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汞(Hg)是一种在大气中具有较长停留时间并能进行长距离传输的全球性有毒重金属[1-4]. 汞具有高挥发性、生物累积性和持久性的特点,对人类健康和环境造成严重危害[5-6]. 汞可通过自然过程(如岩石风化、地热活动和火山爆发)和人类活动(如采矿、工业生产和化石燃料燃烧)排放进入大气[7-9]. 海洋在全球汞循环中发挥着重要作用[10]. 2018年联合国环境规划署报告指出,每年排放到大气中的汞约有3700 t会沉降到海洋[11]. 自工业革命以来,随着人为排放汞的增加,海洋中汞浓度显著增加[12],导致海洋鱼类体内汞浓度升高,对人类健康造成巨大威胁[13-14]. 海洋环境中不同形态的汞在迁移转化过程中始终处于一个动态平衡状态,主要控制过程包括甲基化/去甲基化、吸附/解吸、沉积物-水界面交换、氧化/还原等(图1). 其中,吸附/解吸、沉积物-水界面交换等迁移过程控制着汞在不同介质中的分配,氧化/还原过程控制着水体中Hg0的水平,而甲基化/去甲基化是控制海洋环境中甲基汞循环的关键过程.
汞在环境中的主要赋存形态包括:Hg0、Hg2+和MeHg[15-19]. MeHg是一种毒性强、易富集、具有严重生态风险的有毒污染物[5, 20-22],这种生物可利用性高的汞形态能够在高营养级生物体内累积到较高浓度水平[20-21, 23]. 食用海产品是人类暴露MeHg的主要途径,可以导致神经发育障碍和自身免疫反应等疾病[20-21, 24-25]. 通过各种来源排放到海洋环境中的无机汞在沉积物和水柱中均可甲基化为MeHg. 近海沉积物中产生的MeHg也可通过长距离传输进入大洋. 原位生成是海洋甲基汞的主要来源,原位甲基化和去甲基化被认为是控制海洋环境中甲基汞水平的重要过程[18-19, 26-27]. 汞的甲基化和去甲基化均可以通过生物和化学(光化学和非光化学)途径发生[28-29],微生物途径被认为是海洋生态系统中甲基汞生成降解的主要途径[30-33]. 光化学途径在表层海水汞迁移转化过程中发挥着重要作用[34-35].
目前已有综述对环境汞的甲基化与去甲基化过程进行了总结[36-39],但对海洋环境中汞甲基化/去甲基化涉及相对较少. 本文在总结海洋汞甲基化/去甲基化途径、过程机制和微生物类群等基础上,详细讨论了海洋微生物(细菌、古菌和微藻)、海洋光化学以及海洋非光化学汞甲基化/去甲基化过程机制.
海洋环境汞甲基化/去甲基化研究进展
Progress in mercury methylation and demethylation in marine environment
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摘要: 汞(Hg)是一种在大气中具有较长停留时间并能进行长距离传输的全球性污染物. 甲基汞(MeHg)具有毒性强、易富集、可随食物链放大的特点,是引起环境风险的主要汞形态. 通过各种来源排放到海洋环境中的无机汞在沉积物和水柱中均可甲基化为MeHg,原位甲基化和去甲基化是控制海洋环境中甲基汞水平的关键过程. 虽然目前已有综述总结了环境中汞甲基化/去甲基化,但对海洋生态系统中汞甲基化/去甲基化过程涉及相对较少. 本文在总结海洋汞甲基化/去甲基化过程速率、途径和热点区域基础上,详细讨论了海洋光化学、非光化学以及微生物3种汞甲基化/去甲基化途径机制,并对未来研究方向进行了展望. 在现有研究基础上,未来应在不同汞甲基化/去甲基化途径贡献估算、实际海洋环境汞甲基化/去甲基化基因/微生物作用验证、环境因素对海洋汞甲基化/去甲基化影响方面开展更深入研究. 海洋汞甲基化/去甲基化的研究可为深入理解海洋汞的环境行为与风险和发展有效的汞污染风险防控技术提供科学依据和数据支撑.Abstract: Mercury (Hg) is a global pollutant since it has a long residence time in the atmosphere and can be transported over a long distance. Methylmercury (MeHg) has the characteristics of high toxicity, and bioaccumulation along the food chain. It is the major form of Hg that causes environmental risks. Inorganic Hg discharged into the marine environment through various sources can be methylated into MeHg in sediment and water column. In situ methylation and demethylation are the key processes that control the level of MeHg in marine environments. Although the methylation and demethylation of Hg in the environment have been reviewed in several papers, both processes in the ocean have not been comprehensively reviewed yet. In this review, the rates, pathways and hot spots of Hg methylation/demethylation in marine environments were summarized first. Then, the mechanisms of the three Hg methylation/demethylation pathways in marine environments, i.e., photochemical pathway, non-photochemical pathway, and microbial pathway were discussed in detail. Future perspectives of this research area were also discussed. On the basis of existing research, more in-depth research should be carried out in the future on the contribution of different Hg methylation/demethylation pathways, the verification of the importance of Hg methylation/demethylation genes/microbes in Hg methylation/demethylation in natural marine environments, and the impact of environmental factors on marine Hg methylation/demethylation. The research on methylation/demethylation of Hg in the ocean can provide scientific basis and data support for in-depth understanding of environmental behaviors and risks of mercury in marine environments and the development of effective prevention and control techniques for Hg.
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Key words:
- marine /
- mercury /
- methylation /
- demethylation /
- mechanism.
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表 1 海洋和淡水系统汞甲基化和去甲基化速率
Table 1. Mercury methylation and demethylation rates in various marine and freshwater systems
研究区域
Study area介质类型
Matrices甲基化速率/d−1
km去甲基化速率/d−1
kd参考文献
Reference海洋 Marine 长岛海峡 沉积物 0.014—0.082 a 2.0—20 a [44] 新英格兰海岸大陆架 沉积物 0.02—0.21 a — [45] 大西洋中部大陆架 沉积物 0.002—0.053 a 0.02—0.04 a [46] 大西洋中部斜坡 沉积物 0.02—0.05 a 0.4—6.4 a [46] 墨西哥湾 沉积物 0.02—0.19 a 1.6—2.6 a [47] 切萨皮克湾 沉积物 0.01—0.1 a <1—16 a [48-49] 芬迪湾 沉积物 0.0011—0.0018 a 0.15—0.24 a [50] 旧金山湾 沉积物 0.0003—0.006 a — [51] 赤道太平洋 水体 0.004—0.02 a ND—0.78 a [52] 0.002—0.06 b ND—0.55 b 地中海 水体 0.003—0.04 a 0.064—0.19 a [53] <0.0002 b 0.033—0.064 b <0.0002—0.03 c 0.028—0.084 c 淡水 Freshwater 阿杜尔河 沉积物
水体0.03 a 0.096 a [54] <0.0001—0.005 a 0.031—0.903 a [55] <0.0001—0.004 b <0.02—0.576 b <0.0001—0.002 c 0.059—0.0311 c 哈德逊河 沉积物 0.0001—0.0004 a 0.09—1.59 a [50] 大盐湖 沉积物
水体0.0000012—0.0031 a
0.0000012—0.0011a— [56] 泻湖(法国) 水体 0.008—0.063 a 0.128—0.245 a [53] 0.002—0.017 b 0.01—0.14 b 0.012—0.031 c 0.01—0.11 c 帕塔克森特河 沉积物 0.0004 a — [50] a代表原位汞甲基化/去甲基化速率;b代表非生物汞甲基化/去甲基化速率;c代表微生物汞甲基化/去甲基化速率.
a represents the in situ mercury methylation/demethylation rate; b represents the abiotic mercury methylation/demethylation rate; c represents the microbial mercury methylation/demethylation rate.表 2 海洋环境中已确定为汞甲基化微生物或具有hgcAB同源物的微生物
Table 2. Microorganisms identified as either methylators or having the hgcAB homologue in the marine environment
门
Phylum纲
Class属
Genus参考文献
Reference广古菌门(古菌)
Euryarchaeota甲烷微菌纲
Methanomicrobia甲烷微菌属Methanoregula [94] 甲基甲烷菌属Methanomethylovorans [57] 甲烷叶菌属Methanolobus [57] 甲烷螺菌属 Methanospirillum [59] 变形菌门(细菌)Proteobacteria δ-变形菌纲
Deltaproteobacteria地杆菌属Geobacter [95] 脱硫碱螺旋菌属Desulfonatronospira [57] 脱硫微菌属Desulfomicrobium [96] 互营菌属Syntrophus [95] 脱硫弧菌属Desulfovibrio [97] 脱硫叶菌属Desulfobulbus [59] 脱硫球菌属Desulfococcus [59] 脱硫单胞菌属Desulfuromonas [95] 脱硫杆菌属Desulfobacterium [59] 脱硫葡萄状菌属Desulfacinum [98] 硝化刺菌属Nitrospina [61-62] 厚壁菌门(细菌)Firmicutes 梭菌纲
Clostridia脱硫弯曲孢菌属Desulfosporosinus [59] 脱亚硫酸杆菌属Desulfitobacterium [59] 还原硫素杆菌属Dethiobacter [59] 醋线菌属Acetonema [59] 产乙醇菌属Ethanoligenens [59] -
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