Identification of methanogenic pathways in anaerobic digesters using stable carbon isotopes

Abstract

In a series of anaerobic batch experiments, the stable carbon isotopes, δ13CCH?4? and δ13CCO?2?, were measured in biogas produced from various sources (maize, cellulose, inoculum) to identify the degradation kinetics and specific methanogenic pathways. Isotopic analysis was performed using a new absorption laser spectrometer in addition to conventional MS. A comparison of the isotopic evolution shows large isotope dynamics for maize and cellulose, indicating a temporal change in degradation pathways and/or a change in the relative contribution from different carbon fractions within the substrate. Further batch experiments with isotopically labelled acetate (either 13CH3CO2Na or CH 313CO2Na) were carried out to study the degradation of acetate in inoculum systematically. The results suggest that the acetate is completely oxidized into CO2 which in turn is partly reduced to CH4. Furthermore, the distinct isotopic signature CH4 and CO2 (for acetate‐methyl labelling as well as for acetate‐carboxy labelling) indicate that only a minor part of the produced methane derives from acetate. A substantial fraction of methane may have been produced at an earlier stage of the reaction chain or by other potential methane precursors such as formate or methanol.

Methanosarcinaceae and Acetate-Oxidizing Pathways Dominate in High-Rate Thermophilic Anaerobic Digestion of Waste-Activated Sludge

“…Generally, microbes preferentially utilize 12 C compounds over 13 C compounds (23). According to the literature, hydrogenotrophic methanogenesis exhibits a significantly larger fractionation factor than that of acetate-dependent methanogenesis due to the ability to select 12 C preferentially during methanogenesis (31,32).…”

“…The relative contribution to methanogenesis via the different metabolic pathways can be determined using the stable isotopic signatures of 13 C/ 12 C. The technique has been used mostly to study biogenic methane formation in anoxic environments and has shown that production of ␦CH 4 from reduction of CO 2 exhibits larger fractionation than that from cleavage of acetate (22,23). In the context of an anaerobic digester, it can be used to study the relative contribution from acetate cleavage and oxidation, with oxidation resulting in a lower 13 CH 4 isotopic signature (␦ 13 CH 4 ) due to differential partitioning of bicarbonate to methane during hydrogenotrophic methanogenesis (23).…”

“…In the context of an anaerobic digester, it can be used to study the relative contribution from acetate cleavage and oxidation, with oxidation resulting in a lower 13 CH 4 isotopic signature (␦ 13 CH 4 ) due to differential partitioning of bicarbonate to methane during hydrogenotrophic methanogenesis (23).…”

The required characteristics of ensiled crops used as a feedstock for biogas production: a review

“…Acetotrophic and hydrogenotrophic methanogenic bacteria then convert acetic acid and hydrogen, respectively, into a mixture of methane and carbon dioxide. More recent results suggest that the acetate is completely oxidised into CO 2 which in turn is partly reduced to methane (Laukenmann et al 2010). Information on biogas production from main plant biomass components is given in Table 1.…”

Monitoring CO2 concentration and δ13C in an underground cavity using a commercial isotope ratio infrared spectrometer

“…Commercial IRIS instruments are often designed for atmospheric monitoring and present good accuracy at low and not strongly variable CO 2 concentration [3,7,8]. But there is also a need for carbon isotope monitoring at higher and more variable concentrations, in volcanic [2] or underground [9] environments, as well as for soil [10] and fermentation plants monitoring [11].…”