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Methanogenesis

2016Posted by Ruth Vingerhagen Apr 11, 2016 16:30
My second paper to come out of the Svalbard fieldwork has just been published in JGR Biogeosciences! This work started back in 2011 when I got talking to a colleague at ETH (Susan Lang) who was setting up a method to look at δ13C-DOC in water samples and we decided to test the method on water samples I would collect during the first trip to the field sites. The data showed very low δ13C-DOC which weren't expected as streams normally have C3 values - values from photosynthesis. So during the main field campaign we collect more data and the low values were still there (Fig. 1). What followed was a lot of literature reading to learn about carbon dynamics is freshwater ecosystems.

Fig. 1: The straight lines through the data indicate mixing of two sources of DOC. End-member 1 (EM1) is the 'terrestrial source': carbon produced by photosynthesis (C3 pathway). EM2 is the 'methanotrophy source'. Dryadbreen is the glaciated catchment and Fardalen is the unglaciated catchment.


Permafrost in the Arctic is thought to be a major source of methane, a greenhouse gas, but how much methane is actually released depends on the balance between the microbes which produce methane (methanogens) and those which consume methane (methanotrophs). Dissolved organic carbon is streams is often ultimately derived from photosynthesis (e.g. plants and algae) which leads to a characteristic carbon isotopic composition. However, methanotrophic microbes 'eat' methane which has a distinctly different (lighter) carbon isotopic composition and this can lead to dissolved organic carbon which also has this distinct isotopic value. So far, dissolved organic carbon from Arctic rivers has had the typical 'photosynthetic' carbon isotopic composition, but in this study we measured values which were much lighter and in line with what would be expected from a methane source. This conclusion was supported by the detection of a gene unique to methanotrophs. Previous studies did not detect this methantrophic imprint in the dissolved organic carbon because they focussed on large rivers where the effect of methanogensis is swamped by photosynthetic sources. In contrast, this study focussed small, braided streams, which are found throughout the Arctic. Braided streams enable greater water contact with the permafrost where the methane originates. This study demonstrates that methane is being consumed and additionally the importance of chemosynthetic ecosystems (do not need light, use chemicals as an energy source) in permafrost regions.


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