Supplementation with -GR24 Facilitates the Accumulation of Biomass and Astaxanthin in Two Successive Stages of Cultivation.

The unicellular freshwater green alga has attracted much research attention due to its biosynthetic ability for large amounts of astaxanthin, a blood-red ketocarotenoid that is used in cosmetics, nutraceuticals, and pharmaceuticals. Recently, numerous studies have investigated the functions of natural astaxanthin; however, the high cost of the production of astaxanthin from cultures restricts its commercial viability. There is an urgent need to fulfill commercial demands by increasing astaxanthin accumulation from cultures. In this study, we discovered that treatment of cultures at the beginning of the macrozooid stage (day 0) with 1 μM -GR24, a synthetic analogue of strigolactones (a class of phytohormones), led to significant increases in biomass [up to a maximum dry cell weight (DCW) of 0.53 g/L] during the macrozooid stage and astaxanthin (from 0.63 to 5.32% of DCW) during the hematocyst stage. We elucidated that this enhancement of biomass accumulation during the macrozooid stage by -GR24 is due to its increasing CO utilization efficiency in photosynthesis and carbohydrate biosynthesis. We also found that -GR24 stimulated the overproduction of nicotinamide adenine dinucleotide phosphate (NADPH) and antioxidant enzymes in cultures, which alleviated the oxidative damage caused by reactive oxygen species generated during the hematocyst stage due to the exhaustion of nitrogen supplies. Moreover, -GR24 treatment of synergistically altered the activity of the pathways of fatty acid biosynthesis and astaxanthin esterification, which resulted in larger amounts of astaxanthin being generated by -GR24-treated cultures than by controls. In summary, we have developed a feasible and economic -GR24-assisted strategy that increases the amounts of biomass and astaxanthin generated by cultures, and have provided novel insights into the mechanistic roles of -GR24 to achieve these effects.