Carbon Capture & Utilization Biotechnology

Biological carbon capture and utilization (CCU) converts CO₂ — a waste product of combustion and a greenhouse gas — into useful products using biological systems. Unlike carbon capture and storage (CCS), which buries CO₂ geologically, CCU transforms it into fuels, chemicals, materials, or feed. While CCU does not necessarily deliver permanent carbon removal (the products may be re-emitted when used), it can deliver net emission reductions by displacing fossil-derived products and can contribute to a circular carbon economy.

The biological systems capable of CCU include photosynthetic organisms (plants, algae, cyanobacteria) that use light energy to fix CO₂, and chemotrophic organisms that use chemical energy (hydrogen, methane, reduced sulfur compounds) to fix CO₂. Photosynthetic systems are limited by the availability of light; chemotrophic systems are limited by the availability of the energy source but can operate without light, enabling higher volumetric productivity. Both approaches are being explored at pilot and demonstration scale.

Hrisana Journal welcomes submissions across all aspects of biological CCU — from fundamental research on carbon fixation pathways and metabolic engineering through process development, scale-up, life-cycle assessment, and policy analysis. Our interdisciplinary scope reflects the integrated nature of successful CCU deployment.

Microalgae are photosynthetic microorganisms that can fix CO₂ at higher rates per unit area than terrestrial plants, due to their higher photosynthetic efficiency and the ability to cultivate them at high cell densities. They can be grown on non-arable land using saline or wastewater, avoiding competition with food crops. The resulting biomass can be used for biofuels, animal feed, food ingredients, or high-value products. Microalgae have been widely studied for CO₂ utilization from flue gas, with the CO₂ providing the carbon source for growth.

Cultivation systems include open ponds (raceway ponds), closed photobioreactors (flat panel, tubular, column), and hybrid systems. Open ponds are less expensive but have lower productivity and are susceptible to contamination. Closed photobioreactors achieve higher productivity and better contamination control but are more expensive. The choice depends on the product value, climate, and scale. CO₂ supply to the culture must balance mass transfer (dissolution of CO₂ into the medium) with pH control, as CO₂ dissolution acidifies the medium.

Harvesting microalgae from the dilute culture (typically 0.5-5 g/L dry weight) is energy-intensive and a major cost component. Approaches include flocculation (chemical, biological, or auto-flocculation), flotation, filtration, and centrifugation, often in combination. After harvesting, the biomass must be dried (for stable storage) or processed wet. Downstream processing for specific products — lipid extraction for biodiesel, protein extraction for feed, pigment extraction for high-value products — adds further cost. Despite extensive research, microalgae CCU has not yet achieved commercial viability for bulk products, though high-value applications (nutraceuticals, specialty feeds) support niche markets.

Cyanobacteria — photosynthetic bacteria — offer advantages over eukaryotic microalgae for some CCU applications: they are easier to engineer genetically, grow faster, and can be cultivated in simpler media. Model cyanobacteria such as Synechocystis and Synechococcus have been engineered to produce a range of products including ethanol, lactic acid, succinic acid, 1,3-propanediol, and terpenes. Productivities remain lower than heterotrophic production in yeast or E. coli, but the use of CO₂ and light as feedstocks offers sustainability advantages.

Metabolic engineering of cyanobacteria draws on tools from model bacteria but with some unique challenges. Gene expression tools (promoters, riboswitches, CRISPR-based systems) have been developed, and synthetic biology approaches enable construction of complex genetic circuits. Product export — getting the product out of the cell without requiring cell lysis — is important for continuous production and is being engineered for various products. Strain stability under production conditions, particularly when the product diverts carbon from growth, is a key consideration.

Cultivation of engineered cyanobacteria uses similar photobioreactor technology to microalgae cultivation, with additional considerations for containment of genetically modified organisms. Outdoor cultivation of GM strains faces regulatory hurdles in most jurisdictions, limiting near-term deployment to closed photobioreactors or contained indoor facilities. Despite these challenges, cyanobacteria remain a promising platform for sustainable production of selected high-value products from CO₂.

Hydrogen-oxidizing bacteria (knallgas bacteria) use hydrogen as an electron donor and oxygen as an electron acceptor to fix CO₂ into biomass. The aerobic nature of this metabolism enables higher growth rates than anaerobic chemolithoautotrophs. Cupriavidus necator (formerly Ralstonia eutropha) is a model knallgas bacterium that has been engineered to produce a range of products including PHA bioplastics, fatty acids, and terpenes. The hydrogen and oxygen can be produced by electrolysis of water using renewable electricity, enabling a "power-to-gas-to-product" route that converts renewable electricity into chemical products.

Gas fermentation with anaerobic acetogens such as Clostridium autoethanogenum, C. ljungdahlii, and Acetobacterium woodii uses syngas (CO, H₂, CO₂) or CO₂ plus H₂ as feedstocks to produce ethanol, acetate, and other products. Two commercial facilities (LanzaTech/China Baowu and LanzaTech/ArcelorMittal) are operating at scale, converting steel mill waste gas to ethanol. The technology offers a route to convert industrial waste gases into products, displacing fossil-derived alternatives. Expansion to CO₂ plus renewable hydrogen as feedstock is being explored.

Gas fermentation faces challenges of gas mass transfer (delivering sparingly soluble gases to cells), gas safety (hydrogen-oxygen mixtures are explosive), and the energetics of autotrophic growth. Reactor designs that enhance gas-liquid mass transfer — bubble columns, airlift reactors, monolithic biofilm reactors — are being developed. Integration with upstream gas production (electrolysis, gasification) and downstream product recovery is essential for overall process viability. Hrisana Journal welcomes submissions across these research areas.

Enzymatic CO₂ conversion uses isolated enzymes to convert CO₂ into products. The most explored enzymes are carbonic anhydrase (catalyzing hydration of CO₂ to bicarbonate, useful for CO₂ capture and mineralization), formate dehydrogenase (reducing CO₂ to formate using electrochemical or chemical reducing power), and RuBisCO (carboxylating ribulose bisphosphate in the Calvin cycle). Enzymatic approaches offer high selectivity and mild conditions but are limited by enzyme stability, cofactor requirements, and the cost of reducing power for reduction reactions.

CO₂ mineralization — converting CO₂ into stable carbonate minerals — can be catalyzed by carbonic anhydrase and is being explored for permanent carbon storage. The minerals can be used as construction materials or disposed of, providing a permanent sink for the captured CO₂. The kinetics of mineralization are accelerated by carbonic anhydrase, and immobilized enzyme systems have been demonstrated at pilot scale. The challenge is the availability of suitable cations (calcium, magnesium) at sufficient scale, with industrial waste streams (steel slag, fly ash) being explored as sources.

For researchers working on biological CCU, Hrisana Journal offers a peer-reviewed, open-access venue for sharing your work. Manuscripts should clearly describe the biological system, the CO₂ source, the energy source, the products, the productivities and yields, and the life-cycle implications. Visit our Submit Manuscript page to begin your submission, or review our Author Guidelines for preparation requirements. We also offer a Free Publication Programme for eligible researchers from developing countries.