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群的好氧菌；相較之下，對於此基因群於厭氧菌或是在厭氧環境中的表現，亦或是因其它微生物活動而造成汞氧化態之改變，所知卻是。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。近期有研究指出部分鐵還原菌在不具備汞抗性基因的狀態下，能將汞厭氧還原，而且當這些菌種以固態鐵礦作電子接受者時還原效率最高。然而，目前對於整個過程背後的機制了解卻是有限。為了要釐清此獨特的微生物之汞還原反應機制，在本研究計畫中，我們將同時利用已突變跟未突變的目標微生物，和從地下層底泥中培育出來的鐵還原菌群，調整其所生長所需之營養源的化學，進行汞氧化還原之縮膜試驗，以測試以下的假設：(一)在只能利用固態鐵之環境中，鐵還原菌可能使用其鐵呼吸所需之電子傳遞過程中的酵素將二價汞還原成元素態汞；或者(二)在鐵還原菌一般生長代謝過程中，因對其周圍礦物相的改變而間接導致汞氧化態的變化。實驗除了對氣液兩相中汞之組成作分析外，亦將對鐵礦物相加以定性；此外，微生物菌相的變化與汞抗性基因群的表現也都將被分析。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 governing 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 other possible microbially mediated reactions associated with Hg redox chemistry in anaerobic environments. Recent observations, however, suggest that Hg sensitive iron-reducing bacteria (FeRB) reduce Hg(II) constitutively. In anoxic zones with low levels of Hg, this newly discovered process might more critically affect Hg speciation then the inducible mer-meidated reduction. Relevantly, 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 iron has been considered as one of the most abundant elements for microbes to respire in groundwater environments, we propose to conduct laboratory microcosm experiments to explore the interactions between Hg and dissimilatory iron-reducers. We hypothesize that under environmentally relevant iron-reducing conditions, subsurface microorganisms may reduce Hg(II) by redox-active macromolecules and/or by a coupled biotic/abiotic pathway controlled by the formation of reactive secondary Fe(II)/Fe(III) minerals. To test these hypotheses, by conducting microcosm experiments coupled with molecular biology we will (i) distinguish the roles of electron transport chains from that of biogenic Fe(II) in the reduction of Hg(II) and directly test the role of known components of the electron transport chain of model FeRB in this activity; (ii) investigate the role of biogenic Fe(II)/Fe(III) minerals in Hg(II) reduction by examining production of these minerals during growth of FeRB on ferrihydrite and by exogenously supplied and well characterized Fe(II)/Fe(III) mineral phases; as well as (iii) examine biotic/abiotic pathways for Hg(II) reduction in enrichment cultures derived from subsurface sediments. 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：10008 ~ 10107
