Physiology and adaptive significance of secondary carotenogenesis in green microalgae

Some species of unicellular green algae accumulate under unfavorable conditions high amounts of secondary carotenoids (SCar), which are not involved in photosynthesis and localized outside thylakoids. Such algae are cultivated on an industrial scale for production of carotenoids with valuable pharmacological properties. This review summarizes recent experimental data on the importance of secondary carotenogenesis for stress tolerance of unicellular green algae (Chlorophyta). The peculiarities of SCar biosynthesis, its induction and regulation under the action of various stressors as well as their localization in the cell and physiological effects of the accumulation of high amounts of these pigments are considered. Particular attention is given to the relationships between SCar biosynthesis and stress-induced biosynthesis of neutral lipids as well as the role of ROS as universal inducers and regulators of SCar biosynthesis. Various aspects of SCar protective function in the micfroalgal cells are discussed.