Wetlands are common net producers of the neurotoxin monomethylmercury (MeHg) and are largely responsible for MeHg bioaccumulation in aquatic food-webs. However, not all wetlands net produce MeHg; notable exceptions are black alder (Alnus glutinosa) swamps, which net degrade MeHg. Here we report the mechanisms of MeHg demethylation in one such swamp (EHT), shown to be a sink for MeHg during four consecutive years. The potential demethylation rate constant (k_d) in soil incubations was ∼3 times higher in the downstream (EHT-D: k_d ∼ 0.14 d^–1) as compared to the upstream part of the swamp (EHT-U: k_d ∼ 0.05 d^–1). This difference concurred with increased stream and soil pH, and a change in plant community composition. Electron acceptor and inhibitor addition experiments revealed that abiotic demethylation dominated at EHT-U while an additional and equally large contribution from biotic degradation was observed at EHT-D, explaining the increase in MeHg degradation. Biotic demethylation (EHT-D) was primarily due to methanogens, inferred by a decrease in k_d to autoclaved levels following selective inhibition of methanogens. Though methanogen-specific transcripts (mcrA) were found throughout the wetland, transcripts clustering with Methanosaetaceae were exclusive to EHT-D, suggesting a possible role for these acetoclastic methanogens in the degradation of MeHg.

中文翻译：

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Al木沼泽中甲基汞净降解的机制：产甲烷菌和非生物过程的作用

湿地是神经毒素单甲基汞（MeHg）的常见净生产者，对水生食物网中MeHg的生物蓄积负有很大责任。但是，并非所有湿地都能产生甲基汞。值得注意的例外是黑色al木（Alnus glutinosa）沼泽，其净降解了MeHg。在这里，我们报告了在一个这样的沼泽（EHT）中MeHg脱甲基的机制，该机理被证明是连续四年MeHg的沉没。潜在的脱甲基化速率常数（ķ _d在下游在土壤中温育）是〜3倍更高：（EHT-d ķ _d〜0.14 d ^-1相比，沼泽（的上游部分）EHT-U：ķ _d〜 0.05 d ^–1）。这种差异与溪流和土壤pH值的增加以及植物群落组成的变化有关。电子受体和抑制剂的添加实验表明，在EHT-U处非生物脱甲基作用占主导地位，而在EHT-D处观察到生物降解产生了同样大的额外贡献，这说明了MeHg降解的增加。生物脱甲基化（EHT-D）主要归因于产甲烷菌，其原因是选择性抑制产甲烷菌后k _d降低至高压灭菌的水平。尽管在整个湿地中都发现了产甲烷菌特异性的转录本（mcrA），但转录本却与甲烷藻科群集在一起。 是EHT-D所独有的，表明这些乙酰碎裂性产甲烷菌可能在MeHg的降解中发挥作用。