Anodic respiration of Pseudomonas putida KT2440 in a stirred-tank bioreactor

Anodic batch production of para-hydroxybenzoic acid (pHBA) from citric acid with a genetically modified Pseudomonas putida KT2440 strain was studied in a bio-electrochemical system (BES) based on a standard lab-scale stirred-tank bioreactor at fully controlled anaerobic reaction conditions. Electron transfer to the anode was mediated by addition of K₃Fe(CN)₆ to the medium. Effects of varying anode surface areas (graphite rod, felt and brush), power input (stirrer speed) and mediator concentrations were investigated. The obligate aerobic P. putida grew anaerobically with mediated anodic respiration and pHBA production was observed. Anodic respiration was best applying the graphite rod electrode which showed a maximal current density of 12.5 mA cm⁻². This is the highest measured for non-porous electrodes in BES until now. Increasing the power input to 2.93 W L⁻¹ (700 rpm) and online control of the redox potential E_Medium at 225 mV (vs. Ag/AgCl) in the medium by controlled addition of mediator resulted in a maximal pHBA yield of 9.91 mmolC_{p HBA} molC⁻¹ _citrate which exceeds pHBA yields in the aerobic batch process by 69 % (5.87 mmolC_{p HBA} molC⁻¹ _citrate).