Einflüsse auf die Kohlenstoffisotopenfraktionierung in methanogenen Systemen

Methane is one of most relevant greenhouse gases in the atmosphere. It is formed as the final product of the degradation of organic material in many anoxic habitats. In rice field soils and wetlands methane is predominantly formed by acetoclastic and hydrogenotrophic methanogenic archaea. However, some methanogens can use C1 compounds such as methanol as substrate. To determine the pathways involved in the methane production the carbon isotope signature of the substrates and the released methane can be used. But for quantification it is necessary to know the fraction factors of the involved microbial groups. In the past, most of the data were collected for hydrogenotrophic and acetoclastic pure cultures. However, the level of information regarding the methanogenesis on methanol was very low. Because soil samples are usually incubated in closed systems, we additionally wanted to check, if the previously reported data within open systems for hydrogenotrophic and methylotrophic pure cultures can be transferred to closed systems. During the conversion of methanol to methane in Methanosarcina acetivorans, Methanosarcina barkeri and Methanolobus zinderi similarly large fractionation factors (-83‰ to -72‰), as previously reported for open systems (-83‰ to -79‰), could be observed. The fractionation factors for the investigated hydrogenotrophic methanogens (-38‰ to -31‰), Methanothermobacter marburgensis and Methanothermobacter thermautotrophicus, during the conversion of H2/CO2 to methane in carbonate-buffered and carbonate-free medium likewise confirmed the data obtained in open systems (60 ‰ to 28 ‰). Compared to the hydrogenotrophic methanogens the homoacetogen Thermoanaerobacter kivui which also grows on H2/CO2 in a closed system revealed slightly lower fractionation factors (44‰ to 41‰). A change of the incubation temperature showed for the five examined methanogens and the analyzed homoacetogenic microorganism in different buffer systems no influence on the carbon isotope signatures. In consequence no dependence of the fractionation factor to the incubation temperature could be observed. Furthermore, during the hydrogenotrophic methanogenesis in M. marburgensis a great stability of the fractionation factor with respect to the pH of the medium could be detected. Thus, for the quantification of the in methane production involved pathways in soil samples these factors can be largely neglected. By the use of selective inhibitors the different methanogenic pathways can be differentiated. It was previously believed that addition of methyl fluoride as an inhibitor of acetoclastic methanogenesis results in carbon isotopic signature dominated by hydrogenotrophic produced methane. However, the addition of methyl fluoride does not influence the fractionation factor of methylotrophic methanogenesis and so in the presence of methyl fluoride produced "light" methane could also be produced by methylotrophic and not only by hydrogenotrophic methanogenesis.