台灣北部關渡溼地潮汐對好氧甲烷氧化作用之影響研究

Abstract

好氧甲烷氧化菌利用甲烷作為單一碳源，氧氣為電子接收者，廣泛分佈於土壤表面，在調節溼地甲烷排放量的重要性佔有一席之地。由於時空尺度上的變異，對於氧氣和甲烷濃度在潮汐週期的變化如何與好氧甲烷氧化作用產生共變，其中的機制仍尚未釐清。本研究在台灣北部關渡溼地進行潮汐週期變化內的分析，欲釐清甲烷通量和好氧甲烷氧化作用間的動態關聯，分別在 2016 年 12 月、2017 年 3 月、5 月和 8 月進行四次採樣。在潮汐週期內的不同時間點，測量現地甲烷通量、表層甲烷濃度和氧氣通量，並採集表層沉積物進行實驗室培養實驗，以估算潛在好氧甲烷氧化速率和好氧甲烷氧化菌的動力學特徵。 研究結果顯示氧氣通量在潮汐週期無明顯變化，影響好氧甲烷氧化作用有限，而潮週內 pmoA 數量和潛在甲烷氧化速率的增加，並未總是與甲烷通量下降一致，顯然甲烷通量在潮汐週期內的消長，除了好氧甲烷氧化作用之外，還受無氧環境的甲烷生成影響。三次潮週內所測量和分析的各種隨潮週變化的參數並無一致性，說明暴露於大氣的時間長短不是影響其變化的主要因素。綜合不同月份的潮週現地測量與分析結果，顯示潛在好氧甲烷氧化速率分別與甲烷通量、pmoA 數量都呈現弱相關性，代表好氧甲烷氧化作用在現地潮週變化下與其他特定變動因子的關連性不高；此外，溫度與甲烷通量和潛在好氧甲烷消耗速率皆呈現非高度相關，表示溫度亦非影響潮週內甲烷氧化作用和甲烷通量的主要因子。由培養實驗可知不同月份潮週所採集的樣品會顯現不同的動力學特徵，代表好氧甲烷氧化菌在樣品中的代謝特性明顯不同。受潮汐影響的關渡溼地環境中，微生物與環境之間有著複雜相互影響，好氧甲烷氧化作用對於甲烷通量的影響力在潮週間並不顯著。

Aerobic methanotroph is a group of microorganisms widespread in ground surface using methane as sole carbon source and oxygen as electron acceptor. Apparently, it plays an important role in controlling methane emitted from wetlands. However, the activity of aerobic methanotrophy regulated by temporal fluctuation of oxygen and methane supply in tidal wetlands is not well known. This study aims to examine the dynamics of methane fluxes and potential aerobic methane consumption rates in a tidal wetland of northern Taiwan, where the variation of environmental characteristics, such as sulfate and methane concentration in pore water has been demonstrated during a tidal cycle. Four field campaigns were carried out in December of 2016 and March, May and August of 2017. Methane fluxes, methane concentrations in surface sediments and oxygen profiles were measured at different tidal phases. Besides, batch incubations were conducted on surface sediments in order to quantify potential microbial methane consumption rates and to derive kinetic parameters for aerobic methanotrophy. Our results demonstrated that the oxygen flux into the sediment during a tidal cycle was kept at a similar magnitude, which may infer a limited effect on the aerobic methane oxidation. Besides, the increase of pmoA gene abundance and potential aerobic methane oxidation rate didn’t always coexist with lower methane flux during a tidal cycle, suggesting that the methane flux was controlled by not only aerobic methanotroph but also methanogenesis. No similar pattern could be observed in terms of exposure time among three samplings of different tidal cycles, suggesting the exposure time is not the dominant factor. Weakly and moderately correlations could be found between the potential aerobic methane oxidation rate and either the methane flux or amount of pmoA gene, respectively, which indicated no specific factor could control aerobic methane oxidation and then regulate the methane emission during a tidal cycle. Besides, the sediment temperature was not the main factor affecting the activity of methanotroph and methane flux in a tidal affected wetland. Kinetic characteristics of aerobic methane oxidation were determined from the batch incubations. The maximum potential rate and the half saturation concentration were significantly different among the samples from different sampling campaigns, which inferred the metabolic capability of aerobic methanotrophs have changed through time beside of a tidal period. The interaction of microbes in Guandu wetland was complicated and the rule of aerobic methane oxidation on methane emission may be less significant during a tidal cycle.