Harnessing electrical-to-biochemical conversion for microbial synthesis

Bioelectricity and CO2-to-butyrate production using photobioelectrochemical cells with bio-hydrogel

Bio-photoelectrochemical cell (BPEC) is an emerging technology that can convert the solar energy into electricity or chemicals. However, traditional BPEC depending on abiotic electrodes is challenging for microbial/enzymatic catalysis because of the inefficient electron exchange. Here, electroactive bacteria (Shewanella loihica PV-4) were used to reduce graphene oxide (rGO) nanosheets and produce co-assembled rGO/Shewanella biohydrogel as a basic electrode. By adsorbing chlorophyll contained thylakoid membrane, this biohydrogel was fabricated as a photoanode that delivered maximum photocurrent 126 μA/cm3 under visible light. Impressively, the biohydrogel could be served as a cathode in BPEC by forming coculture system with genetically edited Clostridium ljungdahlii. Under illumination, the BPEC with above photoanode and cathode yielded ∼ 5.4 mM butyrate from CO2 reduction, 169 % increase compared to dark process. This work provided a new strategy (nanotechnology combined with synthetic biology) to achieve efficient bioelectricity and valuable chemical production in PBEC.

Rewiring carbon flow in Synechocystis PCC 6803 for a high rate of CO2-to-ethanol under an atmospheric environment

Cyanobacteria are an excellent microbial photosynthetic platform for sustainable carbon dioxide fixation. One bottleneck to limit its application is that the natural carbon flow pathway almost transfers CO₂ to glycogen/biomass other than designed biofuels such as ethanol. Here, we used engineered Synechocystis sp. PCC 6803 to explore CO₂-to-ethanol potential under atmospheric environment. First, we investigated the effects of two heterologous genes (pyruvate decarboxylase and alcohol dehydrogenase) on ethanol biosynthesis and optimized their promoter. Furthermore, the main carbon flow of the ethanol pathway was strengthened by blocking glycogen storage and pyruvate-to-phosphoenolpyruvate backflow. To recycle carbon atoms that escaped from the tricarboxylic acid cycle, malate was artificially guided back into pyruvate, which also created NADPH balance and promoted acetaldehyde conversion into ethanol. Impressively, we achieved high-rate ethanol production (248 mg/L/day at early 4 days) by fixing atmospheric CO₂. Thus, this study exhibits the proof-of-concept that rewiring carbon flow strategies could provide an efficient cyanobacterial platform for sustainable biofuel production from atmospheric CO₂.

Upcycling air pollutants to fuels and chemicals via electrochemical reduction technology

The intensification of fossil fuel usage results in significant air pollution levels. Efforts have been put into developing efficient technologies capable of converting air pollution into valuable products, including fuels and valuable chemicals (e.g., CO₂ to hydrocarbon and syngas and NO_x to ammonia). Among the strategic efforts to mitigate the excessive concentration of CO₂ and NO_x pollutants in the atmosphere, the electrochemical reduction technology of CO₂ (CO₂RR) and NO_x (NO_xRR) emerges as one of the most promising approaches. It is even more attractive if CO₂RR and NO_xRR are paired with renewables to store intermittent electricity in the form of chemical feedstocks. This review provides an overview of the electrochemical reduction process to convert CO₂ to C₁ and/or C₂₊ chemicals and NO_x to ammonia (NH₃) with a focus on electrocatalysts, electrolytes, electrolyzer, and catalytic reactor designs toward highly selective electrochemical conversion of the desired products. While the attempts in these aspects are enormous, economic consideration and environmental feasibility for actual implementation are not comprehensively provided. We discuss CO₂RR and NO_xRR from the life cycle and techno-economic analyses to perceive the feasibility of the current achievements. The remaining challenges associated with the industrial implementation of electrochemical CO₂ and NO_x reduction are additionally provided.