Addressing the ocean methane paradox: the role of microenvironments in oceanic methane production

为什么世界上的上层海洋会充满甲烷？我们知道是这样，但不明白为什么。有证据表明，一部分甲烷来自含氧水域的原位生产，然而，这似乎与我们所知道的甲烷生成相矛盾；严格的厌氧过程。这种现象被称为“海洋甲烷悖论”。然而，如果在上层海洋中存在厌氧微生物，那么甲烷生成完全有可能在它们内部发生。我们现在认为，海洋浮游动物、它们排泄的粪便物质和其他沉积颗粒可能为远洋水域提供了这些厌氧微生物场所。我们在SAMS的研究小组所做的工作支持了这一假设。我们现在已经清楚地确定了在海洋浮游动物粪便颗粒和沉积颗粒中存在产甲烷菌（产生甲烷的微生物）。这一点，以及显示甲烷浓度升高与这些地点有关的数据，使人们对这种厌氧过程如何在远洋水域积极发生有了更深入的了解。我们也知道产甲烷菌可以使用一系列底物，包括二氧化碳和甲酸盐。然而，我们从浮游动物粪便颗粒中研究的一些产甲烷菌属于Methanolobus属，并且被认为利用一碳（C1）化合物，包括二甲基硫化物（DMS）和甲胺（MAs）。这两种化合物的潜在来源是海洋浮游植物为维持其在海水中的渗透平衡而产生的二甲基磺酰丙酸酯（DMSP）和甜菜碱（GB）。当浮游动物吃浮游植物时，它们可能至少会消耗一些DMSP或GB，然后将其包装成粪便颗粒。DMSP和GB被认为分别通过微生物活性转化为DMS和MAs。因此，放牧代表了这些c1化合物进入浮游动物肠道和粪便颗粒的途径，在那里它们可能成为甲烷生成的底物。据认为，由DMS或MAs产生的气溶胶颗粒可能对自然降水的pH值有所贡献，并因其对云反照率和太阳辐射反射的影响而在气候控制中发挥作用。因此，浮游动物粪便颗粒可能是产生温室气体和消除气候反馈气体的重要场所，对我们理解和模拟海洋在气候变化中所起的作用具有重要影响。我们建议开展一个多学科项目，以进一步了解浮游动物、它们的粪便颗粒和沉积颗粒作为水体中原位甲烷生成的潜在场所的作用。我们的主要目的是阐明藻类衍生化合物在甲烷生成中的作用，确定在这一过程中共生关系的重要性，并研究在这些地点使用替代底物。这将使我们能够确定负责这一过程的主要产甲烷菌群，以及它们如何受到环境和其他微生物的影响。这项工作的先决条件已由SAMS和其他小组证明。然而，这项工作的大部分，虽然令人兴奋，是初步的，过程仍然知之甚少。研究将使用最先进的技术（包括实时PCR，稳定同位素探测，稳定同位素质谱，CARD-FISH）和已建立的分析和微生物学方法（培养和培养独立）进行。此外，通过绑学生的工作，我们希望为这项工作增加令人兴奋的新方面，包括进一步表征分离的产甲烷菌，并使用CARD-FISH和共聚焦显微镜增加对其位置的了解。通过将这些领域的研究与新的方法相结合，我们希望开始解开海洋甲烷悖论。

Why is the world's upper ocean supersaturated with methane? We know that it is, but do not understand why. Evidence shows that a portion of the methane comes from in situ production in oxygenated waters, however that seems to contradict all we know about methanogenesis; a strictly anaerobic process. This phenomenon has been termed the 'oceanic methane paradox'. If, however, there were anaerobic microsites in the upper ocean, then it is entirely possible that methanogenesis could occur within them. We now think that marine zooplankton, their excreted faecal material and other sedimenting particles may provide these anaerobic microsites in pelagic waters. Work conducted by our research group at SAMS supports this hypothesis. We have now clearly identified the presence of methanogens (methane producing microorganisms) within marine zooplankton faecal pellets and sedimenting particles. This, along with data showing that elevated methane concentrations are associated with these sites, has led to greater insights into how this anaerobic process may be actively occurring in pelagic waters. We also know that methanogens can use a range of substrates, including carbon dioxide and formate. However, some of the methanogens we have studied from zooplankton faecal pellets are affiliated with the genus Methanolobus, and are thought to utilize one-carbon (C1)-compounds, including dimethylsulphide (DMS) and methylamines (MAs). Potential sources of these two compounds are dimethylsulphoniopropionate (DMSP) and glycine betaine (GB), which are produced by marine phytoplankton to maintain their osmotic balance in seawater. It is likely that when zooplankton eat phytoplankton they consume at least some of the DMSP or GB, which is then packaged into their faecal pellets. DMSP and GB are thought to be converted into DMS and MAs respectively by microbial activity. Grazing therefore represents a pathway for these C1-compounds to enter into the zooplankton gut and faecal pellets, where they may be substrates for methanogenesis. It is thought that aerosol particles generated from either DMS or MAs may contribute to the pH of natural precipitation and play a role in climate control due to their influence on cloud albedo and reflection of solar radiation. Therefore, zooplankton faecal pellets could be instrumental sites both in the production of a greenhouse gas and the removal of climatic feedback gases, having important consequences for our understanding and modelling of the role the oceans play in climate change. We propose to conduct a multidisciplinary project that will further our understanding of the role of zooplankton, their faecal pellets and sedimenting particles as potential sites of in situ methanogenesis in the water column. Our main purpose is to clarify the role of algal derived compounds in methanogenesis, determine the importance of syntrophic relationships in this process and investigate the use of alternative substrates within these sites. This should enable us to determine the main methanogenic groups responsible for this process and how they are influenced by their environment and other microorganisms. The prerequisites for this work have been demonstrated by the group at SAMS and others. However, much of this work, though exciting, is preliminary and the processes remains poorly understood. Research will be carried out using both state of the art techniques (including real-time PCR, stable isotope probing, stable isotope mass spectrometry, CARD-FISH) and established analytical and microbiological methods (culture & culture independent). In addition, through the work of a tied studentship, we hope to add exciting new aspects to this work including further characterisation of isolated methanogens and an increased understanding of their location using CARD-FISH and confocal microscopy. By combining these areas of research with new methodology we hope to start to unravel the ocean methane paradox.