Environmental Science and Engineering Seminar
Methane, the second principal anthropogenic greenhouse gas after carbon dioxide, is the most abundant reduced organic compound in the atmosphere and plays a central role in atmospheric chemistry. Methane produced by biological processes is considered to contribute about 70 % to the global methane budget. Until recently biological methane formation had been associated exclusively with anoxic environments and methanogenic activity. Almost one decade ago it was reported from laboratory experiments that living plants and plant litter emit methane to the atmosphere under aerobic conditions. These observations caused considerable controversy amongst the scientific community and the general public because of their far-reaching implications. This was mainly for two reasons: firstly, it is generally accepted knowledge that the reduced compound methane can only be produced naturally from organic matter by methanogens in the absence of oxygen, or at high temperatures, e.g. in biomass burning. The fact that no mechanism for an 'aerobic' production of methane had been identified at the molecular level in plants added to the consternation. Secondly, the first extrapolations from laboratory measurements to the global scale indicated that these emissions could constitute a substantial fraction of the total global emissions of methane. This global extrapolation procedure was severely criticised, and other up-scaling calculations suggested a much lower plant source of methane. However, it became also clear, that without further insight into the mechanism of the 'aerobic' production of methane, any up-scaling approach would have considerable uncertainties and thus be of questionable value. Almost ten years after the initial observation of methane emissions from vegetation, there is now convincing evidence of several alternative pathways in the aerobic biosphere including terrestrial plants, soils, fungi, lichens, marine algae and animals. Therefore, in the first part of this presentation I present an overview of the recently discovered methane sources in the biosphere/eukaryotes. These results are also discussed how they might add to our understanding of the global methane cycling; for example how they might contribute to solve the `ocean methane paradox'.
Moreover, a second aspect of the presentation is the investigation of photochemical (UV) induced formation of methane from extraterrestrial matter and its implications for methane observed on Mars, other planets/planetoids and comets of our solar system. In particular, I would like to discuss the following searching questions: Is methane release from extraterrestrial matter by solar irradiation a relevant process that might for example explain formation on Mars and comets? Might it be a relevant process for organic matter degradation in our solar system in general? An overview of the current state of the art and the most recent findings regarding methane formation on Mars will be given in the second part of this presentation.