Effects of nutrient cycling on contaminant fate in aquatic food webs

由于汞（Hg）沉积到水生系统中，经细菌转化为甲基汞（MeHg），并通过水生食物链被生物放大（从猎物到捕食者的浓度增加），从而达到可能对食用鱼类的消费者或鱼类本身有毒的浓度，因此全球一直对汞（Hg）的人为释放日益增加感到担忧。稳定氮同位素（δ ta15n）通常用于了解水生生物的相对营养地位以及汞等污染物通过食物网转移的速率。在不同的生态系统中，水生生物群中的汞浓度与其δ - 15n显著正相关，但这种转移的速率（斜率）在不同系统之间存在差异，原因尚不清楚。汞在水生系统中的命运与硫的非生物和生物循环有关。甲基汞与含s的氨基酸半胱氨酸结合，被细菌吸收，并储存在鱼类和植物的蛋白质中。尽管关于水生食物网的信息很少，但高营养水平的陆生生物在其蛋白质中含有最多的半胱氨酸，而植物中的半胱氨酸含量受水化学的影响。这表明生物体的半胱氨酸含量在水生食物网和跨系统中是不同的，这可能解释了系统间汞生物放大的差异。未来五年，我的国家科学研究委员会发现项目将继续完善我们对通过水生生态系统影响污染物转移的食物网过程的理解。在化学特征（从高到低的营养成分；从酸性到接近中性的pH值）不同的湖泊和溪流内部和之间，我们将研究是否通过其半胱氨酸含量来预测初级消费者中的甲基汞，以及半胱氨酸是否比三角洲15n更能预测甲基汞的食物网转移。研究结果将用于开发和完善模型，以更好地描述该污染物浓度在系统间的可变性，并在化学、物理和生物特性不同的系统中对该过程进行对比。

There is ongoing global concern about increasing anthropogenic releases of mercury (Hg) because it is deposited into aquatic systems, transformed by bacteria to methyl Hg (MeHg), and biomagnified (increases in concentration from prey to predator) through aquatic food chains to concentrations that may be toxic for fish-eating consumers or the fish themselves. Stable nitrogen isotopes (delta15N) are commonly used to understand the relative trophic position of aquatic organisms and the rate at which contaminants such as Hg are transferred through food webs. In diverse ecosystems, mercury concentrations in aquatic biota are significantly and positively related to their delta15N, but the rate (slope) of this transfer varies across systems for reasons that remain unexplained. The fate of Hg in aquatic systems is linked to the abiotic and biotic cycling of sulphur. Methyl Hg binds to the S-containing amino acid cysteine for uptake into bacteria and for storage in proteins of fish and plants. Although little information exists for aquatic food webs, terrestrial organisms at high trophic levels contain the greatest amounts of cysteine in their proteins, and cysteine content in plants is affected by water chemistry. This suggests that cysteine content of organisms varies in aquatic food webs and across systems, perhaps explaining among-system differences in Hg biomagnification. The next five years of my NSERC Discovery program will continue to refine our understanding of the food web processes affecting contaminant transfer through aquatic ecosystems. Both within and across lakes and streams that vary in their chemical characteristics (high through low nutrients; acidic through near neutral pH), we will examine whether MeHg in primary consumers is predicted by their cysteine content, and whether cysteine is better predictor of the food web transfer of MeHg than delta15N. Results will be used to develop and refine models to better describe among-system variability in concentrations of this contaminant, and to contrast this process in systems that vary in their chemical, physical and biological characteristics.