Analysis of the Elodea nuttallii Transcriptome in Response to Mercury and Cadmium Pollution: Development of Sensitive Tools for Rapid Ecotoxicological Testing

Toxic metals polluting aquatic ecosystems are taken up by inhabitants and accumulate in the food web, affecting species at all trophic levels. It is therefore important to have good tools to assess the level of risk represented by toxic metals in the environment. Macrophytes are potential organisms for the identification of metal-responsive biomarkers but are still underrepresented in ecotoxicology. In the present study, we used next-generation sequencing to investigate the transcriptomic response of Elodea nuttallii exposed to enhanced concentrations of Hg and Cd. We de novo assembled more than 60 000 contigs, of which we found 170 to be regulated dose-dependently by Hg and 212 by Cd. Functional analysis showed that these genes were notably related to energy and metal homeostasis. Expression analysis using nCounter of a subset of genes showed that the gene expression pattern was able to assess toxic metal exposure in complex environmental samples and was more sensitive than other end points (e.g., bioaccumulation, photosynthesis, etc.). In conclusion, we demonstrate the feasibility of using gene expression signatures for the assessment of environmental contamination, using an organism without previous genetic information. This is of interest to ecotoxicology in a wider sense given the possibility to develop specific and sensitive bioassays.

污染水生生态系统的有毒金属会被水生生物摄取，并在食物网中富集，进而影响所有营养级的物种。因此，开发可靠工具以评估环境中有毒金属所带来的风险水平，具有重要意义。大型水生植物（macrophytes）是识别金属响应生物标志物（biomarkers）的潜在模式生物，但在生态毒理学（ecotoxicology）领域中仍未得到充分关注。本研究利用下一代测序（next-generation sequencing）技术，对暴露于升高浓度汞（Hg）和镉（Cd）的伊乐藻（Elodea nuttallii）的转录组（transcriptomic）响应展开分析。我们通过从头组装（de novo）得到了超过60000条重叠群（contigs），其中170条基因的表达受汞调控且呈剂量依赖性，另有212条基因的表达受镉调控并呈剂量依赖性。功能分析结果显示，上述基因显著关联于能量代谢与金属稳态（metal homeostasis）相关通路。利用nCounter技术对部分基因子集进行表达分析的结果表明，该基因表达模式可有效评估复杂环境样品中的有毒金属暴露情况，且相较于其他检测终点（如生物富集、光合作用等）具有更高的灵敏度。综上，本研究证实了利用基因表达特征（gene expression signatures）评估环境污染的可行性，且所用的伊乐藻此前并无完整的遗传信息参考。鉴于开发特异性强、灵敏度高的生物检测法（bioassays）的广阔前景，该研究成果对于广义生态毒理学领域具有重要的参考价值。