Scalable Cultivation of Engineered Cyanobacteria for Squalene Production from Industrial Flue Gas in a Closed Photobioreactor.

Economically feasible photosynthetic cultivation of microalgal and cyanobacterial strains is crucial for the biological conversion of CO2 and potential CO2 mitigation to challenge global warming. To overcome the economic barriers, the production of value-added chemicals was desired by compensating for the overall processing cost. Here, we engineered cyanobacteria for photosynthetic squalene production and cultivated them in a scalable photobioreactor using industrial flue gas. First, an inducer-free gene expression system was developed for the cyanobacteria to lower production const. Then, the recombinant cyanobacteria were cultivated in a closed photobioreactor (100 L) using flue gas (5% CO2) as the sole carbon source under natural sunlight as the only energy source. Seasonal light intensities and temperatures were analyzed along with cyanobacterial cell growth and squalene production in August and October 2019. As a result, the effective irradiation hours were the most critical factor for the large-scale cultivation of cyanobacteria. Thus, an automated photobioprocess system will be developed based on the regional light sources.

Metabolic rewiring of synthetic pyruvate dehydrogenase bypasses for acetone production in cyanobacteria.

Designing synthetic pathways for efficient CO2 fixation and conversion is essential for sustainable chemical production. Here we have designed a synthetic acetate-acetyl-CoA/malonyl-CoA (AAM) bypass to overcome an enzymatic activity of pyruvate dehydrogenase complex. This synthetic pathway utilizes acetate assimilation and carbon rearrangements using a methyl malonyl-CoA carboxyltransferase. We demonstrated direct conversion of CO2 into acetyl-CoA-derived acetone as an example in photosynthetic Synechococcus elongatus PCC 7942 by increasing the acetyl-CoA pools. The engineered cyanobacterial strain with the AAM-bypass produced 0.41 g/L of acetone at 0.71 m/day of molar productivity. This work clearly shows that the synthetic pyruvate dehydrogenase bypass (AAM-bypass) is a key factor for the high-level production of an acetyl-CoA-derived chemical in photosynthetic organisms.