汞是具有強烈毒性的微量金屬，且是國際公認需優先管控的全球性污染物。雖然所有的汞物種皆具毒性，但造成最多傷害也最被關注的仍是甲基汞。甲基汞已知主要是由現地環境中的厭氧菌將無機汞攝取進細胞內後轉化生成，但這些可進行汞甲基化作用的菌群中，相較於硫酸鹽還原菌、鐵還原菌與甲烷生成菌，醱酵菌是近期才被確認，因此目前對其如何攝取無機汞、以及甲基化汞的能力為何仍不甚清楚。由於醱酵菌是許多厭氧程序/工法中(如掩埋場、污泥消化、地下水含氯有機溶劑生物復育)的關鍵菌群，因此深入了解此菌群的汞甲基化行為將可更精確的評估特定系統的甲基汞生成與累積潛勢。本研究利用文獻所記載擁有相對較高汞甲基化能力的Desulfitobacterium metallireducens為模式醱酵菌株，藉由調控培養液的組成來了解此菌可能對汞的攝取與甲基化的能力與潛力，過程中也與另一常用且研究較為透徹的鐵還原模式菌株Geobacter sulfurreducens PCA進行比較。研究結果顯示D. metallireducens在有汞-氯/硫化物的環境中其自身汞甲基化能力並不顯著，而即使在有汞-硫醇錯合物(即Hg-cysteine)的狀態下，其汞甲基化的能力表現仍相當有限，皆遠遜於G. sulfurreducens PCA；此外，本研究也對D. metallireducens進行生物性去甲基化之實驗，結果表明此菌有著劇烈的去甲基化作用發生。綜合以上，本研究結果顯示此模式醱酵菌的汞甲基化能力並不出色，且當環境中達一定濃度之甲基汞時則會進行去甲基化，暗示著醱酵菌對於厭氧系統如掩埋場中的汞甲基化生成潛勢增加應不顯著。;Mercury (Hg) is a highly toxic trace element that has been internationally recognized as a global priority pollutant. While all Hg species are toxic, the most harmful and concerned one is methylmercury (MeHg). MeHg is known to be produced in situ by certain anaerobes via a process involving uptake and intracellular methylation of inorganic Hg. Of all the known Hg-methylating microbes, little is understood about how fermentative bacteria carry out this task, compared to other Hg methylators such as sulfate-reducers, iron-reducers, and methanogens. Given that fermenters are key players in anaerobic systems or treatments like landfills, sludge digesters, and bioremediation of chlorinated solvents, it is important to acquire a deeper understanding of Hg methylation processes in such bacteria in order to be able to more accurately predict MeHg impact in a particular system. In this study, Desulfitobacterium metallireducens, which has been reported to have higher Hg methylation capacity than other fermentative bacteria, is used as a model organism, along with another model strain Geobacter sulfurreducens PCA. Results show that D. metallireducens is incomparable with G. sulfurreducens PCA, as it does not exhibit marked Hg methylation in the presence of Hg-chloro or Hg-sulfide complexes, nor in the presence of Hg-thiol complexes (i.e Hg-cysteine). Results of biological Hg demethylation assays also show that this strain has a strong demethylation capacity. Together, these results suggest that potential of MeHg formation and accumulation in systems like landfills may be insubstantial under the condition when fermenters are the dominant microbial guild.
