Environmental irradiance and nitrogen source variability trigger the intracellular carbon and nitrogen reallocation of Gracilariopsis lemaneiformis (Rhodophyta)

A rhodophyte Gracilariopsis lemaneiformis is widely cultivated for food, carbon sequestration and bioproducts, and commonly experiences changing irradiance levels and nitrogen sources that might influence the productivity and biochemical composition of the alga. To explore optimal culture conditions or manipulations to gain best yield of carbohydrates and amino acids of G. lemaneiformis during mariculture, we cultured G. lemaneiformis in four conditions including high irradiance and low irradiance coupled with nitrate and ammonium (denoted as HI-NO3−, LI-NO3−, HI-NH4+ and LI-NH4+). The results showed that irradiance, nitrogen source and their interactions significantly regulated the intracellular carbon and nitrogen metabolism of G. lemaneiformis and the influence of irradiance was greater than of nitrogen source. High irradiance promoted more carbon accumulation of G. lemaneiformis, which was confirmed by higher growth rate, net photosynthesis rates and contents of soluble carbohydrate, carbon and C/N ratios. In contrast, low irradiance is conductive to nitrogen accumulation, as revealed by high contents of nitrogen, phycoerythrin, soluble protein and amino acids. Furthermore, we found nitrate coupled with high irradiance significantly promoted carbon sequestration of G. lemaneiformis, and the ammonium combined with low irradiance significantly promoted nitrogen assimilation and transformation. In addition, arginine exhibited a higher nitrogen storage capacity than phycoerythrin, as evidenced by the higher content of arginine and correspondingly higher nitrogen reserves in G. lemaneiformis under low light coupled with ammonium and nitrogen- enriched environments. Moreover, the content of proline was higher under low irradiance conditions, but its proportion among the eighteen free amino acids was upregulated by high irradiance, indicating its important role in responding to high-light aquaculture environments. These findings demonstrate that irradiance and nitrogen source influence the carbon and nitrogen reallocation of G. lemaneiformis and that the combination of these two variables can induce the production of chemical products for biotechnological, aquaculture, and nutraceutical industry.