Biologically-mediated CO2 capture by Cupriavidus necator for polyhydroxyalkanoates production and biogas upgrading

Cupriavidus necator is a microorganism known for its ability to store polyhydroxyalkanoates (PHA), which are bioplastics that are increasingly being used to replace plastics obtained from chemical compounds of fossil origin. C. necator uses O2 as electron acceptor and H2 as electron donor for CO2 uptake. The conversion of CO2 to PHA by C. necator has been demonstrated in previous studies. This process allows the removal of a greenhouse gas combined with the simultaneous production of a value-added product (PHA). In this study, different conditions and carbon sources were tested for C. necator growth based on previous metabolic flux-balance simulations. Different carbon sources, including glucose, pure CO2, biogas (containing 40% of CO2) and digestate containing a high concentration of volatile fatty acids were used for growing C. necator. Cultures were grown in aerobic and microaerophilic conditions (O2 concentration between 2-5%). The cultures were also exposed to different CH4 and CO2 concentrations to evaluate potential toxic effects on the species under investigation. The maximum growth was achieved at aerobic conditions, both with glucose or CO2 as carbon source. A minimum of around 5% of O2 was needed for the microorganism to grow. CH4 was found not to be toxic for C. necator, so, biogas could be feed to this culture with a minimum amount of O2 in a full-scale process, allowing the biogas upgrading. It is expected that the simultaneous removal of CO2 present in biogas and the externally provided O2 can lead to biogas upgrade to biomethane which can be injected into the gas grid.