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The intrinsic ability of microalgae to accumulate high amounts of carotenoids has made them the preferred aquatic organisms of biotechnological exploration for carotenoid production. To continuously innovate and modify microalgal bioprocesses, aquaculture scientists have been working hard for the past decades in a forwardlooking way, and mixotrophic cultivation of microalgae is deemed as a promising strategy to decrease production cost. This review is intended to summarise the recent research advancement of carotenoids production from mixotrophically cultivated microalgae, starting from the structure, biosynthesis, physiological roles and applications of carotenoids and followed by the production processes both currently established and under development. Most importantly, the microalgal physiology of mixotrophic cultivation is reviewed in depth both in general and specifically for the most studied species, and the prospects of commercially viable mixotrophic microalgal processes for carotenoid production along with the insight of future research are of course discussed. Finally, we conclude that mixotrophy might be a promising strategy for large-scale cultivation of microalgae to produce carotenoids although some technical obstacles need to be overcome.

Although Haematococcus lacustris has been developed for astaxanthin production for decades, the production cost is still high. In order to modify the production processes, we proposed a novel strategy of cultivation, featured by sequential indoor continuous mixotrophic cultivation for the production of green cells followed by outdoor phototrophic induction for astaxanthin accumulation. The continuous mixotrophic cultivation was first optimized indoor, and then the seed culture of mixotrophic cultivation was inoculated into outdoor open raceway ponds for photoinduction. The results showed that mixotrophically grown cultures could efficiently grow without losing their photosynthetic efficiency and yielded higher biomass concentration (0.655 g L−1) and astaxanthin content (2.2% DW), compared to phototrophically grown seed culture controls. This novel strategy might be a promising alternative to the current approaches to advance the production technology of astaxanthin from microalgae.

Response surface methodology (RSM) based on the 2 factorial central composite design (CCD) was employed to evaluate optimum culture conditions (temperature, light irradiance and agitation) to enhance biomass and phycocyanin content of . The predicted maximum biomass and phycocyanin content by RSM was 1.06 g L and 107 mg L, respectively, whereas maximum biomass and phycocyanin content of 1.32 g L and 127 mg L was obtained in the validation experiments under optimized conditions after 10 days of cultivation. Further, influence of optimized conditions (temperature 33±2 ºC, light irradiance 44 μmol photons m s and a flow rate of 2.5 L min) on growth and phycocyanin content of in 7L Panel photobioreactor (PPBR) cultivation was investigated. A 15 days production was carried out and it was observed that a maximum biomass yield of 2.42 g L with a specific growth rate 0.202 day and phycocyanin content of 228 mg L was obtained in the PPBR. The optimum culture conditions obtained through response surface methodology were successfully determined to maximize the biomass and phycocyanin.