BioTechnology Institute, University of Minnesota, St. Paul, MN, USA.
Department of Earth Sciences, University of Minnesota, Minneapolis, MN, USA.
ISME J. 2019 Jul;13(7):1659-1675. doi: 10.1038/s41396-019-0376-1. Epub 2019 Feb 26.
Methylmercury (MeHg) is a bioaccumulative neurotoxin that is produced by certain anaerobic microorganisms, but the abundance and importance of different methylating populations in the environment is not well understood. We combined mercury geochemistry, hgcA gene cloning, rRNA methods, and metagenomics to compare microbial communities associated with MeHg production in two sulfate-impacted lakes on Minnesota's Mesabi Iron Range. The two lakes represent regional endmembers among sulfate-impacted sites in terms of their dissolved sulfide concentrations and MeHg production potential. rRNA amplicon sequencing indicates that sediments and anoxic bottom waters from both lakes contained diverse communities with multiple clades of sulfate reducing Deltaproteobacteria and Clostridia. In hgcA gene clone libraries, however, hgcA sequences were from taxa associated with methanogenesis and iron reduction in addition to sulfate reduction, and the most abundant clones were from unknown groups. We therefore applied metagenomics to identify the unknown populations in the lakes with the capability to methylate mercury, and reconstructed 27 genomic bins with hgcA. Some of the most abundant potential methylating populations were from phyla that are not typically associated with MeHg production, including a relative of the Aminicenantes (formerly candidate phylum OP8) and members of the Kiritimatiellaeota (PVC superphylum) and Spirochaetes that, together, were more than 50% of the potential methylators in some samples. These populations do not have genes for sulfate reduction, and likely degrade organic compounds by fermentation or other anaerobic processes. Our results indicate that previously unrecognized populations with hgcAB are abundant and may be important for MeHg production in some freshwater ecosystems.
甲基汞（MeHg）是一种生物累积性神经毒素，由某些厌氧微生物产生，但环境中不同甲基化种群的丰度和重要性尚不清楚。我们结合汞地球化学、hgcA 基因克隆、rRNA 方法和宏基因组学，比较了明尼苏达州梅萨比铁矿区两个受硫酸盐影响的湖泊中与 MeHg 产生相关的微生物群落。这两个湖泊在溶解态硫化物浓度和 MeHg 产生潜力方面代表了受硫酸盐影响的区域端元。rRNA 扩增子测序表明，两个湖泊的沉积物和缺氧底层水中都含有具有多种硫酸盐还原菌δ变形菌纲和梭菌属的多样群落。然而，在 hgcA 基因克隆文库中，hgcA 序列来自与甲烷生成和铁还原相关的类群，除了硫酸盐还原之外，还有与甲烷生成和铁还原相关的类群，并且最丰富的克隆来自未知组。因此，我们应用宏基因组学来鉴定具有甲基化汞能力的湖泊中的未知种群，并重建了 27 个带有 hgcA 的基因组。一些最丰富的潜在甲基化种群来自通常与 MeHg 产生无关的门，包括 Aminicenantes（以前的候选门 OP8）的相对门和 Kiritimatiellaeota（PVC 超门）和螺旋体的成员，它们在一些样本中占潜在甲基化剂的 50%以上。这些种群没有硫酸盐还原基因，可能通过发酵或其他厌氧过程降解有机化合物。我们的结果表明，具有 hgcAB 的先前未被识别的种群丰富，并且可能在一些淡水生态系统中对 MeHg 产生很重要。
