Mikrobiologische Studien zur anaeroben Oxidation von Methan (AOM)

Abstract

In marine sediments more than 90% of the produced methane is consumed by the anaerobic oxidation of methane with sulfate as terminal electron acceptor (AOM). Due to this, the contribution of the oceans as source of climate-relevant methane is rather low. AOM is catalyzed by consortia of methanotrophic archaea (ANME) and sulfate-reducing bacteria (SRB). The interaction of these two organisms is so far poorly understood. ANME are phylogenetically closely related to methanogenic archaea. Hence, it is thought that the activation of methane acts in a reverse reaction of methanogenesis. The bacterial partner of ANME is also phylogenetically characterized. However both organisms are not isolated yet. Hence, the first aim of this thesis was to achieve (sediment-free) enrichment cultures and the further characterization of the organisms. Therefore AOM-active sediments from different habitats were incubated with methane and sulfate as sole substrates, tested for their growth optima and repeatedly diluted, which yielded finally sediment-free AOM-enrichments. The growth optima of cultures from marine cold seeps were between 4 and 20 °C. Incubations from hydrothermally influenced sediment succeeded the first thermophilic enrichments of AOM with a growth optimum between 50 and 60 °C. These are the first successfully growing enrichments of the ANME 1 cluster. Furthermore presence of AOM was also shown for a terrestrial mud volcano, one of the first described non-marine AOM-habitats. The obtained enrichment cultures from marine cold seeps were used to determine the isotope fraction factors of methane during its anaerobic oxidation. We defined the first experimentally derived carbon and hydrogen isotope fractionation factors for AOM. Those are significantly higher than fractionation factors calculated from isotope profiles of sedimentary marine pore waters. Furthermore, the enrichments were used to test it for the enzymatic back-reaction during AOM, which can be described as exchange reactions between product and substrate pools. The reversibility of AOM was about 13% of the net-metabolic rate. Back reaction, defined as a complete reversal of AOM ( true methanogenesis) was excluded.