Production of Short-chain Fatty Acid from CO2 Through Mixed and Pure Culture in a Microbial Electrosynthesis Cell

Abstract

The continuous accumulation of atmospheric CO₂ requires the development of new technologies for its mitigation. Carbon capture and utilization (CCU) technologies aim to convert CO₂ into precious compounds like chemicals and fuels. Biological fixation is an attractive CCU strategy in terms of cost, sustainability and variety of products. Chemoautotrophic microorganisms such as methanogens and acetogens are able to reduce CO₂ into acetate and methane, respectively. Acetogens bacteria are able to use CO₂ for cell growth through the Wood Liujhundal pathway, moreover, the final precursor (i.e. Acetyl-CoA) of the autotrophic metabolism, is also used in energy metabolism with acetate production as a waste product. Furthermore, it is possible to obtain multicarbon products of autotrophic origin starting from acetyl-CoA and acetate. The biotechnological use of these microorganisms requires the presence of H₂ as substrate, which is used as an electron donor in the pathway. This reaction can be sustained by a biocathode in a microbial electrosynthesis cell, in which the reducing power is generated by a polarized electrode. This study proposes the use of a microbial electrosynthesis cell for conversion to acetate in H-cells by either a mixed culture enriched with Acetobacterium woodii or a pure culture of Acetobacterium woodii, to observe the difference in terms of acetate production and reducing power consumption efficiency. The mixed culture was obtained from a mixture of activated sludge and anaerobic digestate, treated by a protocol capable to select acetogenic microorganisms without the use of specific chemical inhibitors (2-Bromoethanesulfonate). Both inoculums were tested at room temperature (25°C) in the cathodic chamber of the H-cell at potentials in the range of -0.7 to -1.1 V vs SHE. The obtained results showed that the enriched mixed culture produced at -0.7 vs SHE a mixture of volatile fatty acids including C4 and C5 molecules with an overall coulombic efficiency of 50%, while at the potential of -0.9 vs SHE methane constituted the main product of the biocathode. The pure culture, on the other hand, showed a specific production of acetate with a coulombic efficiency of 44% at -0.9 vs SHE and 63% at -1.1 vs SHE. Furthermore, a drastic decrease in biocathode biomass was observed in pure culture, suggesting a higher tendency to form biofilms on the electrode unlike the mixed culture, which showed a standard growth profile in the bulk.