| 摘要: | 環境中汞複雜的循環與轉化反應裡，微生物扮演著極為重要的角色。然而目前對於汞與微生物之間的互動，研究最為徹底的卻只侷限於帶有能將二價汞還原成零價汞之汞抗性基因群的好氧菌；相對而言，此基因群於厭氧菌或是在厭氧環境中的表現卻是所知甚少。即使如此，在受汞汙染的地下水質裡常觀察到飽和之汞蒸氣，推測其原因乃是由於地下水層中受刺激的微生物其活動所引起。此也意味著目前常用的現地生物整治工程在執行時應更為謹慎，因為其方法同樣依賴受汙染的地下層中的厭氧菌，使其刺激後進行污染物的降解與轉化，在這過程中可能會導致汞於地下水中的流佈。基於此，厭氧微生物的活動與汞在含水層中移動的相關性需要更進一步的了解。在本研究計畫中，我們將利用從地下層底泥中培育與篩選出來的脫硝菌，於模擬現地地下水化學的培養液裡，進行脫硝狀態下汞的氧化還原實驗，從中探討汞受厭氧微生物影響之價數轉變機制。以下的兩個假設將被測試：(一)在脫硝環境中，假設汞的還原主要由汞抗性基因群控制；(二)過程中如有汞氧化的現象，假設是由微生物行脫硝反應時所排放之代謝物所引起。實驗系統裡的氣液兩相中汞之組成，除了儀器化學定性定量外，亦將佐以基因改良之生物感應器分析；此外，微生物菌相與汞抗性基因群的表現都將被分析。本研究計畫的成果對於降低未來預測汞於地下含水層中移動時的不確定性將有所助益。 Due to its unique physicochemical property, mercury (Hg) has been considered as a global pollutant of concern. It has been widely recognized that microorganisms play a key role in mediating biogeochemical cycles of Hg in the environment. To date while microbial Hg-reduction has been well documented in aerobic resistant prokaryotes carrying mer operons that are capable of reducing Hg(II) to Hg(0), little work has been undertaken to elucidate mer activities among anaerobes and in anoxic zones. Regardless, it has been suggested that Hg-contaminated groundwater, which may be super-saturated with Hg(0), can result from stimulation of microbial activities in subsurface environments. This strongly imply that conventional in situ bio-remedial actions relying on stimulation of indigenous microbial activities to enhance microbial degradation and reductive transformation of pollutants in contaminated subsurface sediments should be cautioned, as they might inadvertently mobilize Hg into groundwater. Consequently, a better understanding of the processes mediated by anaerobic microbial consortia that control the mobility of Hg in aquifers is crucial for future environmental management and remediation efforts. Because nitrate often appears as a (co-)contaminant in groundwater, we propose to conduct laboratory microcosm experiments to explore the interactions between Hg and dissimilatory nitrate-reducers, thereby investigating the mechanisms of Hg redox transformations mediated by denitrifying activities. We hypothesize that under environmentally relevant nitrate-reducing conditions, Hg(II) reduction is mainly controlled by the mer operons, whereas intermediates along denitrification process may result in Hg(0) oxidation incidents, thus limiting overall Hg mobility. The experiments will be carried out using denitrifying enrichments and pure cultures derived/isolated from subsurface sediments. The cultures will be grown in an artificial groundwater medium, an oligotrophic medium constructed with principal chemical components revealed by on-site monitoring wells to simulate in situ groundwater chemistry. Chemically, we will quantify the speciation of Hg, i.e., Hg(II) and Hg(0), in the system and attempt to identify the compounds responsible for Hg(0) oxidation, if any; microbially, we will characterize the change of bacterial community structure response to Hg toxicity; genetically, we will ascertain how these Hg redox transformations are related to the presence and expression of merA, the gene that encodes for mercuric reductase. The proposed research has the potential to greatly reduce the uncertainty associated with predicting the movement of Hg in the subsurface. Also, information gained from this study will be applicable to the study of other contaminants in natural and engineered systems. 研究期間：10001 ~ 10012 |
