Biotransformation of 4-Hydroxybenzoic Acid under Nitrate-Reducing Conditions in a MEC Bioanode

4-Hydroxybenzoic acid (HBA) is commonly found at high concentrations in waste streams generated by the thermochemical conversion of lignocellulosic biomass to bio-oils and biofuels. The objective of this study was to systematically assess the biotransformation of HBA in the bioanode of a microbial electrolysis cell (MEC) for the production of renewable cathodic H₂. A mixed, denitrifying culture, enriched with HBA as the sole electron donor, was used as the anode inoculum. MEC electrochemical performance, H₂ yield, HBA biotransformation pathways and products, and the bioanode suspended and biofilm microbial communities were examined. In the absence of nitrate, 60%-100% HBA was converted to phenol, which persisted, resulting in very limited exoelectrogenesis. Under nitrate-reducing conditions, complete HBA degradation was achieved in the MEC bioanode with very low phenol production, resulting in the production of cathodic H₂. The predominant bacterial genus in the MEC bioanode (relative abundance 33.4%-41.9%) was the denitrifier Magnetospirillum, which uses the benzoyl-CoA pathway to degrade aromatic compounds. Geobacter accounted for 5.9-7.8% of the MEC bioanode community. Thus, active nitrate reduction in the MEC bioanode led to complete HBA degradation, resulting in a higher extent of exoelectrogenesis and cathodic H₂ production. The results of this study provide mechanistic insights into a productive use of HBA and other phenolic compounds typically found in waste streams resulting from the thermochemical conversion of lignocellulosic biomass to biofuels.