toxicity of MPs in organisms primarily derives from oxidative stress via the generation of ROS. The cumulate ROS subsequently provokes series biological reaction, e.g. signaling pathways elicited by oxidative stress, apoptosis, and inﬂammation (Dou et al., 2019; Jeong et al., 2017). Previous studies suggested that GSH-related antioxidant enzymes could be augmented in response to oxidative stress imposed by xenobiotics, such as MPs (Jeong et al., 2017; Mekahlia et al., 2016). GSH plays as ROS scavenger, and its related antioxidant enzymes take a crucial role in detoxiﬁcation mechanism against oxidative stress (Jeong et al., 2017). As indicated in previous studies that antioxidant enzymes activities could be largely increased due to the presence of metal stressors and micro(nano)particles exposure. In this study, GSH contents were signiﬁcantly higher after exposure to MPs (> 300 mg kg−1), this is in agreement with the study of (Jeong et al., 2017). They showed GSH contents in Paracyclopina nana were increased after exposure to polystyrene microbead. In the case of the measured GSH contents in E. foetida of this work, it diﬀered from the results reported by Lu et al. (2018). They showed that GSH contents in zebraﬁsh was signiﬁcantly lower in the presence of PS microplastic (20 and 200 μg L−1). However, taking into consideration the important role of GSH in diverse biological processes (e.g., maintaining normal cellular redox state, detoxiﬁcation and antioxidant action, Cd chelation, and the synthesis of DNA and function in immune system), the content of GSH may diﬀer between species and under diﬀerent experimental conditions (Adamis et al., 2007; Dou et al., 2019; Jeong et al., 2017; Hutter et al., 1997). In our work, GSH contents increased after exposure to Cd (Fig. 3b), this is in accordance with previous studies on E. foetida (Mekahlia et al., 2016; Li et al., 2016). Furthermore, higher GSH contents were detected after combined exposure to MPs + Cd (Fig. 3b). Which may suggesting that the rise in GSH contents may be closely related to the adaptive mechanism of organisms to mitigate oxidative stress or detoxifying oxidative stresses (Jeong et al., 2017; Mekahlia et al., 2016; Xue et al., 2009). As shown above, E. foetida might suﬀer from sustained oxidative stress, which may induce the synthesis of GSH to alleviate oxidative damages. This may be an explanation for why the trend in GSH contents were similar to that of LPO levels.
4.2. The adverse eﬀ ;ects of MPs and MPs + Cd on E. foetida
4.3. Microplastics ingestion
Not only was the presence of MPs correlated with decreased growth rate and increased mortality of E. foetida, it also ampliﬁed the negative eﬀects. In this study, the LPO levels were greatly increased after combined exposure to MPs + Cd compared with single exposure to either MPs or Cd. Previous studies have indicated that MPs can induce oxidative stress in organisms (Lu et al., 2018; Jeong et al., 2017; Jin et al., 2019), and Cd exposure can increase the level of LPO (Li et al., 2018; Zaltauskaite and Sodiene, 2014). In addition, it was reported that MPs can alter the immune system and cause gastrointestinal tract inﬂammation of soil faunas (Rodriguez-Seijo et al., 2017; Song et al., 2019). This might be related to the higher LPO levels after exposure to
MPs retention has been discovered in aquatic worms (Hurley et al., 2017; Van Cauwenberghe et al., 2015). For instance, (Hurley et al., 2017) identiﬁed on average 129 ± 65.4 particles·g−1 tissue retained within Tubifex worm. However, retention of MPs in terrestrial organisms has seldom been reported. In a similar exposure experiment, Hodson et al. (2017) did not detect MPs particles in earthworm digestate after exposure to PE (on average of 920 μm, 3500 mg kg−1 dw.) for 28 d. As suggested in previous studies, earthworms tend to ingest small MP particles (< 100 μm) (Huerta Lwanga et al., 2017a) and the ecotype of earthworm may inﬂuence their reaction to external pollutants (Zhang et al., 2018). The exposure manner, doses of MPs, MPs particles 7
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size and the ecotype of earthworm may be explanations for the diﬀerences between our study and previous studies (Hodson et al. (2017); Huerta Lwanga et al., 2017a; Rodriguez-Seijo et al., 2017). In addition, in high MPs concentration treatments, particles retained inside of E. foetida were observed to be signiﬁcantly higher after exposure for 42 d than on days 14 and 28 (supplementary Table 3). Our results showed that MPs particles within earthworms increased with exposure duration, except in the lowest dose of MPs treatment (supplementary Table 3). This may indicate that when MPs concentration exceed a certain value in soil environment, E. foetida may accumulate more MPs particles inside the body. MPs percentage in cast signiﬁcantly increased with the doses of MPs concentration in the soil, but no distinct diﬀerences were found on days 14 and 28 (supplementary Tables 4 and 5). The higher MPs concentration in egestion indicates that MPs pollution in the soil environment may account for greater ecological eﬀects than we had previously assumed. Furthermore, the typical surface morphology of MPs particles extracted from E. foetida was clearly diﬀerent from that of MPs particles recovered from soil (Fig. 4c and d). There were noticeable changes in the surface of extracted particles, i.e., holes indicating biodegradation (Fig. 4d). In contrast, no holes were detected on the surface of particles recovered from soil; rather, we found that countless smaller particles or nanoparticles adsorbed on the surface of MPs particles from the soil (Fig. 4c). However, with respect to the ingestion and retention of MPs in soil fauna, further studies are necessary to investigate whether the retained particles will be egested or transferred to other tissues.
Overall, exposure to MPs alone or co-exposure to MPs + Cd for 42 d both can decrease the growth rate and increase the mortality of E. foetida. We found that the presence of MPs (> 3000 mg kg−1) might induce oxidative damage and that MPs enhance the toxic eﬀect of Cd. The results demonstrate that MPs particles could be retained within E. foetida and the number of particles retained increased with the dose of MPs and exposure duration. The accumulation of Cd in E. foetida could be augmented by the existence of MPs, and Cd content was increased with the duration of exposure. These ﬁndings provide a further understanding of the adverse eﬀects of MPs and the synergic pollution of MPs and heavy metals on soil fauna. CRediT authorship contribution statement Yanfei Zhou: Writing - original draft, Methodology, Software. Xiaoning Liu: Investigation, Data curation. Jun Wang: Writing - review & editing, Supervision. Declaration of Competing Interest The authors declare that they have no known competing ﬁnancial interests or personal relationships that could have appeared to inﬂuence the work reported in this paper. Acknowledgments This work was supported by the Funding Project of National Key Research and Development Program of China (2018YFD0900604), Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2018), and Sino-Africa Joint Research Center, Chinese Academy of Sciences (Y623321K01).
4.4. Eﬀect of MPs on accumulation of Cd in E. foetida Cd accumulation in E. foetida was signiﬁcantly raised after co-exposure to MPs + Cd, compared with exposure to Cd alone. Thus, it is possible that E. foetida uptakes metal from the soil, and that MPs can accelerate the accumulation of metal in the body of E. foetida, such accumulation increasing with the increase of exposure duration. To the best of our knowledge, there is only one study focused on the eﬀects of MPs and metal on L. terrestris (Hodson et al., 2017). Our ﬁndings with E. foetida are not in agreement with the study of (Hodson et al., 2017), in which no metal (Zn) accumulation in L. terrestris was found during the 28-day exposure experiment. Two possible explanations for the diﬀerences may be that Zn, as an essential metal for organisms, is likely well regulated by metabolic processes of L. terrestris (Hodson et al., 2017), and, secondly, that the diﬀerent ecotypes of the earthworms may also inﬂuence the accumulation of heavy metals (Zhang et al., 2018). Moreover, it was indicated that MPs can adsorb and transport heavy metal contaminants due to speciﬁc surface characteristics (Hodson et al., 2017; Liu et al., 2019), the accumulated metal ions on the surface of MPs are highly likely to be released inside organisms after ingestion (Hodson et al., 2017; Bakir et al., 2014). Our result showed that the content of Cd in E. foetida is positively correlated with the MPs particles in E. foetida. Hence, MPs particles adsorb Cd ions on its surfaces, and then Cd ions desorb inside E. foetida due to gut surfactants maybe an explanation for our result. In these cases, as shown in the embedded ﬁgure (Fig. 5a), the Cd concentration in E. foetida increased with exposure duration. The accumulated Cd content of each treatment was signiﬁcantly diﬀerent between day 14 and day 42. Notably, we found statistically signiﬁcant increases in Cd concentration in E. foetida after exposure to 14 d, 28 d, and 42 d and with high MPs concentrations (6000 and 9000 mg kg−1) (Fig. 5a). The ability of MPs to increase the accumulation of heavy metals in aquatic organisms has been widely reported (Lu et al., 2018; Barboza et al., 2018a, b). For instance, (Lu et al., 2018) found that after zebraﬁsh was exposed to combined MPs (200 μg L−1) and Cd (10 μg L−1), the Cd content in zebraﬁsh was 85 % higher than that in the Cd treatment. However, further studies are needed to evaluate the biological consequences of diﬀerent polymers and heavy metals on a wide range of soil faunas.
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