Production of pure all-trans retinal from agricultural byproducts by metabolically engineered Corynebacterium glutamicum using two-phase cultivation

Retinoids, including retinal, retinol (vitamin A), and retinoic acid, comprise vitamin A and its biologically active metabolites, which are widely used in pharmaceutical, healthcare, and cosmetic applications. Among these, retinal has attracted increasing attention in skincare applications because of its favorable combination of high biological efficacy with low skin irritation, and its potent antibacterial activity. However, microbial production of retinal generally yields mixtures with other retinoids and requires expensive nutrient media. Here, we report the production of pure retinal from agricultural byproducts using metabolically engineered Corynebacterium glutamicum. A lycopene producer, serving as a retinal precursor, was constructed by blocking the downstream pathway and enhancing carotenoid biosynthesis. Two genes converting lycopene to retinal via β-carotene were introduced to construct a retinal producer. Since retinal is fat-soluble and easily oxidized, a two-phase cultivation system using n-dodecane and butylated hydroxytoluene was employed for in-situ extraction and stabilization. The optimal temperature for retinal production was identified as 21 °C, yielding a 2.6-fold higher volumetric production rate than that at the optimal growth temperature, 30 °C. Using molasses as a carbon and vitamin source, shake-flask cultivation yielded 30.7 mg/L retinal, calculated based on the volume of the aqueous phase, without the formation of other retinoids. Scale-up cultivation in a 2.5-L jar fermenter produced 104.9 mg/L retinal (2,099 mg/L in n-dodecane phase) from molasses, again without forming other retinoids. This study demonstrates a cost-effective and sustainable bio-based strategy for producing pure retinal from renewable resources, paving the way for its industrial bio-based manufacture.