Direct Microbial Conversion of CO₂ to Ethanol

Introduction

Carbon dioxide (CO₂), the most abundant greenhouse gas, is often viewed as waste. But with the rise of carbon capture and synthetic biology, it is increasingly seen as a feedstock for valuable products. One promising approach is the direct microbial conversion of CO₂ into ethanol, using engineered microbes capable of fixing carbon via autotrophic pathways and redirecting it into ethanol biosynthesis.

Unlike traditional ethanol fermentation from sugars, this route does not require biomass hydrolysis or sugar fermentation, making it a low-footprint, carbon-negative alternative. Microorganisms such as acetogens, cyanobacteria, and chemoautotrophs are being engineered or optimized to convert CO₂ and H₂ (or electricity) into ethanol using specialized carbon fixation pathways.

What Products Are Produced?

• Ethanol – Fuel blending (E10–E100), SAF precursor, green solvent
• Oxygen or biomass – Byproducts in photosynthetic systems
• Residual microbial mass – For feed, fertilizer, or protein recovery
• Acetate or other organic acids – Intermediate or side products in some systems

Pathways and Production Methods

1. Carbon Fixation Pathways
• Wood-Ljungdahl (WL) Pathway (Acetogens): CO₂ + H₂ → Acetyl-CoA → Ethanol
• Calvin-Benson-Bassham (CBB) Cycle (Cyanobacteria): CO₂ + light → Sugars → Ethanol
• Reverse TCA (rTCA) or 3-HP/4-HB cycles (in extremophiles, archaea)
2. Electron Sources
• H₂ gas from electrolysis or syngas
• Direct electrons from electrodes in microbial electrosynthesis (MES)
• Light in case of photosynthetic organisms (e.g., cyanobacteria)
3. Ethanol Synthesis Module
• Conversion of Acetyl-CoA → Acetaldehyde → Ethanol
• Catalyzed by pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH)

Catalysts and Key Tools Used

• Key Microbes:

• Clostridium ljungdahlii, Acetobacterium woodii, Moorella thermoacetica – acetogenic CO₂ fermenters
• Synechococcus elongatus, Synechocystis sp. – engineered cyanobacteria
• Cupriavidus necator – chemoautotroph used in CO₂ + H₂ fermentation

• Genetic Tools:

• CRISPR-Cas systems to enhance ethanol modules
• Promoter engineering for gas uptake, ATP regeneration, redox control
• Synthetic operons for complete WL-to-ethanol conversion

• Reactor Systems:

• Gas fermenters (CO₂ + H₂ feed)
• Photo-bioreactors (for cyanobacteria with light + CO₂)
• Bioelectrochemical cells (CO₂ reduction using electric current)

Case Study: LanzaTech’s CO₂ Fermentation to Ethanol

Highlights

• Used Clostridium autoethanogenum engineered for ethanol overproduction
• Feed: industrial CO₂ + H₂ from renewable electrolysis
• Achieved >90% carbon efficiency in pilot plant
• Ethanol used for aviation fuel blending and chemical intermediates

Timeline

• 2015 – Strain development for CO₂-only fermentation
• 2018 – Pilot CO₂-to-ethanol bioreactor trials
• 2021 – Integration with steel plant CO₂ capture
• 2023 – Commercial project with Indian Oil under net-zero roadmap

Global and Indian Startups Working in This Area

Global

• LanzaTech (USA/NZ) – Leading player converting CO, CO₂ to ethanol
• Twelve (USA) – CO₂ electrolysis + microbial modules for ethanol
• Carbon Recycling International (Iceland) – Renewable ethanol from CO₂
• Solar Foods (Finland) – Using C. necator for CO₂-based fuels and proteins

India

• Indian Oil R&D (Panipat) – Pilot microbial CO₂-to-ethanol systems
• CSIR-IMTECH & IISER Pune – Synthetic biology for autotrophic conversion
• Carbon Craft Lab – Exploring CO₂ capture + fermentation linkages
• IIT Bombay – Microbial electrosynthesis consortia for CO₂ fuels

Market and Demand

The carbon-derived ethanol market is emerging, valued at USD 140 million in 2023, projected to reach USD 800 million by 2030 at a CAGR of ~28%.

Major End-Use Segments:

• Fuel blending (E20, E100)
• Aviation biofuels (SAF)
• Green solvents and industrial chemicals
• Cosmetics and pharma-grade ethanol
• Circular carbon materials for polymers

Key Growth Drivers

• Push for carbon-neutral fuels under net-zero policies
• India’s National Green Hydrogen Mission supporting H₂-based microbial systems
• Falling costs of CO₂ capture and electrolytic H₂
• Synthetic biology enabling precise CO₂-to-product pathways
• Industrial decarbonization needs (e.g., steel, cement)

Challenges to Address

• Low ethanol titers in CO₂-fed microbial systems
• Mass transfer limitations of CO₂ gas in bioreactors
• High cost of electrons or hydrogen as electron donors
• Need for robust microbes with high conversion rates
• Regulatory approvals for modified microbes in industrial settings

Progress Indicators

• 2010 – Early CO₂ fixation work in acetogens
• 2015 – Engineered acetogens convert CO₂ + H₂ to ethanol
• 2019 – Cyanobacteria engineered for continuous ethanol secretion
• 2022 – India begins pilot trials for microbial CO₂ conversion
• 2024 – Bioelectrochemical CO₂-to-ethanol platforms enter TRL 6–7

Direct microbial CO₂-to-ethanol via acetogens is at TRL 6–7 for pilot-scale systems; cyanobacterial and MES-based platforms are at TRL 4–6, with rapid development underway.

Conclusion

Direct microbial conversion of CO₂ into ethanol represents a breakthrough in carbon capture and biofuel synthesis, creating value from emissions. As bioprocess design, gas fermentation, and synthetic biology converge, this approach offers a path to carbon-negative ethanol, enabling green fuels and chemicals without using food crops or arable land.

India’s growing interest in industrial CO₂ reuse and renewable hydrogen positions it well to benefit from this clean, scalable, and circular technology—fueling a decarbonized economy one microbe at a time.

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