Contribution of C-13-NMR spectroscopy to the elucidation of pathways of propionate formation and degradation in methanogenic environments

Propionate is an important intermediate in the anaerobic degradation of com plex organic matter to methane and carbon dioxide. The metabolism of propio nate-forming and propionate-degrading bacteria is reviewed here. Propionate is formed during fermentation of polysaccharides, proteins and fats. The s tudy of the fate of C-13-labelled compounds by nuclear magnetic resonance ( NMR) spectroscopy has contributed together with other techniques to the pre sent knowledge of the metabolic routes which lead to propionate formation f rom these substrates. Since propionate oxidation under methanogenic conditi ons is thermodynamically difficult, propionate often accumulates when the r ates of its formation and degradation are unbalanced. Bacteria which are ab le to degrade propionate to the methanogenic substrates acetate and hydroge n can only perform this reaction when the methanogens consume acetate and h ydrogen efficiently. As a consequence, propionate can only be degraded by o bligatory syntrophic consortia of microorganisms. NMR techniques were used to study the degradation of propionate by defined and less defined cultures of these syntrophic consortia. Different types of side-reactions were repo rted, like the reductive carboxylation to butyrate and the reductive acetyl ation to higher fatty acids.