A novel two-stage heterotrophic cultivation for starch-to-protein switch to efficiently enhance protein content of Chlorella sp. MBFJNU-17

This work aimed to firstly establish an efficient and novel two-stage cultivation process to produce microalgal biomass rich in protein using a heterotrophic Chlorella sp. MBFJNU-17 strain. In the first-stage cultivation, to reduce the glucose and urea utilization, microalga achieved a high biomass at 40 g/L glucose and 1 g/L urea; meantime, the expression from starch biosynthesis genes of microalga was up-regulated under nitrogenstarvation conditions for starch accumulation (55.01%). In the second-stage cultivation, based on the overcompensation effect, Chlorella cells after the first-stage cultivation were further treated at 5 g/L glucose and 3 g/L urea to up-regulate starch degradation, central carbon metabolism and urea absorption genes expression to drive intracellular starch-to-protein switch for biosynthetic protein (59.75%). Moreover, microalga performed similar characteristics in a 10-L fermenter by the established process. Taken together, Chlorella sp. MBFJNU-17 was the promising candidate to produce high-value biomass enriched in protein by the established two-stage cultivation.