Metabolic fermentation pathways of microalgae for the synthesis of high-value chemical products

This review critically discusses the biochemical diversity of microalgae, particularly their carbohydrate and protein contents that can be converted via fermentation process into added valuable organic metabolites. It lays on processes such as alcoholic fermentation, dark fermentation and acetone-butanol-ethanol (ABE) fermentation, this review details the enzymatic mechanisms underlying the production of various compounds, including alcohols, organic acids, aldehydes, ketones, esters, and terpenoids/isoprenoids. Special attention is given to the recent progress on metabolic engineering pathways to optimize microbial strains (i.e., Saccharomyces cerevisiae, Clostridium spp.) for higher chemical yield. Fermentation of several microalgae biomass (Chlorella, Arthrospira, Nannochloropsis, Scenedesmus, Chlorococcum) with S. cerevisiae and C. acetobutylicum for ethanol and biobutanol production, respectively, is already well proved at laboratory scale. Genetically modified microorganisms in terms of yield and productivity of high-value compounds is well established. However, the research prospecting includes fermenting carotenoid-rich microalgae such as Dunaliella and Haematococcus, specifically for terpenoid/isoprenoid production remain a gap in the science. Thus, it encompasses biotransformation (fermentation or respiration) of carbohydrates into ethanol, proteins into higher alcohols and volatile fatty acids, lipids into biodiesel, and carotenoid into isoprenoids and terpenoids. These approaches should be correlated to the biomass optimization, including minimization residues - circular economy. It is worth mentioning the synthetic biology and real-time biosensors perspectives, in terms of more robust, flexible, and industrially processes.