義大利西西里島泥火山之微生物族群組成與甲烷循環

Abstract

義大利西西里島西側區域，因 Apennine-Maghrebian 褶皺逆衝斷層帶向北隱沒形成前陸盆地，為西西里島泥火山的主要分布區域。在此區域的 Aragona 泥火山，逸散的氣體以熱裂解及微生物作用產生的甲烷為主。西西里島東側的 Paterno 泥火山，因位於 Mt. Etna 火山南緣，逸散的氣體則主要由來自地函生成的二氧化碳所構成。本研究針對兩處位於不同地質環境的泥火山，藉由地球化學與分子生物分析技術，分析噴泥與沉積物岩芯樣本，以探討在不同甲烷豐度與地質材料特性的泥火山，微生物族群的組成與參與甲烷代謝的微生物地球化學作用。 綜合各項的分析結果顯示，Aragona 泥火山於岩芯 ≥ 8 cm 與 32.5-53.5 cm 區間為甲烷-硫酸鹽過渡帶，其中 ANME-2 與 Desulfarculus/Desulfurivibrio 菌群，其於不同深度的相對含量分布呈現正相關，顯示此兩個區間應以伴隨硫酸還原的厭氧型甲烷氧化作用為主。而 8-32.5 cm 區間則為甲烷生成帶，主要藉由還原二氧化碳與醋酸分解的甲烷菌產生甲烷，並供給淺層與深層的厭氧型甲烷氧化作用所需。西西里島東側 Paterno 泥火山，由於還原態的泥漿補注有限，表層受到較長時間蒸發作用影響，岩芯 0.8-22.8 cm 區間的微生物族群組成以嗜鹽的異營菌、硫氧化菌、好氧甲烷氧化菌為主，其優勢菌種分別為 Halobacteriaceae、Thiohalorhabdus denitrificans 與 Methylohalobius crimeensis。至深部區間，微生物族群的多樣性高，主要由厭氧的發酵菌與硫酸還原菌群構成。 本研究結果顯示兩種不同地質成因泥火山中，不同的流體來源、遷移途徑與環境因子，造成兩個區域微生物族群結構與代謝作用顯著的差異。然而，呈現帶狀分布的複雜微生物組成與微生物作用，並未能有效的完全移除甲烷，反而創造甲烷的淨輸出至大氣並貢獻溫室效應。

The subduction of the African plate underneath the Eurasian plate along the Apennine-Maghrebian fold-and-thrust belt has created fracture network that allows for the upward migration of deeply sourced fluids and sediments to surface environments, producing dense distribution of mud volcanoes (MVs) in western part of Sicily. The MVs in the Aragona area are characterized by high abundances of hydrocarbons generated from thermal decomposition of organic matters and microbial processes. By contrast, the MVs in the Paterno area are located at the southern margin of Mount Etna in eastern Sicily, emitting gases primarily composed of magmatic CO2 and discharging brines. In this study, we employed a combination of geochemical and molecular analyses on samples collected from core sediments and bubbling fluids from MVs associated with these two different geological settings of Sicily to investigate the community assemblages and the role of microbial process in controlling methane emissions and biogeochemical cycling. Our results showed that two sulfate-to-methane transitions at depths of ≥ 8 cm and 32.5-53.5 cm were observed in the Aragona MV. The majority of microbial members at these depth intervals were taxonomically assigned to the ANME-2 group and Desulfarculus/Desulfurivibrio. For comparison, hydrogenotrophic and aceticlastic methanogenesis likely predominated over the other pathways at 8-32.5 cm, supplying methane for the overlying and underlying anaerobic methanotrophy. In the Paterno MVs, fluids and sediments were confined with the bubbling pool with limited discharge to the adjacent mud platform. Surface sediments in the mud platform would have experienced prolonged evaporation and desiccation. Microbial populations at 0.8-22.8 cm were dominated by halotolerant or halophilic aerobic methanotrophs, sulfur oxidizers and heterotrophs affiliated with Methylohalobius crimeensis, Thiohalorhabdus denitrificans and Halobacteriaceae, respectively. At depth, diverse community assemblages were recovered with fermentative and sulfate-reducing bacteria predominating over the others. Overall, the results obtained from this study demonstrated that fluid sources, pathways, and physic-chemical conditions inherited from different geological settings could shape geochemical characteristics, and microbial processes and communities in different MVs of Sicily. The stratified, complexly structured microbial communities and processes did not enable the complete removal of methane in porespace and fracture channel. Instead, net methane emission from these MVs would contribute to the greenhouse warming.