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標題: | 臺灣泥火山與淡水湖泊甲烷循環與微生物作用的特徵分析 Characterization of methane cycle and microbial process in mud volcanoes and freshwater reservoir in Taiwan |
作者: | 林悅婷 Yueh-Ting Lin |
指導教授: | 林立虹 Li-Huang Lin |
關鍵字: | 泥火山,甲烷循環,甲烷叢集同位素,微生物作用, mud volcanoe,methane cycle,methane clumped isotopologues,microbial process, |
出版年 : | 2024 |
學位: | 博士 |
摘要: | 探討環境中甲烷的生成機制,需配合現地地質條件及利用多年的統計累計資料討論同位素與碳氫化合物相對含量,以區分微生物產甲烷作用、有機質熱裂解作用或無機甲烷合成作用 (Milkov & Etiope, 2018)。然而有機質的原始同位素成分、成熟度、移棲,或不同的微生物產甲烷的途徑,都可能導致甲烷同位素成分與傳統定義的的成分相異,造成運用同位素判讀甲烷形成機制的複雜度 (Rowe & Muehlenbachs, 1999; Whiticar, 1999)。而淺層地表中的微生物氧化作用的加入,不論是好氧或厭氧的甲烷氧化菌都是傾向使用較高比例的 12C 或 D 的前趨物 (Whiticar, 1999),甲烷氧在降低甲烷排放到大氣的機制多數仰賴好氧的甲烷消耗作用 (Kotelnikova, 2002),或與金屬或硝酸根耦合的厭氧甲烷消耗作用 (Cui et al., 2015),討論淡水中甲烷的微生物消耗機制,有助於討論不同環境中甲烷循環的調控機制。只仰賴單一種同位素指標來區分環境中的甲烷的形成機制或微生物氧化的對甲烷循環的影響是有難度的。
利用甲烷叢集同位素來討論甲烷的形成機制或受微生物控制的動力學相關的訊號 (Giunta et al., 2022; Young et al., 2017) 已逐漸成熟,對於穩定大陸地塊環境的熱分解作用產生的甲烷,可以提供大部分甲烷生成的地層溫度 (Giunta et al., 2019, Young et al., 2017);或是微生物氧化的受到特定酶影響的碳跟氫同位素分化的特徵 (Liu et al., 2023; Krause et al., 2022),再結合次世代高通量分析技術以 16S rRNA 基因序列跟微生物族群的多樣性分析配合地球化學的結果討論,將能更有針對性的討論特定微生物族群與地球化學間交互作用的相關性。 本研究利用甲烷的叢集同位素的討論及其它多種同位素,討論臺灣陸、海域的泥火山、氣體滲漏的氣體跟流體的來源。在臺灣大部分的泥火山樣本可以藉由甲烷的叢集同位素推算其形成溫度,其形成深度在 2 到 9 公里範圍內;部分甲烷的叢集同位素的樣本則藉由甲烷的四個同位素來討論二個端元成分的混合。另外,針對新養女湖泥火山,討論表層氧氣氧化深部來源的甲烷跟硫化物與微生物作用的相關,研究深層還原物質對微生物地球化學的影響。化學通量計算的結果表示,氧氣的滲透僅限泥漿池表層 4 毫米附近,溶解的硫化物跟甲烷氧化是空氣-流體界面氧氣消耗的重要原因。對於陸域淡水環境的甲烷氧化的富化培養,受到特定酶影響的變化,透過甲烷的叢集同位素結合與甲烷消耗相關的微生物族群及地球化學分析了解有氧與無氧條件下,微生物甲烷氧化的叢集同位素變化的趨勢。以 DNA 和 RNA 分析結果表示 type II 與 type I 的甲烷氧化菌分別在缺氧和高氧氣濃度下占主導優勢;結合其它甲烷代謝相關的叢集同位素訊號,可做為區分特定酶的微生物途徑的指標。 Understanding the generation mechanisms behind environmental methane involves considering the relative contents of isotopes and hydrocarbons alongside local geological setting and accumulated statistical data. This aids in distinguishing between microbial methanogenesis, thermogenesis, or abiotic methane synthesis (Milkov & Etiope, 2018). However, variations in the original isotope composition of organic matter, its maturity, migration, or diverse microbial methanogenesis pathways may lead to methane isotope compositions differing from the established norms. This complexity poses challenges in using isotopes to interpret methane formation mechanisms (Rowe & Muehlenbachs, 1999; Whiticar, 1999). Moreover, the introduction of microbial oxidation in shallow surface layers, whether aerobic or anaerobic methanotrophs, tends to favor higher proportions of 12C or D precursors (Whiticar, 1999). Methanotrophy plays an effective role in reducing the export of methane from subsurface environments to the atmosphere, attenuating greenhouse effect over geological and contemporary time scales. In terrestrial, freshwater environments, sulfate is often scarce, thereby facilitating methane oxidation potentially coupled with the reduction of oxygen or other electron acceptors (Cui et al., 2015; Kotelnikova, 2002). Exploring the microbial methane consumption in freshwater will help clarify the regulatory mechanisms of methane cycle across diverse environments. Clarifying the formation mechanisms, and regulatory factors of methanotrophy using stable isotope approaches remains challenging as methane formation mechanisms, multiple community members or electron acceptors could have generated comparable isotopic patterns. The abundances of methane clumped isotopologues with two rarely substituted isotopes have been theoretically calculated and experimentally tested to estimate the temperature at which methane is formed or thermally equilibrated or isotopic re-equilibration performed by Methanotrophy kinetic controlling processes, especially in sulfate dependent marine methane oxidation. Such a thermometer overcomes the inherent disadvantages of the conventional approach that uses isotopic compositions of two coexisting, equilibrated phases. Advancements in mass spectrometry enable to measure two doubly substituted isotopologues (13CH3D and 12CH2D2) at high precision, providing an opportunity to examine the coherency of methane formation temperature and to deconvolute the origins of methane with additional constraints. Understanding the impact of oxidation is crucial when utilizing △13CH3D and △12CH2D2 values as indicators of the methane affected by microorganisms. Combined with 16S rRNA gene sequences, diversity analysis of microbial population and geochemical analysis, will enable to exploration of relationship between specific microbial communities and geochemistry. This study aims to determine the source of methane from mud volcanoes, seeps, and springs onshore and offshore Taiwan using the abundances of two rare isotopologues of methane, and other isotopic. A portion of the samples were characterized by values falling on the range delineated by the equilibrium line, and predicting a temperature range between 99-260°C. These temperature estimates together with the isotopic compositions suggested that methane was formed by thermal maturation at great depths. Flux assessments suggest the samples collected from the SYNHMV revealed a steep transition in redox conditions, where oxygen infiltration was limited to the upper 4 mm at fluid-air interface, and suggest that the oxidation of dissolved sulfide contributes to oxygen depletion at surface layer. The results of the incubations of mixed methanotrophic populations under anoxic and oxic conditions, alongside earlier findings on anaerobic methane oxidation coupled with sulfate reduction demonstrate that the coupled rare methane isotopologues could be an effectively diagnostic tool. This tool distinguish the pathways and populations of methane oxidations under different environmental. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/91782 |
DOI: | 10.6342/NTU202400227 |
全文授權: | 同意授權(限校園內公開) |
顯示於系所單位: | 地質科學系 |
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