Ekstrom Eileen B, Morel François M M, Benoit Janina M
Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey 08544, USA.
Appl Environ Microbiol. 2003 Sep;69(9):5414-22. doi: 10.1128/AEM.69.9.5414-5422.2003.
Sulfate-reducing bacteria (SRB) in anoxic waters and sediments are the major producers of methylmercury in aquatic systems. Although a considerable amount of work has addressed the environmental factors that control methylmercury formation and the conditions that control bioavailability of inorganic mercury to SRB, little work has been undertaken analyzing the biochemical mechanism of methylmercury production. The acetyl-coenzyme A (CoA) pathway has been implicated as being key to mercury methylation in one SRB strain, Desulfovibrio desulfuricans LS, but this result has not been extended to other SRB species. To probe whether the acetyl-CoA pathway is the controlling biochemical process for methylmercury production in SRB, five incomplete-oxidizing SRB strains and two Desulfobacter strains that do not use the acetyl-CoA pathway for major carbon metabolism were assayed for methylmercury formation and acetyl-CoA pathway enzyme activities. Three of the SRB strains were also incubated with chloroform to inhibit the acetyl-CoA pathway. So far, all species that have been found to have acetyl-CoA activity are complete oxidizers that require the acetyl-CoA pathway for basic metabolism, as well as methylate mercury. Chloroform inhibits Hg methylation in these species either by blocking the methylating enzyme or by indirect effects on metabolism and growth. However, we have identified four incomplete-oxidizing strains that clearly do not utilize the acetyl-CoA pathway either for metabolism or mercury methylation (as confirmed by the absence of chloroform inhibition). Hg methylation is thus independent of the acetyl-CoA pathway and may not require vitamin B(12) in some and perhaps many incomplete-oxidizing SRB strains.
缺氧水体和沉积物中的硫酸盐还原菌（SRB）是水生系统中甲基汞的主要生产者。尽管已有大量研究探讨了控制甲基汞形成的环境因素以及控制无机汞对SRB生物有效性的条件，但分析甲基汞产生的生化机制的工作却很少。乙酰辅酶A（CoA）途径被认为是一种SRB菌株——脱硫脱硫弧菌LS中汞甲基化的关键途径，但这一结果尚未扩展到其他SRB物种。为了探究乙酰CoA途径是否是SRB中甲基汞产生的控制生化过程，对五株不完全氧化的SRB菌株和两株不使用乙酰CoA途径进行主要碳代谢的脱硫杆菌菌株进行了甲基汞形成和乙酰CoA途径酶活性的检测。其中三株SRB菌株还与氯仿一起孵育以抑制乙酰CoA途径。到目前为止，所有被发现具有乙酰CoA活性的物种都是完全氧化菌，它们需要乙酰CoA途径进行基本代谢，同时也会使汞甲基化。氯仿通过阻断甲基化酶或对代谢和生长产生间接影响来抑制这些物种中的汞甲基化。然而，我们已经鉴定出四株不完全氧化菌株，它们显然在代谢或汞甲基化过程中都不利用乙酰CoA途径（氯仿抑制作用的缺失证实了这一点）。因此，汞甲基化独立于乙酰CoA途径，并且在一些甚至许多不完全氧化的SRB菌株中可能不需要维生素B12。
