Nutrient availability-driven shifts in nitrogen and phosphorus trade-offs in Microcystis aeruginosa: Understanding...

Nitrogen (N) and phosphorus (P) concentrations and their ratios jointly regulate the growth and metabolism of microalgae, while influencing aquatic trophic state through uptake and feedback mechanisms. To investigate the dynamic changes in the relative requirements for N and P during microalgal growth, this study established the nutrient equilibrium index (NEI) as a novel metric. The range of N and P absolute concentrations was applied to simulate different NEIs conditions. Local weighted regression (LOESS) was employed to analyze the optimal supply nitrogen -to-phosphorus (N/P) ratios and their trends throughout the growth and biomacromolecule synthesis of Microcystis aeruginosa (M. aeruginosa) under different NEIs. The results showed the optimal N/P ratios for growth decreased gradually from 333.14 at NEI = 1.5 to 13.93 at NEI = 6.9, reflecting the metabolic prioritization adjustments. NEI = 2.4 as a threshold was identified, marking the onset of a shift in regulatory dominance from N to P, with this transition exhibiting biomacromolecule-specific characteristics. Furthermore, NEI = 4.2 was established as a stable threshold for sustained P-dominated regulation. During the decline phase, lipids increased to 48.17 % while carbohydrates stayed at 24.28 %, suggesting that lipid mobilization provided the energy required for continued metabolic activity instead of entering a quiescent state. The results may provide crucial theoretical insights and practical guidance for controlling nitrogen and phosphorus in eutrophic waters or regulating efficient microalgal biomacromolecule synthesis.