Stable isotope fractionation during the methanogenic degradation of organic matter in the sediment of an acidic bog lake, Lake Grosse Fuchskuhle

Lake Grosse Fuchskuhle is an acidic bog lake, which develops an anoxic hypolimnion during the summer season, so that sediment organic matter is degraded anaerobically to CH4 and CO2. The delta C-13 values of organic matter and its degradation products were used for determination of the mass balance, degradation path, and fractionation factors. Addition of methyl fluoride, an inhibitor of acetoclastic methanogenesis, increasingly inhibited CH4 production and resulted in accumulation of acetate (and a little propionate), which then was an additional end product of organic matter degradation. The delta C-13 of acetate-methyl group was slightly lower than that of organic matter, indicating a small fractionation (about 4 parts per thousand) during the fermentative production of acetate. Chemolithotrophic acetogenesis was low (< 9%). The fraction of CH4 produced from hydrogenotrophic methanogenesis was about 55-67%. After incubation for 20 h the average delta C-13 of the degradation products was similar to that of sediment organic matter, both in the absence and the presence of CH3F. The produced total inorganic carbon (TIC) was further converted to CH4 with apparent enrichment factors of 56-67 parts per thousand. The fraction of TIC that was actually converted to CH4 was calculated to be about 33% and 23% in the absence and presence of CH3F, respectively. These data are consistent with the assumption that about half of the organic matter was incompletely degraded, resulting in the production of only CO2 and H-2, afterward converted to CH4. The data further show that the reduction of CO2 to CH4 was partially inhibited in the presence of CH3F.