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Methane oxidation and emission in Lake Lugano (Southern Switzerland) : a lipid biomarker and isotopic approach

机译：卢加诺湖（瑞士南部）的甲烷氧化和排放：脂质生物标志物和同位素方法

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摘要

Methane is an important greenhouse gas in Earth's atmosphere. The sources of atmospheric methane are largely biogenic, being produced under anoxic conditions by methanogenic Archaea. Wetlands, which include lakes, are important contributors to the atmospheric methane budget, since they commonly feature anoxic sediments or bottom water. Methane oxidising bacteria at the interface between oxic and anoxic sediments and water limit the efflux of methane. Furthermore, in the oceans, methane is oxidised anaerobically by Archaea, in a process coupled to sulfate reduction. In freshwater environments, where sulfate concentrations are orders of magnitude lower, this process is not thermodynamically favourable, and archaeal anaerobic oxidation of methane is often absent. It has been proposed in certain lake environments, however, that anaerobic oxidation of methane does take place.ud ud One lake in which anaerobic oxidation of methane was proposed is the northern basin of Lake Lugano, southern Switzerland. Anaerobic oxidation of methane in this basin is explored in chapter 2 of this PhD thesis. Indeed we found methane concentration and carbon isotopic composition profiles characteristic of methane oxidation in the anoxic hypolimnion, more than 30 m below the interface between the oxic and anoxic waters. In addition, microbial biomass at these depths showed carbon isotope signatures of methane-derived carbon (d13C-values as low as -70‰ in C16:1 fatty acids), indicating that methane is used as a carbon source. However, no methane oxidation took place in incubation experiments under anoxic conditions. Addition of alternative potential electron acceptors did not stimulate methane oxidation, and methane oxidation was only observed in the presence of oxygen. Instead, we propose that episodic introduction of oxygenated water into the anoxic hypolimnion sustains a community of aerobic methanotrophs.ud ud Carbon derived from methane oxidation has been shown in several studies to constitute an important carbon input to aquatic ecosystems. In the studies reported in chapters 2 and 3, compound specific stable carbon isotope analysis of lipid biomarkers was used to trace methane-derived carbon through the ecosystems at redox interfaces and in the anoxic hypolimnion of Lake Lugano. In the monomictic southern basin (chapter 3), an anoxic benthic nepheloid layer develops during the period of water column stratification. This layer was found to be derived from microbial production in the hypolimnion. Methane oxidising bacteria constituted up to 30% of total microbial cell numbers in the nepheloid layer, and 77% to 96% of the organic carbon in this layer was methane-derived. High rates of aerobic methane oxidation at the top of the anoxic nepheloid layer led to an oxygen consumption that was greater than the downward diffusion, causing the anoxic nepheloid layer to expand. Bacterial aerobic methanotrophs migrate upwards through the water column with the interface between the oxic hypolimnion and the anoxic nepheloid layer.udud The extent of emission of methane to the atmosphere depends on the totality of sinks and sources in the lake basin. In both the northern and the southern basin of Lake Lugano, large amounts of methane are emitted from the sediments into the bottom water. However, consumption by aerobic methanotrophs at the oxic-anoxic redoxcline is near complete, and during stratified conditions, no methane escapes to the epilimnion. On the other hand, methane super-saturation in the surface water was observed throughout the year. Chapter 4 describes the results of three mapping campaigns of surface water methane concentrations in the northern basin of Lake Lugano, in spring and autumn. Additionally, methane concentration and carbon isotopic composition were measured on depth profiles down to 40 m depth in transects across the lake basin. Methane fluxes to the atmosphere were calculated from surface water concentration and wind speed. At a standardised wind speed of 1.6 m s-1 (average wind speed during the period from May until October) fluxes to the atmosphere were significantly higher in autumn (44 and 97 micromol m-2 d-1 in October 2011 and October 2012, respectively) than in spring (7 micromol m-2 d-1, May 2012). This difference is in part due to higher concentrations in autumn than in spring, and in part a result of a stronger dependence of the transfer velocity on buoyancy flux when the surface water cools. The source of methane in the surface water could not be determined with certainty. It is possible that internal waves at the thermocline induce friction at the sediment-water interface in the littoral zone, which leads to increased outgassing of sedimentary methane. However, the northern basin of Lake Lugano has steep shores along large parts of the basin, which offer little space for deposition of sediments, and the possibility of in situ production of methane in the water column must be considered.ud
机译：甲烷是地球大气中重要的温室气体。大气甲烷的来源主要是生物成因，是在产甲烷条件下产甲烷的古细菌产生的。湿地（包括湖泊）是造成甲烷甲烷排放的重要因素，因为它们通常以缺氧沉积物或底水为特征。在有氧和无氧沉积物与水之间的界面处的甲烷氧化细菌限制了甲烷的外流。此外，在海洋中，甲烷被古细菌厌氧氧化，其过程与硫酸盐还原反应相关。在硫酸盐浓度低几个数量级的淡水环境中，此过程在热力学上不利，因此常不存在甲烷的古细菌厌氧氧化。然而，有人提出在某些湖泊环境中会发生甲烷的厌氧氧化。 ud ud有人提出了甲烷的厌氧氧化的一个湖泊是瑞士南部卢加诺湖的北部盆地。本博士论文的第二章探讨了该盆地中甲烷的厌氧氧化。的确，我们发现了在缺氧低氧化层中甲烷氧化的甲烷浓度和碳同位素组成特征，缺氧和缺氧水之间的界面下方30 m以上。此外，在这些深度的微生物生物量显示出甲烷衍生碳的碳同位素特征（在C16：1脂肪酸中d13C值低至-70‰），表明甲烷被用作碳源。但是，在缺氧条件下的培养实验中没有甲烷氧化。添加替代的潜在电子受体不会刺激甲烷氧化，仅在氧气存在下才能观察到甲烷氧化。取而代之的是，我们建议将含氧的水向缺氧的次碱液中引入是维持有氧甲烷氧化菌的群落。 ud ud甲烷氧化衍生的碳已在多项研究中显示出构成水生生态系统重要的碳输入。在第2章和第3章中报告的研究中，使用脂质生物标记物的化合物特定的稳定碳同位素分析，通过氧化还原界面和卢加诺湖缺氧环境中的生态系统追踪甲烷衍生的碳。在单峰的南部盆地（第3章）中，在水柱分层期间形成了一个缺氧底栖肾上腺体层。发现该层源自次生菌层中的微生物产生。甲烷氧化细菌占肾小球层中微生物细胞总数的30％，并且该层中77％至96％的有机碳是甲烷衍生的。缺氧星状胶体层顶部的好氧甲烷氧化速率高，导致耗氧量大于向下扩散，导致缺氧星状胶体层膨胀。细菌好氧甲烷营养菌通过水柱向上迁移，并在有氧次碱液和缺氧性肾小球层之间形成界面。 ud ud甲烷向大气排放的程度取决于湖盆中汇的总量。在卢加诺湖的北部和南部盆地，大量的甲烷从沉积物中排放到底部水中。然而，在有氧-缺氧的氧化还原茶碱中，好氧甲烷氧化菌的消耗已接近完成，并且在分层条件下，没有甲烷逸出至上苯胺。另一方面，全年观察到地表水中的甲烷过饱和。第4章介绍了春季和秋季在卢加诺湖北部盆地的三个地表水甲烷浓度测绘活动的结果。此外，在整个湖盆横断面上低至40 m深度的深度剖面上测量了甲烷浓度和碳同位素组成。根据地表水浓度和风速计算出向大气中的甲烷通量。在标准风速为1.6 m s-1（5月至10月期间的平均风速）的情况下，秋季的大气通量显着增加（2011年10月和2012年10月分别为44和97 micromol m-2 d-1），分别为春季（7 micromol m-2 d-1，2012年5月）。这种差异部分是由于秋季的浓度高于春季的浓度，部分是由于地表水冷却时传输速度对浮力通量的依赖性更强的结果。无法确定地表水中甲烷的来源。高温跃层内部的波浪可能会在沿海区域的沉积物-水界面引起摩擦，从而导致沉积甲烷的放气增加。但是，卢加诺湖北部盆地沿盆地大部分地区有陡峭的海岸，几乎没有沉积物沉积的空间，因此必须考虑在水柱中原位产生甲烷的可能性。 ud

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    Blees Hendrik Jan Reynier;

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  • 年度 2015
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