Harnessing CO2: A Novel Approach to Single-Cell Protein Production

By Muhammad OsamaReviewed by Laura ThomsonJan 22 2025

A recent research article published in Environmental Science and Ecotechnology introduced a novel hybrid bioreactor system for producing single-cell protein (SCP) using carbon dioxide (CO₂) and electricity.

The bioreactor system combines anaerobic and aerobic processes to enhance the efficiency of microbial electrosynthesis (MES) and address environmental challenges posed by rising CO₂ levels. The researchers emphasized the potential of MES technology to provide sustainable protein sources, contributing to climate change mitigation.

Advancements in MES Technology

MES is an electrochemical process that uses microorganisms to convert CO₂ into valuable organic compounds. It relies on bacteria that use hydrogen gas (H2) electrons or directly transfer electrons to synthesize organic compounds from inorganic sources. MES is gaining attention for its ability to recycle CO₂ and contribute to climate change mitigation.

Unlike traditional CO₂ reduction methods, which produce low-value outputs like acetate and methane, MES enables the production of higher-value compounds like single-cell protein. This is achieved by coupling acetate production with microbial upgrading processes.

MES offers advantages over traditional methods, including better product selectivity, higher energy efficiency, and greater operational stability. However, low productivity in acetate production remains a challenge, as acetate is often seen as a low-value product. Upgrading acetate into higher-value products is essential for MES to be practically viable.

Novel Two-Stage Bioprocess for SCP Production

In this paper, the authors developed and tested a two-stage bioprocess that integrates MES with acetate-utilizing processes to enhance SCP production. The hybrid system consists of an electro-bubble column MES reactor (Reactor 1) and a continuously stirred tank bioreactor (Reactor 2). Reactor 1 converts CO₂ and electricity into acetate using anaerobic homo-acetogens, while Reactor 2 employs aerobic Alcaligenes to produce SCP from the acetate.

The experimental setup included several key components and methodologies. Reactor 1 operated with a synthetic medium containing essential nutrients and inhibitors to prevent methanogen growth. Its anode and cathode were optimized for H₂ production and acetate synthesis. Reactor 2 maintained aerobic conditions for efficient acetate fermentation.

The reactors were interconnected by hollow fiber membranes, enabling continuous circulation of a cell-free medium, which minimized wastewater and nutrient loss. Operational factors such as potential of hydrogen ions (pH), temperature, and nutrient levels were carefully managed to enhance microbial growth and SCP production. The study conducted two experiments to confirm reproducibility and evaluate the system's performance.

Findings of Using New Hybrid Bioreactors

The outcomes demonstrated that the hybrid bioreactor system achieved a maximum cell dry weight (CDW) of 17.4 g/L, with an average production rate of 1.5 g/L/day.

The biomass exhibited a protein content of 74% of the dry weight, highlighting the high nutritional value of the produced SCP. The system yielded 12.8 g/L of SCP, with an average productivity of 1.2 g/L/day, while achieving near-complete utilization of H₂ and CO₂, with gas uptake efficiencies approaching 100%. These results underscore the system's effectiveness in enhancing SCP production while minimizing environmental impact.

Continuous medium recirculation between the reactors maintained optimal microbial growth conditions. The acetate concentration in Reactor 1 was consistently maintained between 2 and 5 g/L, effectively reducing product inhibition and facilitating a rapid startup of Reactor 2 after inoculation.

The researchers noted the stability of the microbial communities within the reactors, with Acetobacterium dominating Reactor 1 and Alcaligenes comprising nearly 96% of Reactor 2's microbial population. This microbial stability was essential for maintaining consistent production rates and process efficiency without extensive sterilization.

The study further analyzed the amino acid composition of the produced SCP, demonstrating similarities to fish meal and better nutritional content than soybean meal. However, a deficiency in sulfur-containing amino acids highlighted the need to optimize the growth medium to enhance its quality.

Applications of the Biohybrid System

This research has significant implications beyond SCP production. The hybrid bioreactor system can be adapted to produce other high-value bioproducts, such as fatty acids and biofuels.

By utilizing CO₂ as a feedstock, the system supports carbon capture and utilization (CCU) strategies, advancing the circular carbon economy and promoting environmental sustainability. Its ability to recycle nutrients and minimize wastewater generation further enhances the process's environmental benefits.

Conclusion and Future Directions

Using a coupled anaerobic-aerobic bioprocess, the novel bioreactor proved effective for producing SCP from CO₂ and electricity. Integrating MES with acetate-utilizing bacteria enhanced productivity while addressing environmental concerns related to CO₂ emissions.

The produced SCP's high protein content and favorable amino acid profile make it a promising alternative for animal feed and human consumption.

Future work should focus on optimizing bioreactor operations, improving electron-to-protein conversion efficiency, and scaling up the system for industrial use. This study represents a significant step toward sustainable protein production and CO₂ utilization, advancing microbial biotechnology while supporting efforts to reduce greenhouse gas emissions.

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