Phenyl Acid Induced Inhibition of Methanogenesis in CO₂ Reducing Organisms

During anaerobic digestion (AD) of lignocellulose- and protein-rich substrates known to contain a high load of aromatic compounds, various undesired intermediates can arise, which can accumulate and cause serious disturbances during the cascade-like AD process. The phenyl acids phenyl-acetic-(PAA), phenyl-propionic-(PPA), and phenyl-butyric acid (PBA) are such intermediates suspected to negatively affect the microbial community, resulting in a decreased biogas yield. In the present study, the impact of PAA, PPA, and PBA on the metabolism of CO₂ reducing methanogens was investigated. The mesophilically incubated Methanococcus vannielii and Methanospirillum hungatei showed a higher sensitivity than the thermophilic Methanoculleus thermophilus, Methanothermobacter thermoautotrophicus, and Methanothermobacter wolfei. A concentration of 35 mM PPA and 35 mM PBA inhibited methanogenesis and the growth of M. hungatei almost completely. PBA had the greatest detrimental impact on methanogens across all tested phenyl acids, followed by PPA and PAA. However, in further investigations, it has to be clarified if and how other microorganisms involved in AD are impacted by phenyl acids. A more detailed knowledge will help to better understand disturbances due to phenyl acid emergence caused by the degradation of lignocellulose- and protein-rich substrates, to ensure a stable AD process even at high loads of these substrates.