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纳米技术的应用几乎触及了现代生活的方方面面,在消费品、化学和能源等领域,纳米材料的使用越来越广泛[1]. 在生产、运输和使用过程中,纳米材料不可避免地释放到水环境中[2],对水生生物造成潜在的危害[3—5]. 研究表明,纳米材料可以被水环境中的单细胞生物吞噬,使纳米材料从低营养向高营养水平转移[6],而纳米材料沿食物链的传递最终会威胁人类健康. 因此,这些单细胞生物被认为是纳米材料进入食物网的重要“入口”之一[7]. 例如嗜热四膜虫(四膜虫),它是一种在水环境广泛存在的处于底层营养级的水生单细胞真核原生生物[8]. 已有研究证明,四膜虫可以直接吞噬纳米和微米级颗粒[9—10],并将其传递到更高营养级的生物[11—12],为纳米材料在微生物环和经典食物网之间的传递提供了重要的联系[13]. 四膜虫还具有繁殖周期短[14]、易于在实验室中培养和保存[15]、对外源物质敏感性强[16]等优点,是良好的生态毒理学研究模式生物[17]. 研究纳米材料对四膜虫的生物效应,对理解纳米材料在水环境中进入食物网的起始环节,探讨纳米材料的环境风险有重要意义.
生物累积是研究纳米材料生物效应的重要基础,而暴露浓度是影响纳米材料生物累积的重要因素之一[18—21]. 相关研究表明纳米材料在环境中的浓度约为pg·mL−1至μg·mL−1[22—23],然而,目前大多数关于纳米材料生物累积的研究都是在比实际环境浓度高几个数量级的暴露浓度条件下进行的[24],这些研究结果不能完全准确地反映真实环境的暴露情况. 此外,单细胞生物群体中,细胞个体吸收纳米颗粒的能力有较大差异[25—26],基于群体平均值和高检出限的研究方法可能导致具有不同生理或吸收特性的稀有细胞群体被忽略[27—28]. 由于受到检测技术的限制,目前在单细胞水平研究环境相关剂量下纳米材料的生物累积特征和毒性效应还具有一定的挑战性.
质谱流式细胞术(CyTOF)是一种以电感耦合等离子体(ICP)与飞行时间质谱(TOF-MS)为检测器的检测技术,具有高通量、低检出限的特点[29],可以在单细胞水平同步进行金属定量和生理指标的检测[30]. 金纳米颗粒(AuNPs)作为一种重要的典型金属纳米颗粒,化学稳定性高,在自然环境或生物体内短时间内不易降解,常用作研究纳米颗粒生物效应的模型颗粒[31—33]. 结合CyTOF可以在单细胞水平同时检测多种金属元素和检出限低的优势[34—35],以及AuNPs稳定性高的特点,本研究以四膜虫作为研究对象,在低剂量暴露下研究了四膜虫对AuNPs的摄取与外排特征以及在单细胞水平上的异质性. 考虑到金属元素在水环境中以多种形态存在[36],本研究还比较了Au(Ⅲ)与AuNPs对四膜虫细胞的单细胞生物效应的差异.
四膜虫摄取与外排AuNPs和Au(Ⅲ)的差异及单细胞异质性分析
Uptake and efflux of AuNPs and ionic Au(Ⅲ) by Tetrahymena thermophila and the single-cell heterogeneity
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摘要: 纳米材料的广泛应用导致其不可避免地对水环境造成污染. 单细胞水生生物体内累积的纳米材料可能会沿着食物链/网向上传递,造成一定的环境风险. 为了探究真实环境中的金属纳米颗粒及其溶解态金属离子对水生生物的影响,本文研究了在低剂量暴露下,单细胞水生生物嗜热四膜虫(四膜虫)对金纳米颗粒(AuNPs)和三价金离子(Au(Ⅲ))的累积特征. 结果表明,1 ng·mL−1暴露剂量下AuNPs和Au(Ⅲ) 对四膜虫的膜通透性未产生明显影响;在持续暴露实验中,发现四膜虫累积Au(Ⅲ)与AuNPs相比有一定的滞后性;撤除暴露(48 h)后,四膜虫细胞内的AuNPs和Au(Ⅲ)会快速外排,但仍有部分AuNPs和Au(Ⅲ)未被排出. 在整个实验过程中,四膜虫累积AuNPs和Au(Ⅲ)在单细胞水平具有较高的异质性,有一小部分个体(分别为6.72%和11.83%)能够累积较多的AuNPs和Au(Ⅲ),而这小部分个体有可能进入食物链/网对高营养级生物产生影响. 借助质谱流式技术能够在单细胞水平同时检测多个金属元素的优势,可以通过适当的金属同位素生物分子标记物,进一步探究部分四膜虫细胞个体Au的异质性累积的分子机制.Abstract: The wide application of nanomaterials inevitably results in the environmental water contamination. Nanoparticles accumulated in unicellular aquatic organisms could be transported to higher trophic levels through the food chain/web, causing certain environmental risks. To explore the effects of metal nanoparticles and metal ions on aquatic organisms in real environments, we investigated the characteristics of gold nanoparticles(AuNPs)and ionic Au(Ⅲ) accumulation by the unicellular aquatic organism Tetrahymena thermophila under low-dose exposure. The results showed that AuNPs and ionic Au(Ⅲ) had no significant effect on the membrane permeability of T. thermophila at 1 ng·mL−1. In the exposure experiment, we found that the accumulation of ionic Au(Ⅲ) by T. thermophila was slower than that of AuNPs. After the exposure was removed(48 h), AuNPs and ionic Au(Ⅲ) in T. thermophila were rapidly excreted, however, some AuNPs and ionic Au(Ⅲ) could not be eliminated. In this study, the heterogeneity of the accumulation of AuNPs and ionic Au(Ⅲ) in T. thermophila was revealed at the single-cell level. A few T. thermophila individuals(6.72% and 11.83%, respectively)could accumulate more AuNPs and ionic Au(Ⅲ) than other T. thermophila cells, and through these individuals, Au may affect the organisms at the higher trophic level. Since CyTOF allows multielement detection of a single cell, the mechanisms underlying different abilities of uptake of Au by T. thermophila cells could be further studied by utilizing proper stable metal isotope bio-markers.
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Key words:
- single-cell analysis /
- bioaccumulation /
- metal nanoparticles /
- mass cytometry /
- cellular heterogeneity.
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图 3 (a)细胞膜正常的四膜虫(PtN)与细胞膜通透的四膜虫(PtP)圈门示意图;(b)不同暴露时间点细胞膜完整的四膜虫比例
Figure 3. (a)Gating scheme of T. thermophila with intact cell membrane(Living cells,PtN)and T. thermophila with permeable cell membrane(Dead cells,PtP);(b)Proportion of T. thermophila cells with intact cell membranes at different exposure times
表 1 不同暴露时间四膜虫细胞中EIAu的范围和平均值
Table 1. Range and mean value of EIAu in T. thermophila cells at different exposure times
暴露时间
Time金纳米颗粒暴露组
AuNPs exposure group三价金离子暴露组
Ionic Au(Ⅲ) exposure group0.5 h 6.0—107.08(4.81) 6.0—66.15(7.79) 6 h 6.0—247.58(35.37) 6.0—94.73(15.97) 24 h 6.0—361.54(40.70) 6.0—102.98(22.60) 48 h 6.0—271.46(32.77) 6.0—214.10(54.93) 60 h 6.0—76.90(3.62) 6.0—65.09(3.28) 72 h 6.0—30.91(1.52) 6.0—63.23(2.67) 96 h 6.0—38.74(1.68) 6.0—46.00(2.31) -
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