厭氧型甲烷氧化作用在台灣西南部陸域泥火山的可能性評估

Abstract

厭氧型甲烷氧化作用（Anaerobic Oxidation of Methane, AOM）是現今科學家認為減緩由海洋環境的甲烷逸散所造成的溫室效應中最有效的一種微生物作用。在海洋沉積物中 AOM發生於硫酸根及甲烷濃度趨近於零的一介面上（sulfate-methane-interface, SMI），由厭氧甲烷消耗菌與硫酸還原菌所組成的共生的微生物族群，消耗由海水向下滲透的硫酸根及深處由生物或地質作用（例如海底泥火山）所產生向上擴散的甲烷。瞭解這樣的微生物作用，是否得以因應地質作用所造成的硫酸根或甲烷供應的變化，有助於我們探討哪些因子控制全球尺度的甲烷溫室效應，並對碳與硫於不同環境間的循環與交互作用提供更深入的認識。 過去的研究顯示台灣西南部的陸上泥火山與海域泥火山的成因有密切的關係。雖然甲烷的通量無論於陸域或海域泥火山皆十分的高，但硫酸根的供應卻可能有數量級之別，因此本研究的目的在於探討AOM 是否得以適應台灣西南部陸上泥火山甲烷供應充足，而硫酸根較為短缺的情況，仍然扮演一重要的甲烷消耗作用。另外，於硫酸根較為短缺的情況下， AOM 是否仍須仰賴如海洋環境中的共生形式以維繫生存，或者發展出新的共生策略。 本研究選擇了三個採樣地點（嘉義中崙濁水潭、高雄燕巢新養女湖及高雄旗山小滾水）進行短岩芯 (30-50 cm) 的採樣，分析孔隙水與氣體化學，以建構可能的地球化學與微生物作用模式。結果顯示無論是孔隙水化學或氣體化學，於不同的採樣點或深度均有顯著的變化。於不同採樣點的比較中，濁水潭的孔隙水硫酸根濃度最高（介於0.5~8.0 mM之間），而甲烷濃度最低（0.1~0.5 mM）；新養女湖與小滾水的硫酸根濃度皆較濁水潭為低（1 mM 以下），而甲烷濃度則遠大於濁水潭（最高可接近 20 mM）。 深度的變化顯示，大部分的岩芯中隨著深度的增加，硫酸鹽均具有不等程度減少，而甲烷濃度呈現不同程度的增加。由硫酸根與甲烷濃度對深度的變化，每一區域的岩芯皆具備 SMI 特徵相似的深度剖面，可能代表 AOM 作用的存在，其中（1）於濁水潭， SMI 可能發生於 4 公分至 12 公分深度；（2）於新養女湖， SMI 則出現於 8 公分至 15 公分深度；（3）於小滾水， SMI 則分佈於 7 公分至 12 公分深度以及 25公分至 32 公分深度。 綜合所有地球化學的特徵於深度的變化也顯示，除了 AOM 與厭氧型的硫酸鹽還原之外，好氧型的甲烷消耗、厭氧下製造甲烷及乙烷等微生物作用亦參與了於陸域泥火山的硫與碳循環。另外由 SMI 地球化學特徵顯示， AOM 作用仍然必須與硫酸鹽還原作用形成共生的關係；現地的甲烷製造所提供的高通量甲烷，以及經由泥火山噴發與地表的蒸發作用所造成的較高濃度硫酸鹽為延續此共生關係最重要的驅動力。

Anaerobic oxidation of methane (AOM) has been regarded as the most effective methane sink to attenuate the greenhouse effect caused by methane emission in marine environments. AOM typically occurs at the sulfate-methane interface (SMI) at which syntrophic microbial communities composed of anaerobic methanotrophs and sulfate reducing bacteria consume sulfate percolated from the top seawater and methane produced by either microbial or geological processes underneath. Unraveling whether such a syntrophic relationships would be subject to the variations in the sulfate and methane supply induced by geological processes would benefit to single out the factors governing the methane greenhouse effect on a global scale, and to provide insights to the carbon and sulfur cycling in different environmental settings. Previous studies indicated that terrestrial and marine mud volcanoes in the southwestern Taiwan might bear the intimate origin. While methane concentrations are generally high in both marine and terrestrial mud volcanoes, sulfate supply could vary orders of magnitude. The purpose of this study was to explore (1) whether AOM could be still sustained or even actively act as an important methane sink in southwestern mud volcanoes where sulfate would be a limited factor for AOM; and (2) whether AOM is dependent upon the syntrophic relationships with sulfate reduction or a new syntrophic strategy is developed to accommodate the restricted sulfate supply. This study chose three sites (Chao-Shih-Tan, Shin-Yang-Nih-Hu, and Shiao-Kun-Shih) for sampling of short push-cores (30-50 cm) from which pore water and gas chemistry of depth intervals were analyzed in order to construct a potential geochemical and microbiological model for terrestrial mud volcanoes. The results indicated that both pore water and gas geochemistry varied substantially both in geographic locations and on vertical scales. For geographical locations, pore water in Chao-Shih-Tan possessed highest sulfate (0.5-0.8 mM) but lowest methane (0.1-0.5 mM). In contrast, lower sulfate (<1 mM), and greater methane (up to 20 mM) were observed in Shin-Yang-Nih-Hu, and Shiao-Kun-Shih. Both pore waters in Chun-Lun and Shiao-Kun-Shih were highly saline (generally above 200mM and up to 650 mM Cl-), whereas moderately saline (generally 100-160 mM Cl-) for Shin-Yang-Nih-Hu. Sulfate and methane abundances declined and enhanced at various degrees, respectively, with the increase of depth. The SMI features based on the sulfate and methane variations over depth were observed in all three locations, suggesting the ubiquitous presence of AOM in southwestern mud volcanoes. The depth intervals of the SMI were assigned at depths of 4 to 12 cm in Chun-Lun, of 8-15 cm in Shin-Yang-Nih-Hu, and of 7-12 cm and 25-32 cm in Shiao-Kun-Shih. Despite AOM and sulfate reduction, aerobic methane oxidation, methanogesis and ethanogenesis were also involved in the carbon and sulfur cycling in the southwestern terrestrial mud volcanoes. The syntrophic relationships between AOM and sulfate reduction was primarily driven by the high methane flux produced from the in situ methanogenesis and elevated sulfate abundances generated during the surface evaporation of the erupted muddy fluids.