Cyanobacteria, photosynthetic microorganisms with the same type of photosynthesis as plants and algae, can be engineered to produce solar chemicals and solar fuels in direct processes from carbon dioxide. I will exemplify this by discussing our strategies to engineer cyanobacteria to produce the alcohol butanol.

Butanol is a four-carbon alcohol (C₄H₉OH) occurring in several structural isoforms. It is an important bulk chemical, as solvent or intermediate in chemical synthesis, and an excellent blend-in fuel. Presently butanol is produced from fossil resources. Additionally, there are biological routes for fermentative butanol production, mainly to produce 1-butanol. Cyanobacteria do not produce butanol naturally, they lack the butanol biosynthetic pathways and corresponding relevant genes.

Introduction of a single gene encoding KivD resulted in isobutanol producing strains of the unicellular cyanobacterium Synechocystis PCC 6803 (1). Knowledge based modelling of the identified bottleneck KivD resulted in strains with significantly increased isobutanol production (2). Using the best isobutanol strain in long-term experiments a cumulative titer of 911 mg per L was observed with a maximal rate of 43.6 mg per L and day (3). Continued engineering increased the capacity of the cells to produce isobutanol significantly (4).

A similar approach to systematically engineer Synechocystis to produce 1-butanol resulted in cells with a cumulative titer of 4.8 g per L and a maximal rate of 302 mg per L and day (5), further doubled to 600 mg 1-butanol per L and day, and a carbon partitioning efficiency of 60% (6). Further advances in developing efficient cyanobacterial cells for production (7) as well as progress towards a complete system will be presented and discussed.

1) Miao et al. 2017. Isobutanol production using Synechocystis PCC 6803 endogenous alcohol dehydrogenases. Metabol Eng Comm 5: 45-53 2) Miao et al. 2018. Protein engineering of a-ketoisovalerate decarboxylase for improved isobutanol production in Synechocystis PCC 6803. Metabol Eng 47: 42-48 3) Miao et al. 2018. Enhancement of photosynthetic isobutanol production in Synechocystis PCC 6803. Biotechn Biofuels 11: 267 4) Xie, Lindblad. 2022. Effects of expressing 2-keto acid pathway enzymes on photosynthetic isobutanol production. Microbial Cell Fact 21: 17; Xie et al. 2023. Sustainable production of photosynthetic isobutanol and 3M1B production in the cyanobacterium Synechocystis PCC 6803. Biotechn Biofuels Bioprod 16: 134; 5) Liu et al. 2019. Modular engineering for efficient biosynthesis of 1-butanol from CO₂ in cyanobacteria. Energy & Environ Sci 12: 2765-2777 6) Wichmann et al. 2021. Engineering Biocatalytic Solar Fuel Production: The PHOTOFUEL Consortium. Trends Biotechn 39: 323-327; Liu et al. 2022. Current advances in engineering cyanobacteria and their applications for photosynthetic butanol production. Curr Opin Biotechn 73: 143-150 7) Xie et al. 2025. dCas12a-mediated CRISPR interference for multiplex gene repression in cyanobacteria for enhanced isobutanol and 3-methyl-1-butanol production. Microbial Cell Fact 24: 104; Bourgade et al. 2025. Development of a CRISPR activation system for targeted gene upregulation in Synechocystis sp. PCC 6803. Commun Biol 8: 772; Xie et al. 2026. Technologies for enhanced photosynthetic butanol production. In: Renewable butanol: Fermentation, Process Technology and Biofuel Applications (Eds: Friedl & Qureshi) 11: 207-22