Protein reserves elucidate the growth of microalgae under nitrogen deficiency.

Nitrogen (N) starvation is commonly used in the microalgae industry to induce biosynthesis of value-added products, such as astaxanthin and triacylglycerols. Microalgal adaptation to N starvation not only has an effect on biomass yield but also relates to a topic of N storage and utilization. Here, we show that only 30 % of saturated N availability and 47 % of maximum protein content is necessary to maintain a chemostat of microalga Graesiella emersonii growing at 0.96 d⁻¹. The additionally absorbed nitrogen are transformed mainly as proteins to maintain reserves in metabolic capacity, as demonstrated by the proteomic resources allocation analysis. Such intracellular nitrogen reserve can be considered as an advanced nitrogen storage and utilization strategy as it shows a highly efficient utilization of nitrogen resources in cells. The results allow us to identify kinetics of microalgal response to N availability that is basic to future metabolic engineering. • Biomass accumulation under nitrogen deficiency is observed in Graesiella emersonii. • Nitrogen are assimilated in excess under nitrogen sufficient conditions. • Excessively absorbed nitrogen are transformed as proteins to maintain metabolic reserves. • Nitrogen-reserving proteins reach striking 26 % of biomass dry weight. • Protein reserve is a more efficient nitrogen storage and utilization strategy. [ABSTRACT FROM AUTHOR]