Cyclic and pseudo-cyclic electron pathways play antagonistic roles during nitrogen deficiency in Chlamydomonas reinhardtii

Nitrogen (N) deficiency is a frequently encountered situation that constrains global biomass productivity. In response to N deficiency, cell division stops and photosynthetic electron transfer are downregulated, while carbon storage is enhanced. However, the molecular mechanism downregulating photosynthesis during N deficiency and its relationship with carbon storage are not fully understood. The Proton Gradient Regulator-like 1 (PGRL1)-involved in cyclic electron flow (CEF) and Flavodiiron proteins involved in pseudo-(CEF) are major players in the acclimation of photosynthesis. To determine the role of PGRL1 or FLV in photosynthesis under N deficiency, we measured photosynthetic electron transfer, oxygen gas exchange and carbon storage in the knockout of Chlamydomonaspgrl1 and flvBmutants. Under N deficiency,pgrl1maintains higher net photosynthesis and O2photoreduction rates, whileflvBshows similar responses compared to control strains. The amount of cytochromeb6fwas maintained at a higher level inpgrl1. The photosynthetic activity ofpgrl1 flvBdouble mutants decreases in response to N deficiency similar to the control strains. Furthermore, the triacylglycerol content ofpgrl1was twice higher than the controls under N deficiency. Taken together, our results suggest that in the absence of PGRL1, FLV-mediated O2photoreduction through PCEF maintains net photosynthesis at a high level, resulting in increased triacylglycerol biosynthesis. This study reveals that PGRL1 and FLV play antagonistic roles during N deficiency. It further illustrates how nutrient status can affect the regulation of photosynthetic energy production in relation to carbon storage and provides new strategies for improving lipid productivity in algae.Significance statementNitrogen (N) deficiency, an often-encountered phenomenon in nature, triggers growth arrest and massive lipid accumulation in microalgae. The downregulation of photosynthesis is necessary to ensure cell viability. We demonstrate that a well-conserved protein in chlorophytes, the Proton Gradient Regulator-like 1 (PGRL1) is a key (down) regulator of photosynthesis. In its absence, cells exhibited sustained photosynthesis and over-accumulated lipids thanks to the Flavodiiron (FLV) protein. We propose that both PGRL1 and FLV, by having antagonistic roles in N deficiency, manage the redox landscape, carbon storage and biomass production. Our work revolves around the current paradigm of photosynthesis regulation during N deficiency and provides a new framework for improving lipid accumulation in microalgae for biotechnological purposes.