Synthesis of antioxidant carotenoids in microalgae in response to physiological stress

Carotenoids act as potential antioxidants, quenching energy of excited singlet oxygen and scavenging free radicals. Among microalgae, Haematococcus, Chlamydomonas, Chlorella, Dunaliella and diatoms and dinoflagellates, such as Phaeodactylum and Isochrysis, are able to synthesize large amount of carotenoids. The main function of carotenoids consists in absorbing light to perform photosynthesis, and some of them are constitutively present in the cells (primary carotenoids). The main primary carotenoids usually found are neoxanthin, violaxanthin, lutein, and β-carotene. To preserve cells from oxidative damage, their production may be increased, while other carotenoids may be synthesized de novo. In particular, under stress conditions such as high light exposure, nutrient starvation, change in oxygen partial pressure, and high or low temperatures, microalgal metabolism is altered and photosynthetic activity may be reduced. In these conditions, photosynthetic electrons transport is reduced, and the intracellular reduction level increase may be associated with the formation of free radicals and species containing singlet oxygen. In order to prevent damage from photooxidation, microalgae are able to adopt strategies to contrast these dangerous oxidant molecules. One of the most active mechanisms is to synthesize large amount of carotenoids, which can act as antioxidants.