Hintelmann Holger
Department of Chemistry, Trent University, Peterborough, ON K9J 7B8, Canada.
Met Ions Life Sci. 2010;7:365-401. doi: 10.1039/BK9781847551771-00365. Epub 2010 Jan 30.
The most important mercury species in the environment is monomethylmercury (MMHg), the topic of this chapter. This organic mercury compound is normally not released into the environment but formed by natural processes. Mercuric mercury (Hg²(+)) is methylated by bacteria and to a lesser extent through abiotic pathways. Highest rates of formation are found in anoxic aquatic environments. Terrestrial systems are mostly irrelevant for MMHg production and not a concern. Most productive environments are sediments, wetlands, and coastal marshes, but also the anoxic hypolimnion of lakes and anaerobic microhabitats like the rhizosphere of floating macrophytes. Prime suspects for methylation are sulfate-reducing bacteria, although also iron reducers have lately been identified as capable mercury methylators. What makes methylmercury such an insidious contaminant is its enormous biomagnification potential. Methylmercury is accumulated by more than seven orders of magnitude from sub ng/L concentrations in water to over 1,000,000 ng/kg in piscivorous fish, which are the main concern from a human health point of view. Since methylmercury is a very potent neurotoxin, particularly small children, pregnant women, and women in childbearing age are advised to either limit their fish consumption to a few meals per week or to select fish species known to have low levels of methylmercury. Formation of methylmercury is counteracted by other bacteria, which are capable of demethylating methylmercury. This process is regulated by an inducible mer operon system and serves as a detoxification mechanism in polluted environments. The other naturally occurring organic mercury species, dimethylmercury (DMHg), is only present at very low levels at great depths in the world oceans. However, it might be an important and very mobile pre-cursor for methylmercury in marine and polar ecosystems.
环境中最重要的汞物种是甲基汞(MMHg),本章将讨论这一主题。这种有机汞化合物通常不会释放到环境中,而是通过自然过程形成的。汞离子(Hg²⁺)会被细菌甲基化,在较小程度上也会通过非生物途径甲基化。甲基汞形成速率最高的环境是缺氧的水生环境。陆地系统对甲基汞的产生大多无关紧要,也不是问题所在。最有利于甲基汞产生的环境是沉积物、湿地和沿海沼泽,还有湖泊的缺氧湖下层以及漂浮大型植物根际等厌氧微生境。甲基化的主要嫌疑对象是硫酸盐还原菌,不过最近也发现铁还原菌也能够甲基化汞。甲基汞之所以是一种极其危险的污染物,是因为它具有极大的生物放大潜力。甲基汞在水中的浓度低于纳克/升,而在食鱼性鱼类体内会累积到超过1000000纳克/千克,生物放大倍数超过七个数量级,从人类健康角度来看,这是主要关注点。由于甲基汞是一种极强的神经毒素,建议幼儿、孕妇和育龄妇女要么将鱼类摄入量限制在每周几餐,要么选择已知甲基汞含量低的鱼类品种。其他一些细菌能够使甲基汞脱甲基,从而抵消甲基汞的形成。这个过程由一个可诱导的汞操纵子系统调控,在污染环境中起到解毒机制的作用。另一种天然存在的有机汞物种二甲基汞(DMHg),仅在世界海洋极深的区域以极低水平存在。然而,它可能是海洋和极地生态系统中甲基汞的一种重要且流动性很强的前体物质。