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Cobalt Limitation of Growth and Mercury Methylation in Sulfate-Reducing Bacteria

机译：还原硫酸盐细菌中钴的生长限制和汞甲基化

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Sulfate-reducing bacteria (SRB) have been identified as the primary organisms responsible for monomethylmercury (MeHg) production in aquatic environments, but little is known of the physiology and biochemistry of mercury (Hg) methylation. Corrinoid compounds have been implicated in enzymatic Hg methylation, although recent experiments with a vitamin B_(12) inhibitor indicated that incomplete-oxidizing SRB likely do not use a corrinoid-enzyme for Hg methylation, whereas experiments with complete-oxidizing SRB were inconclusive due to overall growth limitation. Here we explore the role of corrinoid-containing methyltransferases, which contain a cobalt-reactive center, in Hg methylation. To this end, we performed cobalt-limitation experiments on two SRB strains: Desulfococcus multivorans, a complete-oxidizer that uses the acetyl-CoA pathway for major carbon metabolism, and Desulfovibrio africanus, an incomplete-oxidizer that does not contain the acetyl-CoA pathway. Cultures of D. multivorans grown with no direct addition of Co or B_(12) became cobalt-limited and produced 3 times less MeHg per cell than control cultures. Differences in growth rate and Hg bioavailability do not account forthis large decrease in MeHg production upon Co limitation. In contrast, the growth and Hg methylation rates of D. africanus cultures remained nearly constant regardless of the inorganic cobalt and vitamin B_(12) concentrations in the medium. These results are consistent with mercury being methylated by different pathways in the two strains: catalyzed by a B_(12)-containing methyltransferase in D. multivorans and a B_(12)-independent methyltransferase in D. africanus. If complete-oxidizing SRB like D. multivorans account for the bulk of MeHg production in coastal sediments as reported, the ambient Co concentration and speciation may control the rate of Hg methylation.
机译：还原硫酸盐细菌（SRB）已被确定为负责在水生环境中生产单甲基汞（MeHg）的主要生物，但对汞（Hg）甲基化的生理学和生物化学知之甚少。尽管最近使用维生素B_（12）抑制剂进行的实验表明，未完全氧化的SRB可能不使用类固醇酶进行Hg甲基化，但类固醇化合物与酶促Hg甲基化有关，但由于以下原因，尚无定论：总体增长限制。在这里，我们探讨了在汞甲基化中含有类固醇的甲基转移酶的作用，该酶含有一个钴反应中心。为此，我们对两种SRB菌株进行了钴限制实验：多硫球菌Desulfococcus multivorans，一种使用乙酰辅酶A途径进行主要碳代谢的完全氧化剂，以及非洲脱硫球菌Desulfovibrio africanus，一种不包含乙酰辅酶A的不完全氧化剂。途径。在没有直接添加Co或B_（12）的情况下生长的D. multivorans培养物受到钴的限制，每细胞产生的MeHg比对照培养物少3倍。生长速度和汞生物利用度的差异不能解释Co限制后MeHg产量的大幅下降。相反，无论培养基中无机钴和维生素B_（12）的浓度如何，非洲龙舌兰培养物的生长和Hg甲基化率几乎保持恒定。这些结果与两种菌株中汞通过不同途径甲基化的结果一致：多伏D中含有B_（12）的甲基转移酶和非洲african中有B_（12）的甲基转移酶。如报道的那样，如果完全氧化的SRB（如多角藻）在沿海沉积物中产生大量的甲基汞，则环境Co的浓度和形态可能会控制汞甲基化的速率。

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