Terminale Prozesse des anaeroben Abbaus in sauren Torfmooren der Subarktis und des südlichen Boreals

Due to their role as CO2 sinks and CH4 sources peatlands have contributed fundamentally to the global carbon cycle. Especially peatlands of the northern hemisphere are important reservoirs for soil organic carbon. A global warming will make available these carbon reservoirs for microbial degradation and may stimulate CH4 and CO2 emissions. Physiology and thermodynamics of methanogenesis have been studied extensively. However, the linkage between structure and function of methanogenic communities and the regulatory factors in acidic peatlands are largely unresolved. Here, we studied methanogenic communities and processes coupled to methanogenesis in acidic peatlands of the northern hemisphere (N-Finland, W-Siberia and Estonia). In the first study the relevance of endogenic ethanol as substrate for methanogenesis and Fe(III) reduction in a subarctic peatland in Northern Finland was revealed. Carbon balances showed that 50% of the ethanol pool entered methanogenesis via syntrophic oxidation and 50% Fe(III) reduction. 80% of CH4 originated from H2/CO2. Correspondingly, phylogenetic analysis of the 16S rRNA gene and the gene for the α-subunit of the methyl coenzymeM reductase revealed that a single member of the family Methanobacteriaceae was responsible for methanogenesis. Relatively high methanogenesis even at 4°C indicated the presence of a psychrophilic community. T-RFLP analysis argued for stability of the archaeal community over the whole temperature gradient. However, Real Time PCR revealed a temperature dependent growth of both the bacterial and archaeal community. Both, substrate turnover and growth of the microbial community were strongly temperature dependent Similar to the Finnish peatland, syntrophic processes seemed to be main sources for methanogenic substrates in the Siberian peat as well. Inhibition of acetoclastic methanogenesis with CH3F revealed that >70% of CH4 originated from acetate. Based on the stochiometric ratio of acetate and H2/CO2 originating from syntrophic butyrate oxidation, this process is considered to be the main source for methanogenic precursors. Phylogenetic analysis of the 16S rRNA gene revealed that members of the families Methanosarcinaceae and Methanobacteriaceae were responsible for methanogenesis. All processes were strongly temperature dependent. However, the results argue for stimulation of methanogenesis not directly by temperature, but indirectly by syntrophic processes or even more upstream processes. In peat sampled from an acidic peat bog (Estonia) for the first time syntrophic acetate oxidation was revealed by [2-13C/14C]-acetate labelling experiments. Until today this process has been observed in bioreactors and lake sediments. Phylogenetic analysis of the 16S rRNA gene and of the gene for the α-subunit of the methyl coenzymeM reductase revealed a highly diverse but mainly hydrogenotrophic methanogenic community. This corresponded to the high amount of CH4 originating from H2/CO2. Labelling experiments with 14C-bicarbonate indicated the presence of homoacetogens. Homoacetogenesis was stimulated by H2. Phylogenetic analysis of the gene for the formyltetrahydrofolate synthetase, a key enzyme of the acetyl-CoA pathway, supported the presence of homoacetogens. Since syntrophic acetate oxidizers known today are homoacetogens, oxidizing acetate via the reverse acetyl-CoA pathway, homoacetogens may have been responsible for syntrophic acetate oxidation in the acidic peat bog as well. However, despite the lack of alternative electron acceptors like Fe(III), members of the family Geobacteraceae were detected in the same peat bog. Geobacteraceae are prominent Fe(III) reducers in Fe(III)-rich habitats. However, amendment with ferrihydrite or FeCl3, respectively, showed that Fe(III) reduction played a minor role: Only 4-6% of the available Fe(III) were reduced to Fe(II). Hence, Geobacteraceae are not necessarily involved in Fe(III) reduction and seem to be more widely distributed than assumed until today. In addition, their role in nature has to be reassessed. In summary, the results indicate that syntrophic processes may be a general feature of peatlands in the northern hemisphere. Due to anoxic conditions in peatlands, microbial activity is strongly slowed down. Hence, the microorganisms are permanently exposed to substrate limitation and are forced to act at thermodynamic equilibrium. Hence, due to these conditions, microorganisms may depend on these thermodynamically unfavourable syntrophic processes.