Environmental nitrogen and phosphorus nutrient variability triggers intracellular resource reallocation in Gracilariopsis lemaneiformis (Rhodophyta)

Abstract

The variability in anthropogenic input of nutrients to coastal water has profound impacts on the ecology of Gracilariopsis lemaneiformis, a rhodophyte that is protein-rich and economically valuable for food and industry. However, little is known about how this alga responds to the dynamics of nitrogen and phosphorus (NP) nutrients. In this study, G. lemaneiformis was grown in batch cultures under three nutrient regimes (low-, medium- and high-NP abundances, with a constant N:P ratio of 10) to investigate the physiological, biochemical and transcriptional responses to the variation in N and P nutrient abundances. Under high-NP treatment conditions, G. lemaneiformis exhibited a high growth rate and contents of phycoerythrin, Chl a and soluble protein, but lower contents of soluble sugar and activities of nitrate reductase and glutamine synthetase. Besides, we found a negative correlation between the soluble sugar and soluble protein (R2 = 0.86), suggesting reallocation of intracellular resources modulated by extracellular NP nutrients. Furthermore, the carbon content decreased under high-NP conditions, indicating G. lemaneiformis biomass accumulation at the expense of per unit weight carbon content. In addition, we found that arginine was the main storage form of N in G. lemaneiformis, and its content increased 1017 times in a two-week period under N- and P- enriched conditions. Moreover, the phycoerythrin was the main biologically active N-compound used by G. lemaneiformis for growth, compared to soluble protein and Chl a, under N- and P-deficient conditions. These findings demonstrate that even under constant stoichiometry, the environmental variability of nutrient abundance shapes biochemical landscapes in the algal cell and provides baseline information to help design G. lemaneiformis cultivation to obtain higher protein or higher soluble sugar with respect to NP nutrition.