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

Abstract

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.