Methane formation from algae in oxic seawater

甲烷（CH4）是大气中含量最丰富的还原性有机化合物，是仅次于CO2的第二大人类温室气体，在大气化学中起着重要作用。全球大气CH4收支由许多自然和人为的陆地和水生表面源决定，主要由大气中的一个主要汇（羟基）平衡。直到最近，生物圈中大气CH4的自然来源一直被认为完全来自湿地土壤和稻田、白蚁和反刍动物的肠道、人类和农业废物中严格意义上的厌氧微生物过程，以及生物质燃烧、化石燃料开采和包括泥火山和渗漏在内的地质来源。然而，最近的研究表明，陆生植物，真菌和哺乳动物也可能产生甲烷排放，而无需甲烷菌的帮助下，在有氧条件下。海洋被认为是向大气排放甲烷的一个来源，尽管总净排放量的大小很不确定，而且来源也没有得到很好的说明。为了解释地表沃茨中甲烷的来源，有人认为甲烷生成发生在有机聚集体的缺氧微环境中。也有人提出了其他来源，如藻类就地形成的甲烷，但是，仍然没有从（纯）藻类培养物的实验室实验中得到的藻类衍生的甲烷形成的直接证据，因此，到目前为止，上层水层中甲烷的积累还没有与藻类的直接生产有关。这一研究项目的总体目标是核实（原理证明）和量化若干种海洋藻类如附着植物（如Emiliania huxleyi）产生的甲烷。利用稳定同位素技术将查明藻类产生的甲烷的潜在前体，例如甲基硫化物和亚砜。将调查几个环境因素对藻类产生的甲烷的影响。此外，我们将应用各种微生物测试来筛选可能参与CH4形成的产甲烷菌或细菌。我们还将重点关注对气候变化敏感的因素，如温度、氧气浓度和养分供应。为了实现该项目的目标，设想了一种需要几个学科相互作用的跨学科方法。这些结果有望提高我们对海洋中甲烷形成的地球化学循环的理解，并更好地解释与大气浓度相比，含氧表面沃茨中甲烷富集的现象，即所谓的“海洋甲烷悖论”。

Methane (CH4), the second important anthropogenic greenhouse gas after CO2, is the most abundant reduced organic compound in the atmosphere and plays a central role in atmospheric chemistry. The global atmospheric CH4 budget is determined by many natural and anthropogenic terrestrial and aquatic surface sources, balanced primarily by one major sink (hydroxyl radicals) in the atmosphere. Natural sources of atmospheric CH4 in the biosphere have until recently been attributed to originate solely from strictly anaerobic microbial processes in wetland soils and rice paddies, the intestines of termites and ruminants, human and agricultural waste, and from biomass burning, fossil fuel mining and geological sources including mud volcanoes and seeps. However, recent studies suggested that terrestrial vegetation, fungi and mammals may also produce CH4 emission without the help of methanogens and under aerobic conditions. The oceans are considered to be a source of CH4 to the atmosphere although the magnitude of total net emissions is highly uncertain and sources are not well described. To explain the source of CH4 in surface waters, it has been suggested that methanogenesis takes place in anoxic microenvironments of organic aggregates. Other sources such as in-situ formation of CH4 by algae have also been suggested, however, a direct evidence of algae-derived CH4 formation from lab experiments with (axenic) algae cultures is still missing, and thus so far the accumulation of CH4 in the upper water layer has not yet been related to a direct production by algae. The overall aim of this research project is the verification (proof of principle) and quantification of CH4 production by several species of marine algae such as haptophytes (e.g. Emiliania huxleyi). Potential precursors such as (e.g. methyl sulfides and sulfoxides) of algae-derived CH4 will be identified by using stable isotope techniques. Several environmental factors will be investigated with respect to their effect of algae-derived CH4 production. Furthermore, we will apply various microbiological tests to screen for methanogenic Archaea or bacteria potentially involved in CH4 formation. We will also focus on factors sensitive to climate change such as temperature, oxygen concentration and nutrient availability. An interdisciplinary approach requiring the interaction of several disciplines is envisaged to realise the aims of the project. The results are expected to improve our understanding of biogeochemical cycling of CH4 formation in the oceans and to better explain the CH4-enrichment of oxygenated surface waters compared to atmospheric concentration, so-called 'Oceanic methane paradox'.