Biological upgrading of C1-C2 products of electrocatalytic CO 2 reduction to C4-C6 carboxylates

BackgroundMicrobial chain-elongation by Clostridium kluyveri using the products (acetate and ethanol) derived from the electrocatalytic CO2 reduction reaction (CO2RR) represents a unique sustainable strategy for producing C4-C6 chemicals from CO2. However, direct integration of electrocatalytic effluents with anaerobic bioprocesses is often impeded by the physiological incompatibility between electrocatalytic product streams and microbial metabolism. Specifically, CO2RR effluents commonly contain formate, which cannot be utilized by C. kluyveri for chain elongation and therefore reduces the overall carbon efficiency of CO2 conversion to C4-C6 chemicals. Moreover, both formate and the elevated phosphate concentrations typical of electrochemical reaction solutions may inhibit microbial growth. ResultsWe show that formate at concentrations of up to 50 mM did not inhibit the growth of or the chain elongation by C. kluyveri. Based on this finding, we developed a modular two-step bioprocess. In the first step, the acetogen Clostridium ljungdahlii converts formate in CO2RR product mixtures into acetate, thereby generating additional substrates for second-step C. kluyveri-driven chain elongation, thus increasing the CO2RR carbon-conversion efficiency to C- C6 chemicals. To address the issue of C. ljungdahliis inhibition by high phosphate concentrations in electrocatalytic solutions, we explored the use of C. ljungdahlii biofilms for the first, i.e. the formate-conversion, step. C. ljungdahlii biofilms exhibit tolerance to concentrated electrolytes, enabling the conversion of up to 50 mM formate in CO2RR solutions. ConclusionsThe demonstrated two-step process constitutes the basis for the development of a robust and carbon-efficient biological process for the scalable upgrading of C1-C2 CO2RR products into higher-value C4-C6 chemicals.