Improvement of Squalene Production from CO₂ in Synechococcus elongatus PCC 7942 by Metabolic Engineering and Scalable Production in a Photobioreactor

The push-and-pull strategy for metabolic engineering was successfully demonstrated in Synechococcus elongatus PCC 7942, a model photosynthetic bacterium, to produce squalene from CO₂. Squalene synthase (SQS) was fused to either a key enzyme (farnesyl diphosphate synthase) of the methylerythritol phosphate pathway or the β-subunit of phycocyanin (CpcB1). Engineered cyanobacteria with expression of a fusion CpcB1-SQS protein showed a squalene production level (7.16 ± 0.05 mg/L/OD₇₃₀) that was increased by 1.8-fold compared to that of the control strain expressing SQS alone. To increase squalene production further, the gene dosage for CpcB1·SQS protein expression was increased and the fusion protein was expressed under a strong promoter, yielding 11.98 ± 0.49 mg/L/OD₇₃₀ of squalene, representing a 3.1-fold increase compared to the control. Subsequently, the best squalene producer was cultivated in a scalable photobioreactor (6 L) with light optimization, which produced 7.08 ± 0.5 mg/L/OD₇₃₀ squalene (equivalent to 79.2 mg per g dry cell weight). Further optimization for photobioprocessing and strain development will promote the construction of a solar-to-chemical platform.