汞於脫硝狀態下之氧化還原作用;Mercury redox transformations under denitrifying conditions

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Title:	汞於脫硝狀態下之氧化還原作用;Mercury redox transformations under denitrifying conditions
Authors:	李靖婷;Li,Jing-Ting
Contributors:	環境工程研究所
Keywords:	脫硝狀態;汞氧化還原;汞抗性操作子;硫醇基團;denitrifying conditions;Hg redox transformations;mer operon;thiol compounds
Date:	2016-08-26
Issue Date:	2016-10-13 15:26:13 (UTC+8)
Publisher:	國立中央大學
Abstract:	汞雖不具任何生物性功能，但在其複雜的環境生地化循環過程中，多數關鍵性的反應乃受到微生物直接或間接的操控。截至目前為止，汞與微生物之間的交互作用研究最為透徹的，乃是原核生物細胞內「汞抗性操作子」(mer operon)中可將二價汞還原成元素汞的汞還原酶(MerA)運轉機制，然此抗汞機制於脫硝狀態下的表現為何、對於汞最後的化性與移動性的影響為何，文獻上幾無相關資訊可得。由於地下水受硝酸鹽污染已成為世界各地最常見的地下水污染問題之一，且多種污染物共存於地下環境的情況並非罕見，再加上近期文獻指出元素汞在缺氧環境中可因天然有機物、微生物分泌物或其細胞膜套上的硫醇官能基氧化成二價汞，因此釐清地下含水層於脫硝狀態下汞受到MerA及硫醇化合物的衝擊程度即顯得有其必要性與時效性。為此，本研究利用基因全組具有汞抗性操作子的脫硝菌Cupriavidus metallidurans CH34，以及剔除其汞抗性基因群所成的變種菌C. metallidurans AE104為模式生物，設計厭氧汞添加培養試驗，深入了解「汞物種」、「脫硝菌」、以及「汞抗性操作子」彼此間在硝酸鹽還原狀態封閉系統中的相互影響作用，盼進一步推估汞在地下飽和層於此狀態下可能的污染擴散。實驗結果證實，雖然在高濃度二價汞的情況下CH34會立即誘導汞抗性操作子以進行汞還原作用，但隨著系統內菌數的增加使得附屬於細胞被膜上的硫醇濃度跟著上升，反而造成原本經還原作用所產生的元素汞發生再氧化的現象，且當進一步以元素汞作為汞來源時仍可觀察到此氧化現象，證實主要是因細胞膜上的硫醇作用所致。本研究結果意味著若地下含水層中的汞在脫硝狀態時即使有汞還原酶的存在，最終仍可能因為元素汞與細胞硫醇基鍵結生成錯合型二價汞，而降低整體汞的流佈與傳輸，亦即有無汞還原酶。;Although mercury (Hg) is a toxic element with no known biological role, it has been widely acknowledged that of various Hg biogeochemical transformations that undergo once Hg is released to the environment, microorganisms play a key role in controlling many of these important reactions by direct or indirect means. To date, the most well characterized interaction between Hg and microbes has been conferred by the Hg resistance (mer) operon, which is an inducible genetic system that encodes a mercuric reductase (MerA) catalyzing the reduction of oxidized Hg, i.e., Hg(II), to elemental Hg, i.e., Hg(0). However, while mer-mediated microbial reduction of Hg(II) has been extensively studied in aerobic resistant prokaryotes, at present little is known about mer activities under denitrifying conditions, particularly how the expressions of merA affect the ultimate chemistry and mobility of Hg in a confined (closed) system such as the aquifer. Given that (i) nitrate has been considered as one of the most commonly occurring pollutants in worldwide groundwater contamination scenarios, (ii) coexistence of two or multiple pollutants in the aquifer is not rare, and (iii) lately anaerobic oxidation of Hg(0) by thiol moieties associated with natural organic matter, cell exudates and the cell envelope has been reported, it is critical and timely to investigate how the behavior of Hg is influenced by MerA and thiolates concurrently in anoxic groundwater environments. In this study, laboratory incubation experiments were conducted using Cupriavidus metallidurans CH34, a denitrifying bacterium that harbors mer operon, and its mutant strain AE104 that lacks mer operon in the genome as the model organisms grown under nitrate-reducing conditions with spike of either Hg(II) or Hg(0) to investigate the interactions among Hg species, denitrifying cells and MerA, and the resulting effect that can be useful in predicting Hg movement in the aquifer. Results showed that while Hg(II) was reduced rapidly to Hg(0) by MerA in the presence of elevated Hg(II) levels, the extent on which Hg(0) was re-oxidized back to Hg(II) was proportional to the growth of cells, presumably due to the increase in the quantity of cell associated thiols. This thiol-induced oxidative complexation between Hg(0) and thiols on cells was further confirmed when Hg(0) was introduced into the culture as one of the Hg sources for the bacteria to react with. These results suggest that under nitrate-reducing conditions, the presence of MerA most likely has little impact on Hg mobility in the saturated zone of aquifers.
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