Induction of Carotenoid Synthesis in Microalgae with Reference to Their Production Outdoors

Plants, algae, and photosynthetic bacteria all contain carotenoids, which are lipid-soluble natural compounds. They can act as both light-harvesting complex and photoprotectors. Due to their nature, they are able to neutralize the effect of the presence of singlet oxygen and free radicals, acting as quenchers; for this function, an important and crucial role as an antioxidant has been attributed to a large number of carotenoids. Their production has been studied in several microalgal species, which represent a natural source of these antioxidants. In particular, Haematococcus, Chlamydomonas, Chlorella, Dunaliella, diatoms such as Phaeodactylum and Isochrysis, and dinoflagellates are able to synthesize large amounts of carotenoids. Among the most powerful antioxidant carotenoids, the xanthophylls loroxanthin, neoxanthin, lutein, violaxanthin, antheraxanthin, zeaxanthin, and α-carotene and β-carotene are the ones most synthesized under photo-oxidative stress conditions. Under physiological stresses, such as high light exposure, nutrient limitation-starvation, excessive low-high temperatures, the photosynthetic activity decreases, and different metabolic pathways are activated. The study of the physiological response to different stresses helps to understand the mechanisms which regulate the accumulation of antioxidant compounds. This information can be useful for optimizing the growth conditions of microalgal strains, the high carotenoid producers, for increasing their productivity, in terms of both antioxidants and biomass, and for the scale-up of the process from laboratory to outdoor cultures.