Microbial Metabolism of the Plant Phenolic Compounds Ferulic and Syringic Acids under Three Anaerobic Conditions

Abstract

Ferulic and syringic acids are methoxylated aromatic compounds that often serve as models of the subunits of lignin. Although these compounds have important implications for global carbon cycles, there is limited information on their fate in anoxic environments. Enrichment cultures were established on these two model compounds under methanogenic, sulfidogenic, and denitrifying conditions, using a Raritan River (New Jersey) marsh sediment as the inoculum. All cultures completely degraded ∼1.5 mm of both substrates. Methane production in the methanogenic cultures corresponded to the stoichiometric values expected for complete mineralization to CO₂ and CH₄. Sulfate and nitrate reduction in their respective cultures were both greater than 60% of the amounts predicted for complete mineralization. Aromatic intermediates of ferulic and syringic acid metabolism were identified, and pathways of degradation under sulfidogenic and denitrifying conditions are proposed. Syringic acid is sequentially O-demethylated to gallic acid under both sulfate and nitrate-reducing conditions before ring cleavage occurs. Ferulic acid undergoes propenoate side chain reduction, O-demethylation, removal of an acetate moiety from the side chain, and decarboxylation to form catechol. Catechol is further degraded under sulfidogenic conditions. Under denitrifying conditions, ferulic acid undergoes loss of an acetate moiety, prior to O-demethylation, to form protocatechuic acid, the last product detected before ring cleavage.