Methanogenesis from Propionate in Sludge and Enrichment Systems

Abstract

The biological formation of methane from organic matter is a complex microbiological process involving many physiologically dependent relationships between and among a diversity of heterotrophic fermentative and methanogenic bacteria. The methanogenic metabolism of all organic matter leads to the formation of the same types of intermediates namely H₂, C0₂, and formate, acetic, propionic, and butyric acids. These compounds are, in turn, converted directly or indirectly to methane by methanogenic bacteria alone or acting together with non-methanogenic heterotrophs. These latter organisms may be syntrophic partners of the methanogens or simply members of a broader food-chain. Estimates on the sum_:total of the fermentative contributions by these few metabolic intermediates account for 100% of the methane formed in a typical digestor. The importance of acetate as a direct methanogenic intermediate is already well established (5,6). Evidence points to a more complicated role played by the metabolism of butyrate and propionate in this fermentation. The main focus of the present paper is to examine the role of propionate in methanogenesis not only to re-assess its importance as a source of methane or methanogenic precursors in digestors but also to examine the biochemical and physiological basis for its conversion to methane.