Cells of the green microalga Haematococcus pluvialis were induced to accumulate the ketocarotenoid pigment, astaxanthin. This induction was achieved by the application of the following environmental conditions: light intensity (170 μmol m~⁻²s⁻¹), phosphate starvation and salt stress (NaCl 0.8%). These conditions retarded cell growth as reflected by a decrease in cell division rate, but led to an increase in astaxanthin content per cell. Accumulation of astaxanthin required nitrogen and was associated with a change in the cell stage from biflagellate vegetative green cells to non-motile and large resting cells. It is suggested that environmental or nutritional stresses, which interfere with cell division, trigger the accumulation of astaxanthin. Indeed, when a specific inhibitor of cell division was applied, a massive accumulation of astaxanthin occurred.

Microalgae are considered a promising source for various high-value products, including carotenoids and omega-3 and omega-6 polyunsaturated fatty acids (PUFAs). Excluding production by heterotrophic fermentation, only two microalgal high-value products are successfully marketed at a relevant scale: β-carotene from Dunaliella salina , and astaxanthin from Haematococcus pluvialis . In addition, Chlorella and Spirulina biomass are marketed in large volumes as nutraceuticals, and phycocyanin extracted from cyanobacteria has gained major market share recently. Additional algal strains of industrial potential have been described for the production of high-value products, such as carotenoids and PUFAs, or for biofuels production, and novel promising strains continue to be reported. However, phototrophic production of algal products is considered 2–5 times more expensive than competing pathways for both high-value products and bulk biomass. Recent—and often still unpublished—advances have been made in deciphering the genomes and transcriptomes of multiple high-value algal species and their metabolic pathways toward carotenoid,lipid, and PUFA biosynthesis have been resolved. Together with recent progress in microalgae transformation and genetic engineering, it is now possible to increase production efficiencies for high-value products, bulk biomass, and biofuels in microalgae by metabolic engineering. Furthermore, encouraging progress has been achieved in expressing high-value proteins in several microalgae species. This review describes major, recent advances in the understanding and engineering of microalgal metabolic pathways towards developing competitive production pathways. Such technologies, supported by adequate biorefinery technologies and highly sustainable cultivation options, can significantly contribute to enabling sustainable production of high-value biobased chemicals, while also offering opportunities for increasing sustainable food and fuel supplies. Microalgae thus offer the unique opportunity to shift significant agricultural production volumes into unproductive land using non-potable water while reducing global resource depletion and pollution from unsustainable farming and fishing practices.