Exoelectrogenic activity of hydrocarbonoclastic strains and toluene degradation in bioelectrochemical systems using a pure culture of Cupriavidus metallidurans CH34

Bioelectrochemical systems (BESs) have proven to be a useful tool for bioremediation and have been applied to achieve the oxidation of organic compounds (e.g. hydrocarbons) at the anode. Microbial metabolism can be stimulated in a BES when overpotential is applied, increasing the rate of pollutant degradation. The aim of this work was to test the exoelectrogenic capacity of five hydrocarbonoclastic strains of the genera Cupriavidus and Pseudomonas and to determine if the applications of overpotential stimulates microbial metabolism, analyzing current density and substrate consumption. Exoelectrogenic activity was studied with succinate as sole carbon source. Three of the five strains (Cupriavidus metallidurans CH34, Pseudomonas sp. DN34 and Pseudomonas sp. DN36) showed exoelectrogenic activity. Comparing the current densities obtained with the three exoelectrogenic strains, Pseudomonas sp. DN36 reached a maximum of 1.00 mA/m2, followed by Cupriavidus metallidurans CH34 (0.65 mA/m2) and Pseudomonas sp. DN34 (0.23 mA/m2). C. metallidurans CH34, Pseudomonas sp. DN34 and Pseudomonas sp. DN36 reached a 91.74%, 89.38% and 35.28% of succinate removal respectively in 42 h. The application of overpotentials between the anode and the cathode lead to an increase in substrate consumption, turbidity and current density production. Although whole genome sequencing reveal that C. metallidurans CH34 lacks in the upper pathway of toluene degradation in anaerobic conditions, C. metallidurans CH34 significantly reduced (from 100 to 42 ppm in 27 days) the concentration of toluene under denitrifying conditions, while the abiotic control maintained toluene concentration around 100 ppm. A Microbial Fuel Cell (MFC) and a Microbial Electrolysis Cell (MEC) were set with a pure culture of C. metallidurans CH34 and with toluene as sole carbon source. In the MEC, overpotential (+800 mV) was applied between the anode and the cathode. When C. metallidurans CH34 was inoculated in a MFC containing toluene as sole carbon source, current densities reached 0.80 mA/m2. Conversely, in the abiotic control no current was detected. Toluene concentration in the control remained stable (38 ppm) while in inoculated MFC toluene decreased by 36.6% in 10 days. In the biotic MEC, current densities increased from 21 to 30 mA/m2 in 20 days, while in the control, current densities remained constant at 10 mA/m2. After the initial adsorption of toluene on the anodes of both inoculated and sterile control MECs, 63.3% of the initial toluene in the biotic MEC was removed, while it remained stable in the abiotic reactor. The current density increased when toluene was further spiked in the reactors. These data showed that C. metallidurans CH34 was able to degrade toluene under anaerobic conditions and to transfer electrons to a solid electrode.