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  • 學位論文

溫度調控微生物硫酸還原作用之硫同位素分化研究:以Thermodesulfobacterium與Desulfovibrio菌株為例

Temperature controls of the sulfur isotope fractionation during sulfate reduction by Thermodesulfobacterium and Desulfovibrio strains

指導教授 : 林立虹
共同指導教授 : 王珮玲
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摘要


硫酸還原菌會將海水中硫酸鹽還原成硫化氫,最終形成黃鐵礦被保存在沈積物中,而硫酸鹽還原過程中同時伴隨有機物的氧化分解,因此硫酸還原作用被視為影響碳-硫-鐵循環的重要過程之一。由於硫酸還原菌所進行的硫酸還原作用造成硫同位素的分化,且分化程度與環境因子有關,因此地質記錄中硫酸鹽類礦物沈澱與沈積型黃鐵礦的硫同位素成份可提供重要的地球環境變動訊息。 硫酸還原菌造成硫同位素分化程度差異的原因仍然不清楚,可能與物種差異和細胞硫酸鹽還原速率有關,或是受到溫度、硫酸鹽濃度和電子供應者等因素所影響。為解釋複雜的分化情形,前人提出細胞內部代謝步驟的概念模型,並加入質量流的計算,用以推估微生物代謝過程對硫同位素分化程度的影響,其中硫酸鹽進出細胞的通量與細胞內部硫酸鹽轉換的通量是影響硫同位素分化程度的重要因子。 本研究利用培養嗜熱菌 Thermodesulfobacterium commune 接近株與嗜溫菌 Desulfovibrio gigas 探討溫度如何影響硫酸還原菌的生理代謝,並進而影響硫同位素的分化程度。Thermodesulfobacterium commune 接近株的培養溫度範圍為 34-79 oC,最適生長溫度為 72 oC,而最高細胞硫酸還原速率發生在 77 oC,其同位素分化範圍在8.2-31.6 ‰ 之間,在大多數培養溫度下硫同位素分化在8.2-18.9 ‰ 之間上下變動,僅在68 oC出現一高值。Desulfovibrio gigas 的培養溫度範圍為 10-41 oC,最適生長溫度為33 oC,而最高細胞還原速率發生在41 oC,其同位素分化範圍在 10.3-29.7 ‰ 之間,最大分化程度出現在最高溫以及最低溫的培養,而最小分化程度則出現在中間溫度。兩菌株所產生的分化範圍與前人研究相近,但與同種屬菌株相比則其最大分化皆大於前人研究結果,顯然菌株培養的條件不同便會造成同位素分化程度的不同。此外,此二菌株之分化程度隨溫度的變化趨勢並不相同,其變化並不符合前人根據細胞生理特性隨溫度變化之理論分化模型。將多重硫同位素分析結果與前人模式所建立的多重硫同位素分化網格比較,顯示該網格並無法包絡本研究的分析結果,表示前人模式所建立的多重硫同位素分化網格仍須重新檢討。 綜合上述討論可知,微生物硫酸鹽還原作用造成的硫同位素分化程度雖已有各種實驗觀察和模型加以解釋,但仍有諸多不一致的分析結果,其中溫度雖為單一環境因子,但對菌株的生理學特性可能並無系統性的影響,也因此難以預測其造成硫同位素分化的程度變化。未來若能結合不同菌株之生理學特性對於環境因子的反應與多重硫同位素分析,將可更進一步探討硫同位素分化程度與硫酸還原菌之特定生長條件之間的關係。

並列摘要


Microbial sulfate reduction is a major mechanism driving anaerobic mineralization of organic matter in global ocean. While sulfate-reducing prokaryotes are well known to fractionate sulfur isotopes during dissimilatory sulfate reduction, unraveling the isotopic composition of sulfur-bearing minerals preserved in sedimentary records could provide invaluable constraints on the evolution of seawater chemistry and metabolic pathways. Variations in sulfur isotope fractionations are partly due to inherent differences among species and also affected by environmental conditions (e.g. sulfate abundance and temperature). Sulfur isotope fractionations caused by microbial sulfate reduction have been interpreted to be caused by a sequence of enzyme-catalyzed kinetic isotope fractionation steps. The fractionation factor mainly depends on (1) the sulfate flux into and out of the cell, and (2) the flux of sulfur compound transformation between the internal pools. This study examined the multiple sulfur isotope fractionation patterns catalyzed by a thermophilic Thermodesulfobacterium-related strain and a mesophilic Desulfovibrio gigas over a wide temperature range. The Thermodesulfobacterium-related strain grew between 34 and 79 oC with an optimal temperature at 72 oC and the highest cell-specific sulfate reduction rate at 77 oC. The isotope fractionation (ε34Ssulfate-sulfide) ranges between 8.2 and 31.6‰ with a maximum at 68 oC. The D. gigas grew between 10 and 45 oC with an optimal temperature at 30 oC and the highest cell-specific sulfate reduction rate at 41 oC. The isotope fractionation ranges between 10.3 and 29.7 ‰ with higher fractionations at both lower and higher temperatures. The isotope fractionation causing by these two strains is similar to previous reports, but the maximum fractionation is greater than that by the same species. Apparently, the differences in growth conditions may cause the different isotope fractionation. In addition, the change of fractionation with temperature is different for the two strains and cannot be predicted by a standard model considering physiological characteristics of cells. The result of multiple sulfur isotope measurements in this study cannot be described by a sulfate reduction network, which calculated the Δ33S and δ34S values by assuming the equilibrium fractionation among internal steps. Indeed, the sulfate reduction network has to be reevaluated. Although there are many experiments and several models to study the sulfur isotope fractionation by microbial sulfate reduction, but the result is not conclusive. Temperature is one of the most important environmental factors, but it may not make systemic influence on the physiology of strains and also the isotope fractionation. Further studies regarding physiological responses to environmental factors with the multiple sulfur isotope analysis may probably offer a linkage between sulfate isotope fractionation and growth conditions by sulfate reducing microorganisms.

參考文獻


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