Carotenoid Synthesis and Accumulation in Microalgae Under Environmental Stress

Abstract

In oxygenic phototrophs including cyanobacteria, algae, and land plants, carotenoids serve the light-harvesting function together with chlorophylls and photoprotective functions. High-intensity irradiation can cause excess excitation pressure on photo system II (PSII) of chloroplasts and can generate reactive oxygen species resulting in the photodamage to PSII. In general, to prevent the photodamage to photosynthetic apparatus, carotenoids dissipate excitation energy of singlet-state chlorophylls as heat (xanthophyll-dependent non-photochemical quenching) and also quench triplet-state chlorophylls in antenna complexes of PSII. Carotenoids also quench singlet oxygen in the reaction center of PSII. Light is one of the most striking environmental cues that alter the expression of carotenoid biosynthetic genes and induce the carotenogenesis in microalgae. Light-induced accumulation of carotenoids is reported in certain algae such as Haematococcus pluvialis, Dunaliella salina, Chlorella zofingiensis, and Euglena gracilis. Besides light stress, various abiotic stresses, such as heat, low temperature, drought, salinity, and oxidative stress, can enhance the extent of photoinhibition in photosynthetic organisms and can also induce the accumulation of carotenoids in microalgae. Here, we summarize studies on the regulation of biosynthesis and accumulation of carotenoids in eukaryotic microalgae in response to environmental stimuli especially focusing on the light and temperature. In addition, here we show our recent findings on the carotenoid biosynthesis in Euglena gracilis in response to increasing light intensity and light-induced stress enhanced by decreasing temperature.