Phosphorus Limitation Differentially Affects Growth, Phosphate Consumption, and Carbon Allocation in Chlamydomonas reinhardtii Batch Cultures Under Photoautotrophic and Mixotrophic Conditions

Phosphorus availability strongly influences the growth, metabolism, and storage accumulation of microalgae, yet its interaction with trophic conditions remains insufficiently understood. This study examined the effects of reduced initial phosphate concentration in the medium on the growth, phosphate consumption, and accumulation of lipids, starch, and proteins in photoautotrophic (continuous lightning, no acetate) and mixotrophic (continuous lightning + acetate) cultures of Chlamydomonas reinhardtii. Reduced phosphate concentration affected mixotrophic cultures more severely than photoautotrophic ones. The absence of phosphate resulted in a 1.7-fold decrease in the growth rate of cell density in mixotrophic cultures, while in autotrophic cultures, the decrease was only 1.15-fold. Mixotrophic cells exhibited 1.7-fold faster phosphate uptake per cell, while under photoautotrophic conditions cells absorbed phosphate more slowly, but over a longer period, resulting in 8-fold greater absorbed phosphorus quota per cell. Preceding phosphorus limitation stimulated rapid phosphate consumption at the early growth phase. Cells demonstrated complicated storage management during culture development. Acetate stimulated early lipid accumulation, while starch reserves supported rapid initial growth and improved recovery after subculturing. Phosphorus starvation promoted the accumulation of storage compounds in cells, especially at stationary phase. Lipid accumulation per culture volume remained relatively constant despite marked increases in cellular lipid content (5.8- and 1.7-fold for mixotrophic and autotrophic conditions), whereas starch accumulated substantially both per cell (12- and 3.8-fold for mixotrophic and autotrophic conditions) and volumetrically. Overall, it could be concluded that trophic conditions strongly affect phosphorus requirements, phosphate consumption, and carbon allocation, thereby shaping the adaptive response of C. reinhardtii to phosphorus limitation.