Cyanobacterial conversion of CO2 to bio-based diols and downstream valorization: biological basis, upgrading strategies, and applications.

Cyanobacteria have attracted growing interest as photosynthetic cell factories for the conversion of carbon dioxide (CO2) into renewable chemicals because they directly use solar energy and inorganic carbon under mild aqueous conditions. Recent advances in chassis development, genome engineering, carbon-partition control, and photobioreactor design have expanded the scope of cyanobacterial CO2-to-chemical biomanufacturing. Among the reported diols, 1,2-propanediol (1,2-PDO), 1,3-propanediol (1,3-PDO), and 2,3-butanediol (2,3-BDO) represent distinct development cases. In particular, 1,3-PDO provides an established benchmark for fermentative diol production from organic feedstocks, whereas 2,3-BDO offers additional opportunities for stereochemistry-sensitive upgrading but requires further validation in several downstream applications. These stages are often studied separately, and their interdependence remains a major challenge for process integration. This review summarizes the biological and engineering basis of cyanobacterial diol production, product recovery and purification, major upgrading strategies, and application prospects in pharmaceuticals, energy-storage materials, hydrogels, fuels, polymers, and related functional products. Key challenges in biological productivity, selective upgrading, catalyst stability, feed quality, and process sustainability are also discussed. The review distinguishes demonstrated cyanobacterial production from downstream transformations reported for other feedstocks and identifies the requirements for connecting them.