Remarkable metabolic reconfiguration due to three classical nitrogen sources in Gracilariopsis lemaneiformis (Rhodophyta)

The macroalga Gracilariopsis lemaneiformis undergoes dynamic nitrogen (N) fluctuations during cultivation, driven by climate change and eutrophication . However, our understanding of how G. lemaneiformis responds to different N environments remains limited. This study investigates the physiological and molecular metabolic responses of G. lemaneiformis to various N chemical forms (ammonium, nitrate, and urea). Our results indicate that G. lemaneiformis preferentially utilizes ammonium, consuming it more rapidly than nitrate and urea. Furthermore, the transcript profiles of N transporters exhibited distinct regulatory patterns between inorganic N and urea. Metabolic flux analysis revealed significant changes under different conditions of N. Notably, ammonium treatment upregulated the expression of photosynthesis-related proteins in PS II and led to an accumulation of amino acids with higher N content, such as arginine and citrulline . In contrast, nitrate treatment downregulated carbon (C) fixation while promoting the accumulation of amino acids with higher C/N ratios, particularly leucine and valine . Urea treatment resulted in elevated levels of polyamines and aromatic amino acids, which displayed the highest C/N ratios. As a result, the overall C/N ratio in G. lemaneiformis corresponded with that of the dominant amino acids across the three N treatments. Our findings revealed that ammonium, nitrate, and urea differentially affect C and N allocation in G. lemaneiformis . Furthermore, strategic use of specific forms of N can significantly enhance the production of industrially valuable metabolites, highlighting the potential of this macroalga as a sustainable resource for aquaculture and nutraceutical applications. • Ammonium is preferentially utilized in G. lemaneiformis . • Inorganic nitrogen and urea transporters display different regulation patterns. • Total carbon content and carbon fixation pathway were down-regulated by nitrate. • Specific amino acids were mediated by three nitrogen sources in G. lemaneiformis . • Total C/N ratio matches the specific amino acids' C/N ratio in each nitrogen.