Human-induced expansion of oxygen-deficient zones can have dramatic impacts on marine systems and its resident biota. One example is the formation of the potent neurotoxic methylmercury (MeHg) that is mediated by microbial methylation of inorganic divalent Hg (Hg^II) under oxygen-deficient conditions. A negative consequence of the expansion of oxygen-deficient zones could be an increase in MeHg production due to shifts in microbial communities in favor of microorganisms methylating Hg. There is, however, limited knowledge about Hg-methylating microbes, i.e., those carrying hgc genes critical for mediating the process, from marine sediments. Here, we aim to study the presence of hgc genes and transcripts in metagenomes and metatranscriptomes from four surface sediments with contrasting concentrations of oxygen and sulfide in the Baltic Sea. We show that potential Hg methylators differed among sediments depending on redox conditions. Sediments with an oxygenated surface featured hgc-like genes and transcripts predominantly associated with uncultured Desulfobacterota (OalgD group) and Desulfobacterales (including Desulfobacula sp.) while sediments with a hypoxic-anoxic surface included hgc-carrying Verrucomicrobia, unclassified Desulfobacterales, Desulfatiglandales, and uncharacterized microbes. Our data suggest that the expansion of oxygen-deficient zones in marine systems may lead to a compositional change of Hg-methylating microbial groups in the sediments, where Hg methylators whose metabolism and biology have not yet been characterized will be promoted and expand.

中文翻译：

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波罗的海缺氧水域促进下层沉积物中未表征的汞甲基化微生物

人为造成的缺氧区扩张会对海洋系统及其居民生物群产生巨大影响。一个例子是在缺氧条件下由无机二价汞 (Hg ^II )的微生物甲基化介导的强神经毒性甲基汞 (MeHg) 的形成。缺氧区扩大的一个负面后果可能是由于微生物群落有利于微生物甲基化汞而导致甲基汞产量增加。然而，关于 Hg 甲基化微生物的知识有限，即来自海洋沉积物的那些携带对介导这一过程至关重要的hgc基因的微生物。在这里，我们旨在研究hgc的存在来自波罗的海四种表层沉积物的宏基因组和宏转录组中的基因和转录物，具有不同浓度的氧气和硫化物。我们表明潜在的 Hg 甲基化因子在沉积物中因氧化还原条件而异。具有氧化表面的沉积物具有主要与未培养的脱硫杆菌门（OalgD 组）和脱硫杆菌目（包括脱硫杆菌属）相关的 hgc 样基因和转录物，而具有缺氧-缺氧表面的沉积物包括hgc-携带疣微菌、未分类的脱硫杆菌、脱硫腺菌和未鉴定的微生物。我们的数据表明，海洋系统中缺氧区的扩大可能导致沉积物中汞甲基化微生物群的组成发生变化，其中代谢和生物学尚未表征的汞甲基化将得到促进和扩大。