Metabolic responses to nitrogen deficiency and rapamycin treatment in freshwater microalgae strains

Nutrient deprivation causes significant stress to microalgae, which responds by altering their metabolic pathways. Following N deprivation, the accumulation of starch and triacylglycerols (TAGs) are significantly altered following reprogramming of cellular metabolism. In addition, the adaptation of cell growth and proliferation to environmental changes is essential for the surviving of biological systems. The evolutionary conserved Ser/Thr protein kinase (Target of Rapamycin - TOR) is crucial for the signaling pathway that integrates the sensing of growth signals to the regulation of metabolism and cellular growth. The TOR pathway controls cell growth by promoting anabolic processes, including protein synthesis and ribosome biogenesis, and inhibiting catabolic processes such as autophagy. Therefore, a greater understanding of the regulation of growth and metabolic pathways under N deprivation and TOR inhibition is necessary to understand the physiology and the mechanisms involved in the biosynthesis of storage compounds in Brazilian native microalgae. In this study, native microalgae, Scenedesmus obliquus BR003, Chlorella vulgaris BR017, Chlamydomonas sp. BR020 and Monoraphidium sp.BR023, and one consider model, Chlamydomonas reinhardtii CC503 were selected to evaluate the growth parameters, biochemical composition, and metabolite profile under different nitrogen concentrations and TOR pathway inhibition by rapamycin. Whereas N depletion leads to typical stress-related responses in all strains, including reduction of cell growth, chlorophyll and protein, differential accumulation of primary compounds, membrane lipid composition and fatty acid were also observed. The metabolic and physiological analysis showed not only differential sensitivity to the absence of N in relation to N-replete and N- saturated treatments but also differences between strains. The response of growth, dynamics, metabolism profile, and lipid profile to TOR-inhibition by rapamycin in the strains showed a decrease in both cell growth and strong changes in carbon and nitrogen partitioning in the direction of rapid conversion into carbon and nitrogen storage through an accumulation of starch, triacylglycerol and amino acids. Interestingly, the strains showed different metabolite levels, confirming the difference in reserve compound accumulation of each strain. Based on the results, this study indicates that, due to their metabolic profile, microalgae have different responses in cell growth and metabolism, thus, a better understanding of the metabolic pathways in different strains is needed for biotechnological use. Keywords: Chlorophytes. Chlamydomonas reinhardtii. Primary- and lipid- metabolism. Rapamycin. N-deprivation.