Assessment of Voltage Influence in Carbon Dioxide Fixation Process by a Photo-Bioelectrochemical System under Photoheterotrophy.

Bioelectrochemical systems are a promising technology capable of reducing CO2 emissions, a renewable carbon source, using electroactive microorganisms for this purpose. Purple Phototrophic Bacteria (PPB) use their versatile metabolism to uptake external electrons from an electrode to fix CO2. In this work, the effect of the voltage (from -0.2 to -0.8 V vs. Ag/AgCl) on the metabolic CO2 fixation of a mixed culture of PPB under photoheterotrophic conditions during the oxidation of a biodegradable carbon source is demonstrated. The minimum voltage to fix CO2 was between -0.2 and -0.4 V. The Calvin-Benson-Bassham (CBB) cycle is the main electron sink at these voltages. However, lower voltages caused the decrease in the current intensity, reaching a minimum at -0.8 V (-4.75 mA). There was also a significant relationship between the soluble carbon uptake in terms of chemical oxygen demand and the electron consumption for the experiments performed at -0.6 and -0.8 V. These results indicate that the CBB cycle is not the only electron sink and some photoheterotrophic metabolic pathways are also being affected under electrochemical conditions. This behavior has not been tested before in photoheterotrophic conditions and paves the way for the future development of photobioelectrochemical systems under heterotrophic conditions.