Overcoming the biomass-lipid tradeoff through gradual nitrogen limitation in semicontinuous cultivation of Chlorella thermophila JPG-1

Microalgae are promising feedstocks for sustainable biofuel and bioproduct production; however, lipid accumulation often reduces biomass productivity, limiting commercial viability. This study evaluated the potential of semicontinuous cultivation and gradual nitrogen limitation to enhance lipid and carotenoid production in Chlorella thermophila JPG-1 while maintaining biomass growth. Compared with batch cultivation, the semicontinuous regime increased biomass productivity by 7% and improved lipid accumulation under nitrogen-limited conditions. Among the four nitrogen-limitation strategies evaluated, gradual stepwise nitrogen depletion was the most effective, maintaining biomass productivity while increasing lipid content by 74% and carotenoid content by 2.6-fold relative to the control. The effects of salinity and light intensity were subsequently assessed under this optimized nitrogen-limitation regime. Mild salinity (0.1 mM NaCl) further enhanced biomass productivity, lipid content, and carotenoid accumulation by 35%, 7%, and 8%, respectively, whereas higher salinity levels adversely affected culture performance. Similarly, an optimized light intensity of 200 μmol m−2 s−1 increased biomass productivity, lipid content, and carotenoid accumulation by 10%, 17%, and 22%, respectively. These findings demonstrate that gradual nitrogen limitation in a semicontinuous cultivation system can effectively stimulate lipid and carotenoid biosynthesis while sustaining biomass production. The study also identifies mild salinity and optimized irradiance as promising cultivation parameters that may further improve metabolite yields. Overall, the proposed strategy provides a practical framework for enhancing the productivity of C. thermophila for biofuel and high-value bioproduct applications.