Global prevalence and distribution of genes and microorganisms involved in mercury methylation.

A global metagenome assessment reveals a low risk of methylmercury production in humans and a high potential in Arctic permafrost. Mercury (Hg) methylation produces the neurotoxic, highly bioaccumulative methylmercury (MeHg). The highly conserved nature of the recently identified Hg methylation genes hgcAB provides a foundation for broadly evaluating spatial and niche-specific patterns of microbial Hg methylation potential in nature. We queried hgcAB diversity and distribution in >3500 publicly available microbial metagenomes, encompassing a broad range of environments and generating a new global view of Hg methylation potential. The hgcAB genes were found in nearly all anaerobic (but not aerobic) environments, including oxygenated layers of the open ocean. Critically, hgcAB was effectively absent in ~1500 human and mammalian microbiomes, suggesting a low risk of endogenous MeHg production. New potential methylation habitats were identified, including invertebrate digestive tracts, thawing permafrost soils, coastal “dead zones,” soils, sediments, and extreme environments, suggesting multiple routes for MeHg entry into food webs. Several new taxonomic groups capable of methylating Hg emerged, including lineages having no cultured representatives. Phylogenetic analysis points to an evolutionary relationship between hgcA and genes encoding corrinoid iron-sulfur proteins functioning in the ancient Wood-Ljungdahl carbon fixation pathway, suggesting that methanogenic Archaea may have been the first to perform these biotransformations.

一项全球宏基因组评估显示，人类体内甲基汞生成风险较低，而北极永久冻土中甲基汞生成潜力较高。 汞（Hg）甲基化会产生具有神经毒性且极易生物累积的甲基汞（MeHg）。近期发现的汞甲基化基因hgcAB具有高度保守性，这为广泛评估自然界中微生物汞甲基化潜力的空间分布及特定生态位模式奠定了基础。我们在3500多个公开可得的微生物宏基因组中探究了hgcAB的多样性与分布情况，涵盖了广泛的环境，从而对汞甲基化潜力有了全新的全球视角。研究发现，hgcAB基因几乎存在于所有厌氧（而非需氧）环境中，包括公海的含氧层。关键的是，在约1500个人类和哺乳动物的微生物组中几乎检测不到hgcAB，这表明内源性甲基汞生成的风险较低。研究还确定了一些新的潜在甲基化栖息地，包括无脊椎动物消化道、正在融化的永久冻土土壤、沿海“死亡地带”、土壤、沉积物以及极端环境，这意味着甲基汞有多种途径进入食物网。一些能够甲基化汞的新分类群浮出水面，其中包括尚无培养代表菌株的谱系。系统发育分析表明，hgcA与编码在古老的伍德 - 隆达尔碳固定途径中发挥作用的咕啉铁硫蛋白的基因之间存在进化关系，这表明产甲烷古菌可能是最早进行这些生物转化的微生物。