| 摘要: | 部分異化性金屬還原菌(dissimilatory metal-reducing bacteria, DMRB)如Shewanella及Geobacter等可在低汞濃度下進行與mer無關的生物性汞直接還原作用，但其背後運轉的機制截至目前為止仍未確認。由於DMRB是沈積物系統(即底泥與地下含水層環境)中掌控污染物質移動與轉化最重要的微生物族群之一，因此深入了解此還原作用的機制層對於未來的相關系統的環境管理與整治將有所助益。有鑑於此，本研究以 Shewanella的Mtr (即metal-reducing)胞外呼吸作用的電子傳遞途徑為模型，利用基因圖譜已全面解密的S. oneidensis MR-1野生菌株，以及分別剔除其細胞外膜表面的多血紅素細胞色素蛋白(即MtrA、MtrB、MtrC及OmcA)的生成基因與黃素類化合物(即flavins)的分泌基因而得的突變菌為試驗模式生物mtrA、mtrB、mtrC、omcA、mtrC/omcA與bfe，進行低汞濃度的暴露試驗，以測試「Shewanella菌屬可利用胞外電子傳遞時所需的外膜細胞色素蛋白將Hg(II)還原成Hg(0)，且此還原效率可因所分泌的內源性黃素類化合物的協助而增強」之假說與機制。實驗結果證實S. oneidensis MR-1的細胞外膜色素蛋白雖可將胞外水溶液中的Hg(II)還原為Hg(0)，但其反應效率與程度遠不及內源性的黃素類化合物的作用；且當MR-1與外源性的電子梭物質AQDS一起培養時，Hg(II)的還原作用只在厭氧狀態且AQDS達一定濃度下才較為顯著。這些結果說明Shewanella在進行胞外的重金屬轉化時，主要依賴的還是自身所分泌的黃素類電子穿梭物；此外，當環境中存在著具有微生物可利用性的氧化還原活躍介質時，可能會更進一步影響汞在環境中的形態與分佈。;Shewanella oneidensis MR-1 is an important model strain of dissimilatory metal-reducing bacteria (DMRB) that has been shown to reduce low-levels ( 0.3 M) of aqueous inorganic Hg(II) under both aerobic and anaerobic conditions via pathways independent of mer activities. Yet, the electron transport mechanism that underpins this Hg redox transformation process is unclear. Given that DMRB like Shewanella are prevalent in soil and sedimentary environments, as well as that Hg(II) reduction by DMRB may be a critical process in these settings that controls the Hg content in the system profile, a better understanding of the processes modulated by DMRB that govern the fate and transport of Hg in the environment is crucial for future environmental management and remediation efforts. In this study, by using MR-1 and its mutant strains lacking the gene(s) encoding the protein(s) associated with the Mtr respiratory pathway, including ∆omcA, ∆mtrA, ∆mtrB, ∆mtrC and ∆omcA/∆mtrC, and the mutant that lacks the primary bacterial FAD exporter (i.e., ∆bfe), laboratory incubation experiments were carried out to test the hypothesis that while the outer-membrane multi-heme cytochromes of S. oneidensis MR-1 may directly involve in the reduction of Hg(II) to Hg(0), this Hg redox transformation process is facilitated in the presence of endogenous (i.e., flavins) and exogenous (i.e., AQDS) electron shuttles. Results show that (i) Hg redox chemistry in the MR-1 culture was a function of Hg-to-cell ratios, suggesting that this (ratio) factor may need to consider when assessing the spread and impact of Hg in the aquifer; (ii) although outer-membrane cytochromes directly participated in the reduction of Hg(II) of Hg(0), endogenous flavins might play a more important role in this process; (iii) the importance of electron shuttling in the conversion of Hg(II) to Hg(0) by MR-1 was further supported by the results of short-term incubations showing that synthetic flavins and AQDS significantly facilitated Hg(II) reduction with an effect of riboflavin > FMN > AQDS; (iv) in the absence of terminal electron acceptors, MR-1 not only remained viable but also maintained the capacity of Hg(II) reduction after incubation with externally provided electron shuttles for 12 hr. |
