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.