Optimization of stage conversion time and modification of cell metabolism to enhance lipid production of Auxenochlorella pyrenoidosa in two-stage cultivation

[Display omitted] •The stage conversion time was optimized in the two-stage culture of A. pyrenoidosa.•The commonly used stage conversion time in two-stage cultivation is challenged.•Growth potential should also be considered when choosing the stage conversion time.•Lipid productivity was promoted in the second stage with adequate phosphorus.•The supply of phosphorus redirected carbon flux toward growth and lipid accumulation. Traditionally, the time of maximum biomass concentration in stage I is the widely adopted stage conversion time in two-stage microalgae culture. This study challenges this conventional approach, demonstrating that the optimal stage conversion time in stage I is 72 h rather than 120 h for achieving maximum biomass concentration. A comparison of cell characteristics revealed that algal cells at 72 h exhibited better growth potential, leading to a higher biomass concentration after transfer to stage II and, consequently, increased lipid productivity. Moreover, the use of phosphorus repletion (5-fold) in stage II directed carbon flux toward biomass growth and lipid accumulation, thereby enhancing lipid productivity. By optimizing the stage conversion time and implementing phosphorus repletion, the mean lipid productivity of Auxenochlorella pyrenoidosa cultured under autotrophy-nitrogen starvation and autotrophy-high light conditions increased by 31 % and 60 %, respectively. This study underscores the importance of reevaluating the currently widely used stage conversion time.