Smithsonian Environmental Research Center, Edgewater, Maryland, USA
Smithsonian Environmental Research Center, Edgewater, Maryland, USA.
mBio. 2018 Apr 10;9(2):e02403-17. doi: 10.1128/mBio.02403-17.
Methylmercury (MeHg) production was compared among nine cultured methanogenic archaea that contain , a gene pair that codes for mercury (Hg) methylation. The methanogens tested produced MeHg at inherently different rates, even when normalized to growth rate and Hg availability. Eight of the nine tested were capable of MeHg production greater than that of spent- and uninoculated-medium controls during batch culture growth. , an strain with a fused gene pair, was unable to produce more MeHg than controls. Maximal conversion of Hg to MeHg through a full batch culture growth cycle for each species (except ) ranged from 2 to >50% of the added Hg(II) or between 0.2 and 17 pmol of MeHg/mg of protein. Three of the species produced >10% MeHg. The ability to produce MeHg was confirmed in several methanogens that had not previously been tested ( SANAE, , GKZPZ, and E1-9c). Maximal methylation was observed at low sulfide concentrations (<100 μM) and in the presence of 0.5 to 5 mM cysteine. For , the addition of up to 5 mM cysteine enhanced MeHg production and cell growth in a concentration-dependent manner. As observed for bacterial Hg methylators, sulfide inhibited MeHg production. An initial evaluation of sulfide and thiol impacts on bioavailability showed methanogens responding to Hg complexation in the same way as do The mercury methylation rates of several methanogens rival those of the better-studied Hg-methylating sulfate- and iron-reducing , specifically methanogenic organisms, play a role in mercury methylation in nature, but their global importance to MeHg production and the subsequent risk to ecosystems are not known. Methanogenesis has been linked to Hg methylation in several natural habitats where methylmercury production incurs risk to people and ecosystems, including rice paddies and permafrost. In this study, we confirm that most methanogens carrying the gene pair are capable of Hg methylation. We found that methylation rates vary inherently among methanogens but that several species are capable of MeHg production at rates that rival those of the better-know Hg-methylating sulfate- and iron-reducing bacteria. Methanogens may need to be considered equally with sulfate and iron reducers in evaluations of MeHg production in nature.
比较了 9 种含有编码汞（Hg）甲基化的基因对的培养产甲烷古菌之间的甲基汞（MeHg）生成。测试的产甲烷菌以固有不同的速率产生 MeHg，即使归一化为生长速率和 Hg 可用性也是如此。在批培养生长过程中，9 种测试中有 8 种能够产生比消耗和未接种培养基对照更多的 MeHg。具有融合基因对的 菌株无法产生比对照更多的 MeHg。每个物种（除 外）通过完整的批培养生长周期将 Hg 转化为 MeHg 的最大转化率为添加的 Hg（II）的 2%至>50%或 0.2 至 17 pmol MeHg/mg 蛋白之间。三种物种产生了>10%的 MeHg。在以前未经过测试的几种 产甲烷菌中证实了产生 MeHg 的能力（SANAE、、GKZPZ 和 E1-9c）。在低浓度的硫化物（<100 μM）和存在 0.5 至 5 mM 半胱氨酸的情况下观察到最大甲基化。对于 ，半胱氨酸的添加量高达 5 mM 以浓度依赖的方式增强了 MeHg 的产生和细胞生长。与细菌 Hg 甲基化剂一样，硫化物抑制 MeHg 的产生。对半胱氨酸和硫醇对生物利用度影响的初步评估表明，产甲烷菌对 Hg 络合的反应方式与 相同。几种产甲烷菌的汞甲基化率与研究较好的 Hg 甲基化硫酸盐和铁还原菌相当，具体来说，产甲烷生物在自然界中对汞甲基化起作用，但它们对 MeHg 产生的全球重要性以及随后对生态系统的风险尚不清楚。在包括稻田和永冻层在内的几个自然栖息地中，产甲烷作用与 Hg 甲基化有关，在这些自然栖息地中，甲基汞的产生会给人类和生态系统带来风险。在这项研究中，我们证实携带 基因对的大多数产甲烷菌都能够进行 Hg 甲基化。我们发现甲基化率在产甲烷菌之间固有地存在差异，但有几个物种能够以与研究较好的 Hg 甲基化硫酸盐和铁还原细菌相当的速率产生 MeHg。在评估自然环境中 MeHg 的产生时，产甲烷菌可能需要与硫酸盐和铁还原菌同等考虑。
