Design and implementation of aerobic and ambient CO<sub>2</sub>-reduction as an entry-point for enhanced carbon fixation.

The direct reduction of CO₂ into one-carbon molecules is key to highly efficient biological CO₂-fixation. However, this strategy is currently restricted to anaerobic organisms and low redox potentials. In this study, we introduce the CORE cycle, a synthetic metabolic pathway that converts CO₂ to formate at aerobic conditions and ambient CO₂ levels, using only NADPH as a reductant. Combining theoretical pathway design and analysis, enzyme bioprospecting and high-throughput screening, modular assembly and adaptive laboratory evolution, we realize the CORE cycle in vivo and demonstrate that the cycle supports growth of E. coli by supplementing C1-metabolism and serine biosynthesis from CO₂. We further analyze the theoretical potential of the CORE cycle as a new entry-point for carbon in photorespiration and autotrophy. Overall, our work expands the solution space for biological carbon reduction, offering a promising approach to enhance CO₂ fixation processes such as photosynthesis, and opening avenues for synthetic autotrophy.