Methylmercury (MeHg), a neurotoxic compound biomagnifying in aquatic food webs, can be a threat to human health via fish consumption. However, the composition and distribution of the microbial communities mediating the methylation of mercury (Hg) to MeHg in marine systems remain largely unknown. In order to fill this knowledge gap, we used the Baltic Sea Reference Metagenome (BARM) dataset to study the abundance and distribution of the genes involved in Hg methylation (the hgcAB gene cluster). We determined the relative abundance of the hgcAB genes and their taxonomic identity in 81 brackish metagenomes that cover spatial, seasonal and redox variability in the Baltic Sea water column. The hgcAB genes were predominantly detected in anoxic water, but some hgcAB genes were also detected in hypoxic and normoxic waters. Phylogenetic analysis identified putative Hg methylators within Deltaproteobacteria, in oxygen-deficient water layers, but also Spirochaetes-like and Kiritimatiellaeota-like bacteria. Higher relative quantities of hgcAB genes were found in metagenomes from marine particles compared to free-living communities in anoxic water, suggesting that such particles are hotspot habitats for Hg methylators in oxygen-depleted seawater. Altogether, our work unveils the diversity of the microorganisms with the potential to mediate MeHg production in the Baltic Sea and pinpoint the important ecological niches for these microorganisms within the marine water column.

甲基汞（MeHg）是一种在水生食物网中生物放大的神经毒性化合物，可通过食用鱼对人类健康构成威胁。然而，在海洋系统中介导汞（Hg）甲基化为MeHg的微生物群落的组成和分布仍然未知。为了填补这一知识空白，我们使用了波罗的海参考元基因组（BARM）数据集来研究与汞甲基化有关的基因的丰度和分布（抗体基因簇）。我们确定了相对丰度抗体81个微咸的基因组中的这些基因及其分类身份，涵盖了波罗的海水域中的空间，季节和氧化还原变异性。的抗体 基因主要在缺氧水中检测到，但是有些 抗体在缺氧和常氧水中也检测到基因。系统发育分析确定了在缺氧水层的三角洲细菌中的假定的汞甲基化分子，同时也鉴定了螺旋藻类和Kiritimatiellaeota类细菌。较高的相对数量抗体与缺氧水中的自由生活群落相比，在海洋颗粒的基因组中发现了这些基因，这表明此类颗粒是缺氧海水中汞甲基化剂的热点栖息地。总而言之，我们的工作揭示了微生物的多样性，具有介导波罗的海甲基汞生产的潜力，并指出了海水域内这些微生物的重要生态位。