Effect of Undissociated n-Caproic Acid on Methanogen Activity and Subsequent Recovery: Methane Anabolism and Community Structure

The anaerobically fermented medium chain carboxylic acid (MCCA) has been demonstrated to inhibit methanogenesis in an anaerobic chain elongation fermentation system. However, the inhibition mechanism and recovery performance remain unknown. This study systematically investigated the short-term effects of undissociated n-caproic acid (UnCA) on methanogenesis in granular sludge, focusing on methane anabolism and community structure. Results showed that 2.0 mM UnCA inhibited CH4 yield by 94.3% and acetate consumption by 88.8%, which was ascribed to the variations of intracellular H+ and cell membrane damage. Specifically, UnCA altered multiple processes of methane anabolism such as energy metabolism and electron transport by increasing the intracellular H+ level. In addition, the hydrophobicity of UnCA destroyed the integrity of the cell membrane. However, the methane synthesis enzymes appeared not to be affected as indicated by the stable abundance and activities and also the subsequent reversible 66.6–82.8% recovery of the CH4 yield. Meanwhile, the UnCA supplementation transformed the dominant species from acetoclastic methanogenic to hydrogenotrophic ones and also enriched Bathyarchaeia which had potential CH4 production capacity. These findings are expected to enhance the understanding of the MCCA inhibition on methanogenesis, promoting the establishment of an efficient and stable MCCA production system.