Regular articleAnodic respiration of Pseudomonas putida KT2440 in a stirred-tank bioreactor

- A fully controlled stirred-tank bioreactor was used as a bio-electrochemical system.
- Electro-fermentative para-hydroxybenzoic acid production was optimised.
- Mass transfer limits production rates in the electro-fermentative process.
- Redox potential in the medium needs to be controlled.
- para-Hydroxybenzoic acid yields exceeded pHBA yields from an aerobic batch process.

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 EMedium at 225 mV (vs. Ag/AgCl) in the medium by controlled addition of mediator resulted in a maximal pHBA yield of 9.91 mmolCpHBA molC−1citrate which exceeds pHBA yields in the aerobic batch process by 69 % (5.87 mmolCpHBA molC−1citrate).

Controlling the redox potential in the medium is important to achieve steady para-hydroxybenzoate (pHBA) production in a bio-electrochemical system with Pseudomonas putida KT2440 ΔpobA/pSEVA-ubiC. The working electrode potential did not influence the anodic respiration but the ratio of oxidised to reduced mediator concentration did. Using a fully controlled stirred-tank bioreactor with redox potential control in the medium, pHBA concentration could be increased by 69% compared to the aerobic process and current densities of 12.5 mA cm−2 were reached.194