Effects of nutrient cycling on contaminant fate in aquatic food webs

人类活动不断增加的汞排放引起了全球的关注，因为汞会沉积到水生系统中，通过细菌转化为甲基汞，并通过水生食物链转化为生物汞（从猎物到捕食者的浓度增加），达到可能对食用鱼类的消费者或鱼类本身有毒的浓度。稳定氮同位素（δ 15 N）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δ 15 N显著正相关，但这种转移的速率（斜率）因系统而异，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。甲基汞与含硫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。虽然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，植物中的半胱氨酸含量受水化学的影响。 这表明，生物体内的半胱氨酸含量在水生食物网和不同系统中各不相同，这或许可以解释汞生物放大作用在水生系统中的差异。 在接下来的五年里，我的NSERC探索计划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。无论是内部和跨湖泊和溪流，其化学特性不同（高通过低营养，酸性通过近中性pH值），我们将研究是否甲基汞在初级消费者预测其半胱氨酸含量，以及半胱氨酸是否是更好的预测食物网转移的甲基汞比delta15N。结果将被用来开发和完善模型，以更好地描述这种污染物的浓度的微生物系统的变异性，并在不同的化学，物理和生物特性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.