Liu Jibao, Bao Zhen, Wang Chenlu, Wei Jinyi, Wei Yuansong, Chen Meixue
State Key Joint Laboratory of Environmental Simulation and Pollution Control, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China; Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China.
Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China.
Water Res. 2022 Nov 1;226:119204. doi: 10.1016/j.watres.2022.119204. Epub 2022 Oct 5.
Municipal sewage especially the produced sewage sludge is a significant source releasing mercury (Hg) to the environment. However, the Hg speciation especially methylmercury (MeHg) transformation in sewage sludge treatment process remains poorly understood. This study investigated the transformation of Hg speciation especially MeHg in sludge composting. The distribution of Hg transformation related gene pairs hgcAB and merAB, and their putative microbial hosts were comprehensively analyzed. Both Hg (from 3.16±0.22 mg/kg to 3.20±0.19 mg/kg) and MeHg content (from 4.77±0.64 ng/g to 4.36±0.37 ng/g) were not obviously changed before and after composting, but about 19.69% of Hg and 27.36% of MeHg were lost according to mass balance calculation. The metagenomic analysis further revealed that anaerobes (Desulfobacterota and Euryarchaeota) were the mainly putative Hg methylators especially carrying high abundance of hgcA gene in the initial periods of composting. Among the 151 reconstructed metagenome-assembled genomes (MAGs), only 4 hgcA gene carriers (Myxococcota, Firmicutes, Cyclobacteriaceae, and Methanothermobacter) and 16 merB gene carriers were identified. But almost all of the MAGs carried hgcB gene and merA gene. The merA gene was widely distributed in genomes, which indicated the widespread functionality of microbes for reducing Hg(II) to Hg(0). The hgcA carrying microbes tends to present the similar metabolic pathways including methanogenesis and sulfur metabolism. Besides, both the irregular distribution of hgcA in various species (including Actinobacteria, Archaea, Bacteroidetes, Desulfobacterota, Euryarchaeota, and Nitrospirae, etc.) and opposite evolution trends between hgcA gene abundance and its host genome abundance can be an indication of horizontal gene transfer or gene deletions of hgcA during composting. Our findings thus revealed that sludge composting is not only a hotspot for Hg speciation transformation, but also a potential hotspot for MeHg transformation.
城市污水尤其是产生的污水污泥是汞（Hg）向环境释放的重要来源。然而，污水污泥处理过程中汞的形态尤其是甲基汞（MeHg）的转化仍知之甚少。本研究调查了污泥堆肥过程中汞形态尤其是甲基汞的转化。全面分析了与汞转化相关的基因对hgcAB和merAB的分布及其假定的微生物宿主。堆肥前后汞含量（从3.16±0.22毫克/千克至3.20±0.19毫克/千克）和甲基汞含量（从4.77±0.64纳克/克至4.36±0.37纳克/克）均无明显变化，但根据质量平衡计算，约19.69%的汞和27.36%的甲基汞损失。宏基因组分析进一步表明，厌氧菌（脱硫杆菌门和广古菌门）是主要的假定汞甲基化菌，尤其是在堆肥初期携带高丰度的hgcA基因。在151个重建的宏基因组组装基因组（MAG）中，仅鉴定出4个hgcA基因携带者（黏球菌门、厚壁菌门、环杆菌科和嗜热栖热菌属）和16个merB基因携带者。但几乎所有的MAG都携带hgcB基因和merA基因。merA基因在基因组中广泛分布，这表明微生物将Hg（II）还原为Hg（0）的功能广泛存在。携带hgcA的微生物往往呈现出相似的代谢途径，包括甲烷生成和硫代谢。此外，hgcA在各种物种（包括放线菌门、古菌、拟杆菌门、脱硫杆菌门、广古菌门和硝化螺旋菌门等）中的不规则分布以及hgcA基因丰度与其宿主基因组丰度之间相反的进化趋势，可能表明堆肥过程中hgcA发生了水平基因转移或基因缺失。因此，我们的研究结果表明，污泥堆肥不仅是汞形态转化的热点，也是甲基汞转化的潜在热点。
