Effects of nutrient cycling on contaminant fate in aquatic food webs

全球持续关注汞(Hg)人为释放的增加，因为它被沉淀到水生系统中，被细菌转化为甲基汞(MeHg)，并通过水生食物链生物放大(从猎物到捕食者的浓度增加)到可能对食鱼消费者或鱼类本身有毒的浓度。稳定氮同位素(Delta15N)通常用于了解水生生物的相对营养位置以及汞等污染物通过食物网转移的速度。在不同的生态系统中，水生生物群中的汞浓度与其delta15N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磺的非生物和生物循环有关。甲基汞与含有S的氨基酸半胱氨酸结合，可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，但营养水平高的陆地生物的蛋白质中含有最多的半胱氨酸，植物中的半胱氨酸含量受到水化学的影响。这表明，生物体的半胱氨酸含量在水生食物网和不同系统之间存在差异，这可能解释了系统间汞生物放大作用的差异。我的NSERC探索计划的下一个五年将继续完善我们对影响污染物通过水生生态系统转移的食物网过程的理解。在化学性质不同的湖泊和溪流中(从高营养到低营养；从酸性到近中性pH)，我们将检查是否通过半胱氨酸含量来预测主要消费者的甲基汞，以及半胱氨酸是否比delta15N更好地预测甲基汞的食物网络转移。结果将用于开发和改进模型，以更好地描述这种污染物浓度的系统间变异性，并在化学、物理和生物特性不同的系统中对比这一过程。

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.