Unravelling the ocean methane paradox

为什么世界上层海洋的甲烷会过饱和？我们知道这是事实，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，这似乎与我们所知的甲烷生成--严格意义上的厌氧过程--相矛盾。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场所，那么它们完全有可能在其中发生产甲烷作用。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淀物可能为远洋水域提供了这些厌氧微生物场所。我们在SAMS的研究小组进行的工作支持这一假设。我们现在已经清楚地确定了海洋浮游动物粪便颗粒和沉淀物中存在产甲烷细菌。这一点，再加上公布的数据显示，甲烷浓度升高可能与这些地点有关，这使得人们对这种厌氧过程如何在远洋水域活跃地发生有了更深入的了解。我们还知道，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分离出的一些产甲烷菌属于甲烷菌属，这些微生物被认为利用一碳(C1)化合物，包括二甲硫化物(DMS)和甲胺(Mas)。这两种化合物的潜在来源是二甲基磺酸碘丙酸酯(DMSP)和甘氨酸甜菜碱(GB)，它们是由海洋浮游植物产生的，以维持它们在海水中的渗透平衡。很可能，当浮游动物吃浮游植物时，它们至少会消耗一部分DMSP或GB，然后将这些物质包装成它们的粪便颗粒。DMSP和GB被认为是通过微生物活动分别转化为DMS和MAS的。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，在那里它们可能是产甲烷的底物。认为DMS或MAS产生的气溶胶粒子可能通过影响云的反照率和太阳辐射的反射而对自然降水的pH值做出贡献，并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的作用具有重要影响。我们建议进行一个多学科的项目，研究浮游动物和粪便颗粒作为水柱中潜在的产甲烷地点。我们的主要目的是阐明藻类衍生化合物在这一过程中的作用，并确定主要的产甲烷基团。这项工作的先决条件已经得到证明，现在有证据表明，a)粪便颗粒中可能存在厌氧条件，b)可存活的产甲烷菌可能与浮游动物及其粪便颗粒有关，c)这些地点含有相对较高浓度的DMSP、DMS和Mas，以及d)海洋甲烷的产生部分与颗粒物质有关。然而，到目前为止，这些研究都是孤立地进行的，对这一过程仍然知之甚少。我们将采用不同的方法进行研究，包括浮游植物养殖研究、浮游动物放牧实验、沉积物捕捉器研究，这些研究将与分子生物学研究相结合，包括使用稳定同位素探测技术。通过将这些研究领域与新方法相结合，我们希望最终揭开海洋甲烷悖论的面纱。

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 bacteria) within marine zooplankton faecal pellets and sedimenting particles. This, along with published data showing that elevated methane concentrations may be 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 isolated from zooplankton faecal pellets are affiliated with the genus methanolobus, and these microbes 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 graze upon 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 investigate zooplankton and faecal pellets as potential sites for methanogenesis in the water column. Our main purpose is to clarify the role of algal-derived compounds in this process and identify the main methanogenic groups responsible. The prerequisites for this work have been demonstrated and there is now evidence that a) anaerobic conditions are possible within faecal pellets, b) viable methanogens may be associated with zooplankton and their faecal pellets c) these sites contain relatively high concentrations of DMSP, DMS and MAs, and d) oceanic methane production is, in part, associated with particulate material. So far however, these studies have been conducted in isolation and the process remains poorly understood. We will conduct research using different approaches including phytoplankton culture studies, zooplankton grazing experiments, sediment trap studies, and these will be coupled with molecular biological investigation including the use of stable isotope probing techniques. By combining these areas of research with new methodology we hope to finally unravel the ocean methane paradox.