Sun-Powered Microbes Turn CO2 Into Industrial Organic Acids

Cyanobacteria, the ancient photosynthetic microbes that first flooded Earth’s atmosphere with oxygen more than three billion years ago, are being reimagined as microscopic chemical factories. A comprehensive review published in Blue Biotechnology by Ruchi Pathania of the University of Florida and Amit Srivastava of the Czech Academy of Sciences maps the rapidly advancing field of organic acid production in these organisms, charting a path from laboratory curiosities to industrial biomanufacturing platforms that could help decarbonize the chemical industry. Unlike conventional fermentation hosts such as yeast and Escherichia coli, which must be fed sugars derived from crops, cyanobacteria pull carbon dioxide directly from the air and convert it into valuable molecules using nothing more than sunlight and minimal nutrients.

The commercial stakes are considerable. The global organic acids market is projected to reach 36.86 billion dollars by 2026, and in 2004 the United States Department of Energy identified twelve high-priority building-block chemicals obtainable from biomass, roughly seventy percent of which are organic acids. These include succinic acid, 3-hydroxypropionic acid, itaconic acid, and levulinic acid, compounds that serve as precursors for biodegradable plastics, solvents, pharmaceuticals, and food additives. Traditionally these chemicals come from petrochemical synthesis or heterotrophic fermentation, both of which carry substantial carbon footprints. Cyanobacteria offer a third route: photoautotrophic biosynthesis that consumes the greenhouse gas responsible for climate change as its raw material.

The metabolic logic behind this capability is elegant. Cyanobacteria fix atmospheric carbon dioxide through the Calvin-Benson-Bassham cycle, generating central intermediates such as pyruvate, phosphoenolpyruvate, and acetyl-CoA that feed into the tricarboxylic acid cycle. From these nodes, carbon can be channeled into acetate, lactate, citrate, succinate, fumarate, and malate. Intriguingly, cyanobacteria possess an atypical, incomplete TCA cycle lacking the canonical 2-oxoglutarate dehydrogenase complex. Instead, they bypass this step via 2-oxoglutarate decarboxylase and succinic semialdehyde dehydrogenase, an evolutionary adaptation that appears designed to preserve carbon skeletons for biosynthesis rather than burning them off as carbon dioxide. Some strains also harbor the glyoxylate shunt and a gamma-aminobutyric acid shunt, alternative routes that connect carbon metabolism with nitrogen assimilation and redox regulation.

The organisms also display a striking diurnal rhythm that researchers are learning to exploit. During daylight, cyanobacteria assimilate carbon dioxide into glycogen stores; when darkness falls and oxygen dwindles, they ferment these reserves, secreting organic acids and hydrogen. Under dark, anoxic conditions, Synechocystis sp. PCC 6803 has been shown to release more than 400 milligrams per liter of acetate, along with lactate, succinate, and malate, independent of nitrogen source. This natural overflow metabolism acts as a redox-balancing mechanism, dumping excess reducing equivalents when the cell’s usual sinks, such as glycogen and protein synthesis, are unavailable. Engineers are now working to redirect this flux deliberately toward single target products under continuous illumination.

Case studies demonstrate how far metabolic engineering can push these yields. In Synechocystis PCC 6803, inserting a codon-optimized phosphoketolase from Pseudomonas aeruginosa boosted acetate production fortyfold, and co-expressing phosphotransacetylase raised titers to 2.3 grams per liter, an eightyfold increase over the starting strain. Lactate engineering has been even more dramatic: combining overexpression of malic enzyme and d-lactate dehydrogenase with deletion of the acetate kinase gene produced a record 26.6 grams per liter of d-lactate from high-density cell cultures after seventy-two hours of fermentation. Because lactate is the monomer for polylactic acid, a biodegradable plastic used in packaging and 3D printing, such strains could eventually supply the bioplastics industry with carbon-negative feedstock.

Succinate, a top Department of Energy platform chemical used to make 1,4-butanediol, gamma-butyrolactone, and biodegradable polymers, has seen similar success. In Synechococcus elongatus PCC 7942, researchers used CRISPR interference to repress glycogen synthesis and succinate dehydrogenase while overexpressing key carboxylation and decarboxylation enzymes, achieving the highest cyanobacterial succinate titer reported to date: 4.8 grams per liter in twenty-eight days, rising to 8.9 grams per liter with reinoculation. A separate photomixotrophic strategy engineered the same strain to import glucose at low pH and export succinate at high pH, exploiting opposing proton symporters to reach 5.0 grams per liter in ten days. Even citrate, long considered too tightly regulated to accumulate, has been produced photoautotrophically in PCC 7002 using a theophylline-responsive riboswitch to throttle TCA flux, yielding more than a hundredfold increase over wild type.

Redox engineering has emerged as a central theme in these efforts. Cyanobacteria inherently produce abundant NADPH through photosynthesis, yet many fermentative enzymes prefer NADH, creating a cofactor mismatch that limits yields. Solutions include overexpressing transhydrogenases such as PntAB to interconvert the two pools, and site-directed mutagenesis to swap enzyme cofactor preferences, as demonstrated with an NADH-dependent lactate dehydrogenase from Lactobacillus bulgaricus that was redesigned to accept NADPH. Environmental tuning matters too: raising cultivation temperature from 30 to 37 degrees Celsius enhanced d-lactate and succinate production in PCC 6803, while mildly acidic conditions increase passive secretion of weak acids by boosting membrane permeability. Nitrogen starvation can push glycogen-deficient strains into a photo-catalytic state, continuously excreting alpha-ketoglutarate and pyruvate even without growth.

Formidable obstacles remain before these microbes can compete with petrochemical plants. Cyanobacteria grow slowly, allocate only a minor fraction of fixed carbon to target products, and suffer from genetic instability, low transformation efficiency, and byproduct formation as carbon leaks into competing pathways. Product recovery is another bottleneck: organic acids exist in ionized form at neutral pH, and traditional acidification with mineral acids generates mountains of inorganic salt waste. Photobioreactors struggle with uneven light penetration, limited gas transfer, and biofouling, and techno-economic and life-cycle assessments for engineered cyanobacterial systems remain scarce, making it difficult to judge commercial feasibility. Most studies to date remain at proof-of-concept or laboratory scale.

The road ahead is nonetheless rich with opportunity. Emerging tools include CRISPR-based gene repression and activation, riboswitches and light-inducible promoters that decouple growth from production, and adaptive laboratory evolution to breed acid-tolerant strains. Machine learning-guided flux modeling and multi-omics integration are converting strain design from trial-and-error into predictive optimization, while next-generation photobioreactors with optical waveguides and gas-permeable membranes promise better scalability. Co-cultures with heterotrophs, photomixotrophy using waste-derived carbon, and multi-product biorefineries coupling organic acids with biofuels and bioplastics could further improve economics. Greener recovery technologies, including membrane filtration, electrodialysis, and in situ product removal, are being developed to replace salt-generating acidification. If these threads converge, cyanobacteria could transform sunlight and carbon dioxide into the molecular backbone of a circular, fossil-free chemical economy, turning the planet’s oldest oxygen-makers into its newest industrial workhorses.

Subject of Research: Metabolic engineering of cyanobacteria for sustainable photosynthetic production of organic acids

Article Title: Advances in organic acid production using cyanobacteria: strategies and applications

Article References: Pathania, R., & Srivastava, A. (2025). Advances in organic acid production using cyanobacteria: strategies and applications. Blue Biotechnology, 2(1), Article 22. https://doi.org/10.1186/s44315-025-00045-7

Image Credits: AI Generated

DOI: 10.1186/s44315-025-00045-7

Keywords: cyanobacteria, organic acids, metabolic engineering, synthetic biology, carbon dioxide fixation, succinate, lactate, TCA cycle, bioplastics, photobioreactors, CRISPRi, sustainable biomanufacturing