台灣西南海域冷泉系統之好氧甲烷氧化菌族群組成及表現

Abstract

甲烷氧化菌是唯一將甲烷作為碳源及能量來源的微生物，在環境中扮演調節甲烷通量的重要角色，過去研究顯示在台灣西南海域之沉積物與海水交界面存在高甲烷通量，值得探討甲烷氧化菌在此系統中的特徵和作用。本研究選擇台灣西南海域四方圈合海脊、MV1 泥火山及 G96 逸氣通道作為研究區域，以水下無人載具 (Remotely Operated Vehicle, ROV) 進行海底觀測及採樣，透過氣體化學分析以了解甲烷濃度變化，並藉由分子生物技術，進行 16S rRNA 基因與 pmoA 功能性基因的定序及族群豐度分析，以了解環境中好氧甲烷氧化菌 (aerobic methanotrophs) 族群結構及基因表現。 透過沉積物孔隙水甲烷濃度變化顯示有深部來源的甲烷，而在沉積物表層與其上方 1 m 的海水甲烷濃度可相差高達 6 個數量級，推測沉積物與海水交界面存在顯著的甲烷消耗機制。根據 16S rRNA 基因序列結果顯示環境中包含 Type I 及 Type II 的好氧甲烷氧化菌，主要以 Type I 的 Methylomonaceae 菌科佔大宗，並且以底層海水樣本有最高的相對豐度，顯示海洋深層環境相對適合好氧甲烷氧化菌生存及表現；透過群集分析可看出在海水及沉積物樣本間族群結構明顯不同，但仍發現存在 16 個共有的好氧甲烷氧化菌 OTUs。即時定量聚合酶鏈鎖反應 (quantitative polymerase chain reaction, qPCR) 的分析結果顯示在 G96 的表層沉積物樣本中有最多的 pmoA 數量，比較各採樣點的表層沉積物可發現 pmoA 數量與甲烷含量有關；然而在海洋水柱樣本中只有在出現高流體噴氣的 MV1 可看到顯著的 pmoA 數量變化，在剖面中甲烷濃度下降區域存在較高的基因表現量，此外在各採樣地區底層海水與表層沉積物孔隙水的甲烷濃度差值，也與底層海水中 pmoA 基因表現量呈現正相關，顯示海洋環境中微生物作用對於甲烷調控的重要性。

Methanotrophs have the unique ability to grow on methane as their sole carbon source and energy. The activity of methanotrophs contributes significantly to the global methane budget. High methane concentrations have been observed in both seafloor sediment and water column samples offshore southwestern Taiwan. Still, the characteristics and roles of methanotrophs are not well known in this system. In this study, we used Remotely Operated Vehicle (ROV) to conduct seafloor observation and sampling in Four Way Closure Ridge (FWCR), MV1 mud volcano and G96 gas seep offshore southwestern Taiwan. We first analyzed the gas components to understand the concentraction of methane (in water column, bottom water, and sediment pore water). Furthermore, we used the sequences of 16S rRNA and the functional gene pmoA to study the community structure and the abundance of aerobic methanotrophs in cold seep systems. The difference of methane concentration in sediment pore water and seawater within 1 m from the seabed can be as high as six orders of magnitude, indicating a significant methane consumption mechanism. Molecular analysis revealed that type I methanotrophs, family Methylomonaceae, was predominant in the study area. The relative abundance was higher in the bottom water, inferring a suitable place for the survival and performance of methanotrophs. Community structures were significantly different between sediment and seawater samples, but 16 identical OTUs were shared in both sediment and water column environments. Aerobic methanotrophs were quantified by using quantitative real-time polymerase chain reaction (qPCR) targeting the pmoA genes. The highest copy number of methanotrophs were present in the surface of sediments associated with the cold seep system and the abundances of methanotrophs in the surface sediment were related to the methane concentrations. However, in the water column samples, only MV1 demonstrated significant changes in pmoA copy number along with the depth, showing higher performance of aerobic methanotrophs at the methane depletion depth. Besides, a positive correlation was revealed between the difference in methane concentration of bottom water and sediment porewater and the pmoA gene expression. It shows the importance of microbial activities for methane regulation in the marine environment.