Methane formation from algae in oxic seawater

甲烷（CH4）是大气中含量最多的还原性有机化合物，在大气化学中起着核心作用，是仅次于CO2的第二大人为温室气体。全球大气CH4收支由许多自然和人为的陆地和水生地表源决定，主要由大气中的一个主要汇（羟基自由基）平衡。直到最近，人们才认为生物圈大气中甲烷的自然来源完全来自湿地土壤和稻田中的严格厌氧微生物过程、白蚁和反刍动物的肠道、人类和农业废物、生物质燃烧、化石燃料开采和地质来源，包括泥火山和渗漏。然而，最近的研究表明，陆地植被、真菌和哺乳动物也可能在没有产甲烷菌的帮助下，在好氧条件下产生CH4。海洋被认为是大气CH4的一个来源，尽管总净排放量的大小高度不确定，来源也没有得到很好的描述。为了解释地表水中CH4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。其他来源，如藻类原位生成CH4也被提出，然而，从（无氧）藻类培养的实验室实验中，藻类生成CH4的直接证据仍然缺失，因此到目前为止，上层水层中CH4的积累尚未与藻类直接产生有关。本研究项目的总体目标是验证（原理证明）和量化几种海洋藻类如联生藻（如Emiliania huxleyi）产生的CH4。藻类衍生CH4的潜在前体（如甲基硫化物和亚砜）将通过稳定同位素技术进行鉴定。将研究几种环境因素对藻类产生CH4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形成的产甲烷古生菌或细菌。我们还将关注对气候变化敏感的因素，如温度、氧气浓度和养分有效性。设想采用跨学科的方法，需要几个学科的相互作用来实现项目的目标。这些结果有望提高我们对海洋中CH4形成的生物地球化学循环的理解，并更好地解释与大气浓度相比，含氧地表水中CH4的富集，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'.