Unravelling the ocean methane paradox

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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.