Artificial Carbon Fixation Outperforms Nature

One-carbon (C1) feedstocks, such as formate, are promising renewable substrates for the sustainable microbial production of food, fuels, and chemicals. Some bacteria naturally metabolize formic acid to produce valuable compounds. To harness this potential, scientists are exploring hybrid processes that first fix CO₂ into formic acid through physicochemical methods and then further convert it using microbial pathways.

In nature, CO₂ is primarily fixed through the Calvin cycle, a key component of photosynthesis. However, this pathway is inefficient. Researchers have developed artificial cycles that outperform the Calvin cycle in vitro, though integrating these cycles into living organisms remains challenging.

Beau Dronsella (Max Planck Institute of Molecular Plant Physiology, Potsdam, Germany, and Max Planck Institute for Terrestrial Microbiology, Marburg, Germany), Nico J. Claassens (Wageningen University, The Netherlands), and colleagues have replaced the native, energy-inefficient Calvin–Benson–Bassham (CBB) cycle in Cupriavidus necator—a Gram-negative soil bacterium that naturally uses the CBB cycle to convert formic acid—with the more energy-efficient reductive glycine pathway (rGlyP), optimizing growth on formate and CO₂. The rGlyP is the most ATP-efficient aerobic pathway for formate assimilation.

Integrating rGlyP into the C. necator genome involved modular and evolutionary engineering. The team transferred the entire reductive glycine pathway into the bacterium’s genome and optimized its efficiency. Mobile DNA elements, capable of random genomic integration, were loaded with parts of the metabolic pathway. The team then enhanced genomic modifications through laboratory evolution, selecting for improved growth on formic acid.

In chemostats—bioreactors used to maintain a continuous culture of microorganisms under steady-state conditions—the engineered C. necator strain achieved a 17% higher biomass yield than the wild type and a yield higher than any previously reported for growth on formate via engineered pathways or the CBB cycle. However, the engineered strain was so far only half as fast as the natural one.

According to the researchers, this study represents a significant step forward for the emerging field of synthetic biology. Its implications extend beyond C. necator, as future studies may focus on implementing even more efficient synthetic CO₂ fixation pathways, such as the crotonyl-CoA-ethylmalonyl-CoA-hydroxybutyryl-CoA (CETCH) cycle, in autotrophic hosts. The study also underscores the potential of using these techniques for optimizing synthetic metabolism in other microbial species, including more difficult-to-engineer bacteria as well as eukaryotes such as yeast and microalgae.