肠道微生物对鱼体内甲基汞去甲基化的影响

Consumption of fish is one of the most significant sources of human exposure to mercury. Methylmercury is accumulated in fish generally through food chain and the accumulation level correlates with trophic levles. However, it is unable to explain some differences in methylmercury levels in fish simply by food chain biomagnification. Mercury demethylation inside the fish body has been reported recently and is perhaps one of the pathways that affects the accumulation of methylmercury. The project is based on the hypothesis that microbes inside the intestine of fish may affect methylmercury bioaccumulation through regulation of the transformation between methylmercury and inorganic mercury species. This study will utilize zebrafish as a model, combining the incubation of microbes in the presence of stable isotope labeled mercury diet to investigate the effects of intestinal microbes to mercury species transformation inside fish body. The roles of microbes in wild fish intestine collected from different mercury-contaminated water will also be investigated and the relationships between microbe distribution and efficiency in mercury methylation/demethylation in wild fish will be carried out to further validate the hypothesis that the intestinal microbes could contribute to the species transformation of methylmercury inside fish body. The study is important for the understanding of methylmercury bioaccumulation and transfer in food chain and will provide scientific basis for aquatic ecological risk assessment.

鱼是人类汞暴露的主要途径之一。鱼体内的甲基汞主要通过食物链富集，大多数情况下遵循高营养级富集水平高于低营养级的规律。但是，单纯从食物链富集的角度很难解释一些调查发现的相同营养级鱼体中甲基汞含量的差异。有研究表明鱼体内存在汞的去甲基化过程，该途径可能是影响鱼体甲基汞生物富集的一个原因。本项目提出鱼类肠道微生物可能参与鱼体甲基汞形态转化进而影响鱼类甲基汞生物富集的假设，拟以斑马鱼为实验室模拟对象，通过饵料添加特异性汞转化相关微生物，结合汞稳定同位素示踪技术，模拟研究肠道微生物在鱼体内汞形态转化过程中的作用。同时选取汞污染程度不同环境中各营养级鱼类，采用同位素示踪方法，研究不同鱼类肠道的汞甲基化、去甲基化能力，分析其与肠道微生物群落结构的关系，明确肠道微生物介导的汞形态转化在鱼体甲基汞富集中的作用。本研究对于阐明汞的生物富集与食物链传递具有重要的意义，将为科学评价水环境汞生态风险提供科学依据。

食用鱼肉是汞进入人体的主要途径之一。鱼体内的甲基汞主要通过食物链传递富集，大多数情况下遵循从低营养级向高营养级逐渐增高的规律。然而相同营养级鱼体中甲基汞水平存在较大差异，单纯从食物链富集的角度很难解释。有研究表明鱼体内存在汞的去甲基化过程，我们提出鱼类肠道微生物可能参与鱼体甲基汞形态转化进而影响鱼类甲基汞生物富集的假设。基于此，我们建立了一种基于液相分子杂交的功能基因捕获检测方法，用于沉积物、鱼体肠道内容物中汞甲基化基因hgcAB、汞去甲基化基因merAB的定性、定量分析。我们发现沉积物中hgcA丰度与甲基汞比例呈正相关，表明hgcA介导的汞甲基化在沉积物甲基汞生成过程中起主导作用。鱼体肠道微生物中存在汞形态转化基因hgcAB、merAB，但其相对丰度较低，与鱼体甲基汞比例并无相关性，表明鱼体肠道中存在汞形态转化过程，但是该过程对鱼体甲基汞富集过程的贡献较小。通过测定沉积物和鱼体肠道中16s rRNA序列分析其微生物群落结构，我们发现高甲基汞位点沉积物中优势菌属为芽单胞菌属、瘤胃梭菌属、酸化菌属、地杆菌属；低甲基汞位点沉积物优势菌群为硫功能菌。从鱼体肠道微生物的聚类分析结果并未发现与甲基汞比例相关的菌属，进一步提示我们鱼体肠道微生物介导的汞形态转化过程对于鱼体甲基汞生物富集的影响较小。本研究中对甲基汞生成能力的预测以及鱼体内汞形态转化的研究对于阐明汞的生物富集与食物链传递具有重要意义，将为科学评价环境中汞生态风险提供科学依据。