The discovery of methane (CH 4 ) accumulation in oxic marine and limnic waters has redefined the role of aquatic environments in the regional CH 4 cycle. Although CH 4 accumulation in oxic surface waters became apparent in recent years, the sources are still subject to controversial discussions. We present high-resolution in situ measurements of CH 4 concentration and its stable isotope composition in a stratified mesotrophic lake. We show that CH 4 accumulation in surface waters originates from a highly dynamic interplay between (oxic) CH 4 production and emission to the atmosphere. Laboratory incubations of different phytoplankton types and application of stable isotope techniques provide a first unambiguous evidence that major phytoplankton classes in Lake Stechlin per se produce CH 4 under oxic conditions. Combined field and lab results show that the photoautotroph community is an important driver for CH 4 production and its highly dynamic accumulation in oxic surface waters.

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Mammalian formation of methane (methanogenesis) is widely considered to occur exclusively by anaerobic microbial activity in the gastrointestinal tract. Approximately one third of humans, depending on colonization of the gut by methanogenic archaea, are considered methane producers based on the classification terminology of high and low emitters. In this study laser absorption spectroscopy was used to precisely measure concentrations and stable carbon isotope signatures of exhaled methane in breath samples from 112 volunteers with an age range from 1 to 80 years. Here we provide analytical evidence that volunteers exhaled methane levels were significantly above background (inhaled) air. Furthermore, stable carbon isotope values of the exhaled methane unambiguously confirmed that this gas was produced by all of the human subjects studied. Based on the emission and stable carbon isotope patterns of various age groups we hypothesize that next to microbial sources in the gastrointestinal tracts there might be other, as yet unidentified, processes involved in methane formation supporting the idea that humans might also produce methane endogenously in cells. Finally we suggest that stable isotope measurements of volatile organic compounds such as methane might become a useful tool in future medical research diagnostic programs.

A fast and sensitive method for the continuous determination of methane (CH 4 ) and its stable carbon isotopic values (d 13 C-CH 4 ) in surface waters was developed by applying a vacuum to a gas/liquid exchange membrane and measuring the extracted gases by a portable cavity ring-down spectroscopy analyser (M-CRDS). The M-CRDS was calibrated and characterized for CH 4 concentration and d ments indicate either local methane production/oxidation or physical variations in the thermocline. Therefore, these results illustrate the need of fast and sensitive analyses to achieve a better understanding of different mechanisms and pathways of CH 4 formation in aquatic environments.

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