Mass spectrometric identification of 13C-labeled metabolites during anaerobic propanoic acid oxidation

Biowaste digestion is a possibility to gain biogas as a renewable fuel source. However, the anaerobic food chain may be disrupted by, e.g., substrate overload or by inhibitors, leading to the accumulation of volatile fatty acids (VFAs), predominantly of propanoic acid (PA). VFA Accumulation may cause a rapid pH decrease, less biogas production, or even a total inhibition. To maintain high biogas productivity or to prevent a collapse of methanogenesis, metabolic properties of the degrading microorganisms must be elucidated, e.g., by investigation of the established pathways for degradation of VFAs. A Dani 3950 headspace system (HS), a Varian 431 gas chromatograph (GC), and a Varian 210 mass spectrometer (MS) have been combined to quantify and specifically identify metabolites of PA oxidation. The use of [1-(13)C]-labeled PA as a carbon source for microorganisms allows differentiation between the methyl-malonyl-CoA or the C(6)-dismutation pathway, both resulting in AcOH production. Appearance of the (13)C-moiety either in the COO or Me group of AcO can easily be detected by MS. The methyl-malonyl-CoA pathway was successfully identified as the only pathway of PA degradation by organisms in a lab-scale anaerobic digester. A similar approach can be applied to any degradation pathway involving VFAs.