# Phenyl Acid Induced Inhibition of Methanogenesis in CO2 Reducing Organisms

## TL;DR

This study shows that phenyl acids, produced during anaerobic digestion of certain substrates, can strongly inhibit methane-producing microbes, especially at higher concentrations.

## Contribution

The study identifies phenyl-butyric acid as the most harmful phenyl acid to methanogens and compares the sensitivity of mesophilic and thermophilic species.

## Key findings

- Phenyl-butyric acid (PBA) had the strongest inhibitory effect on methanogens compared to other phenyl acids.
- Mesophilic methanogens were more sensitive to phenyl acids than thermophilic ones.
- 35 mM concentrations of PPA and PBA almost completely inhibited the growth and activity of Methanospirillum hungatei.

## Abstract

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.

In the presented study, the impact of phenyl acids on hydrogenotrophic methanogenic pure cultures was evaluated. Mesophilic‐incubated methanogens showed a higher sensitivity against phenyl acids than thermophilic‐incubated methanogens. Phenyl‐butyric acid had the highest adverse effect on the metabolism of the tested methanogens, and 35 mM phenyl‐propionic‐ and phenyl‐butyric acids could prevent the metabolic activity of certain methanogenic pure cultures.