Engineered Bacteria Recycle CO2 In Situ to Boost Succinic Acid Production to Near-Industrial Levels

Researchers in China have engineered a bacterium to recycle its own carbon dioxide in real time, coaxing it to produce succinic acid查看药品详情 at titers and productivities that approach the thresholds demanded by commercial fermentation. The study, published in Biotechnology for Biofuels and Bioproducts, describes a genetically rewired strain of Actinobacillus succinogenes (搜索)搜索该词条 that uses formate—a cheap, one-carbon compound that can be made electrochemically from CO2—as both a fuel and an internal source of the gas the microbe needs to build succinate molecules. The result is a self-reinforcing metabolic loop that lifts succinate production to 96.64 grams per liter, a productivity of 1.38 grams per liter per hour, and an overall yield of 0.78 grams of succinate per gram of total consumed carbon source.

Succinic acid查看药品详情 sits high on the wish lists of green chemists. The four-carbon dicarboxylic acid is a building block for polymers such as polybutylene succinate, plasticizers, solvents, and a range of fine chemicals, and the U.S. Department of Energy has long listed it among the top value-added chemicals obtainable from biomass. In principle, fermentation offers a doubly attractive route: instead of cracking petroleum, cells convert sugars into succinate while pulling carbon dioxide out of the environment, because the reductive branch of the tricarboxylic acid cycle that leads to succinate requires a molecule of inorganic carbon at the carboxylation step. In practice, however, the biology has been stubborn. Supplying enough dissolved CO2 or bicarbonate to a fermenter is technically awkward—the gas poorly dissolves in the broth, and bubbling it in is expensive—and the cells' internal redox balance often tips toward unwanted byproducts such as acetate, formate, lactate, and ethanol, siphoning carbon away from the target product.

A Synergistic Metabolic Engineering Strategy

The team, led by Mingyang Zhao, Yaqin Sun, and Zhilong Xiu at the MOE Key Laboratory of Bio-Intelligent Manufacturing at Dalian University of Technology (搜索)搜索该词条, attacked both problems simultaneously. Their approach co-expresses two genes in A. succinogenes: the endogenous gene fdoG, which encodes the large catalytic subunit of formate dehydrogenase (搜索)搜索该靶点, and a heterologous gene for phosphoenolpyruvate carboxylase (搜索)搜索该靶点 (PPC), an enzyme that fixes bicarbonate onto the central metabolite phosphoenolpyruvate to form oxaloacetate, the gateway to the reductive TCA pathway that ends in succinate.

The elegance of the design lies in the coupling between the two enzymes. When formate dehydrogenase (搜索)搜索该靶点 oxidizes formate, it releases CO2 directly inside the cell and, at the same time, generates reducing power in the form of NADH. Both products are exactly what the succinate pathway needs: the freshly generated CO2 feeds the PPC-driven carboxylation reaction, and the NADH drives the downstream reductive steps catalyzed by malate dehydrogenase and fumarate reductase.

This tight spatial and stoichiometric coupling means the engineered strain no longer depends on CO2 diffusing in from the gas phase. Instead, the microbe carries what the authors describe as an endogenous CO2 generator, effectively decoupling succinate production from external mass-transfer limitations that have plagued conventional fermentations. Real-time off-gas analysis provided physiological confirmation that the engineered pathway functions as intended, revealing the signature of active in-situ CO2 generation and consumption within the cells.

Redirecting Carbon Flux

To verify that the metabolic rewiring had genuinely redirected carbon flow, the researchers performed carbon flux distribution analysis across the engineered strain's central metabolism. Compared with the wild-type organism, the engineered strain showed a 12.11 percent increase in the fraction of carbon flux directed toward succinate, accompanied by a significant reduction in byproduct accumulation.

That shift matters commercially as much as scientifically: in fermentation economics, downstream separation of organic acid byproducts such as acetate and lactate is one of the costliest steps, so every percentage point of carbon that stays on the succinate branch translates directly into cheaper purification and better overall process efficiency. The co-expression strategy, in other words, did not simply add a new reaction to the network—it rebalanced the entire carbon and electron economy of the cell toward the product of interest.

Process Optimization and Results

With the core design validated, the team turned to process engineering to squeeze out the full potential of the formate-utilization machinery. They developed an optimized fed-batch fermentation protocol in which glucose serves as the main carbon skeleton donor while formate is delivered in carefully timed pulses. The pulsed feeding strategy is chemically deliberate: formate is simultaneously a substrate and a potential stressor, so drip-feeding it at intervals maintains the co-substrate at levels high enough to sustain formate dehydrogenase (搜索)搜索该靶点 activity and NADH regeneration without overwhelming the cell's tolerance or letting the pH drift.

Under these conditions, the engineered strain, designated AS-PF, reached a succinate titer of 96.64 grams per liter at a volumetric productivity of 1.38 grams per liter per hour, with a yield of 0.78 grams per gram based on the combined glucose and formate consumed. Productivity above 1 gram per liter per hour and titers approaching 100 grams per liter are widely cited benchmarks for economically viable organic acid fermentation, placing this strain within striking distance of industrial relevance.

Why A. succinogenes?

The choice of A. succinogenes as the chassis is itself significant. The bacterium is a natural succinate overproducer, a rumen isolate that already possesses a highly active reductive TCA branch, including phosphoenolpyruvate carboxykinase, malate dehydrogenase, and fumarate reductase, and it tolerates high concentrations of the acid product better than many engineered alternatives such as Escherichia coli or yeast. But its natural metabolism also leaks carbon into acetate, formate, and lactate through pyruvate formate-lyase, acetate kinase, and L-lactate dehydrogenase—enzymes whose fluxes the flux analysis could quantify.

By installing the FdoG-PPC loop on top of this native architecture, the researchers essentially plugged a leak and boosted the pump at the same time: formate that would otherwise be excreted or wasted becomes a recycled carbon-and-electron carrier, and phosphoenolpyruvate that might have drained into byproduct pathways is pulled into carboxylation by the overactive PPC.

Broader Implications for C1 Valorization

Beyond the immediate numbers, the study carries broader implications for the emerging field of C1 valorization—the effort to convert single-carbon compounds such as CO2, carbon monoxide, methane, methanol, and formate into products of higher value. Formate is attracting particular attention as a "liquid hydrogen" carrier and CO2-derivative because it can be produced efficiently by electrochemical reduction of CO2 using renewable electricity, and it is stable, water-soluble, and easy to store and transport.

Coupling electrochemical formate production with a fermentation host that consumes formate as its CO2 source creates what is sometimes called a hybrid electro-fermentation chain: renewable electricity fixes atmospheric CO2 into formate, the microbe oxidizes that formate to release CO2 and NADH in situ, and that same CO2 is immediately re-fixed into a four-carbon product. Each turn of the loop effectively doubles down on the carbon captured at the electrochemical stage, and the reducing equivalents liberated along the way improve the thermodynamics of succinate synthesis.

The work was supported by the National Key Research and Development Program of China, and the authors, whose study was conducted at Dalian University of Technology (搜索)搜索该词条's School of Bioengineering, report no competing interests. Their published findings arrive as a peer-reviewed, open-access early-release article, citable under its permanent DOI while the final version of record is prepared.

What remains to be seen is how the platform scales beyond the laboratory fermenter. Formate cost, tolerance limits of the host at larger volumes, and the integration of pulsed feeding with industrial-scale gas and pH control will all shape the economics. Still, the conceptual achievement is clear: rather than fighting the poor solubility of CO2 with bigger compressors and finer spargers, the Dalian team taught the microbe to brew its own carboxylation substrate from a renewable one-carbon liquid. If formate-driven in-situ CO2 recycling proves robust at scale, it could become a template not only for succinic acid查看药品详情 but for an entire family of fermentation products whose biosyntheses hinge on the same bottleneck—getting enough inorganic carbon, and enough reducing power, to the right place inside the cell at the right time.