Aggressive nitrogen assimilation during exponential growth in Chlamydomonas reinhardtii

Bioavailable nitrogen (N) is a key limiting factor for global biomass production. In dynamic environments, organisms must employ effective nitrogen use strategies (NUS) to capitalize on episodic N availability. The recent discovery of intracellular guanine crystals in diverse algal species has prompted questions about their roles in NUS. To investigate NUS in Chlamydomonas reinhardtii, a model photosynthetic eukaryote, we compared N-to-biomass conversion rate of mixotrophic batch cultures fed with three common N sources, ammonium, nitrate, and urea. Saturating growth was achieved at 4 mM ammonium, 4 mM nitrate, and 2 mM urea, indicating comparable molar N utilization efficiency. Residual N measurements revealed that approximately 1.2 unit of optical density (O.D. at 680 nm) biomass was produced per mM N under sub-saturating conditions, while biomass accumulation per N decreased at above-saturating conditions. To estimate N storage capacity, we tracked N uptake kinetics in high-density culture following an N pulse, showing rapid assimilation of 3~6 mM N per O.D. biomass. N source-specific transcriptome revealed N source-specific regulation of assimilation pathways and transporter genes in support of effective NUS in C. reinhardtii. These findings support a model in which C. reinhardtii can rapidly acquire and store N during exponential growth, enabling sustained growth and metabolism during periods of N scarcity. Observed variation in N storage capacity across growth stages and N conditions predicts the regulatory mechanisms governing N partitioning and storage. This study highlights flexible NUS in microalgae, offering insights for improving N assimilation capacity and resilience in agricultural and aquacultural crops.