Arginine culture turns on the elusive nitrogen starvation signal during robust phototrophic growth in Chlamydomonas

Under nitrogen (N) starvation, photosynthetic organisms search for other N sources while slowing down photosynthesis by downregulating light harvesting and electron transport to balance the carbon/nitrogen ratio and eventually stopping growth due to N limitation. To investigate the elusive N starvation-specific signaling mechanisms, we sought a way to induce N starvation responses without limiting photosynthesis or cell growth. In the chlorophyte Chlamydomonas reinhardtii, gametogenesis is exclusively induced during N starvation except in arginine culture. We showed that the arginine-grown culture turned on N starvation responses including hundreds-fold induction of N starvation-induced genes, reduced chlorophyll content, and increased carbon storage in the form of lipid droplets. Arginine culture supported robust phototrophic growth but not heterotrophic growth, indicating that arginine catabolism contributes CO2 to Rubisco without directly fueling ATP synthesis. Based on in silico analysis, we propose the possible routes of arginine catabolism that may bypass critical steps for monitoring of cellular N status and thereby trigger N starvation responses. Our results describe a study system where the N starvation responses are constantly induced without compromising photosynthesis or growth, paving ways to discover the mechanisms that sense and respond to cellular N status in eukaryotic phototrophs.