Changes in nitrogen pools in Ulva fenestrata (Chlorophyta) and Gracilaria pacifica (Rhodophyta) under nitrate and ammonium enrichment

The accumulation of nitrogen in different cellular pools by the macroalgae Ulva fenestrata (Postels and Ruprecht) (Chlorophyta) and Gracilaria pacific a (Abbott) (Rhodophyta) was studied in a laboratory experiment. After 8 or 9 days of nitrogen enrichment, nitrate, ammonium, free amino acid (FAA), pr otein, chlorophyll (chl), phycoerythrin (PE), and insoluble nitrogen pools were extracted and analyzed, and their relative contribution to total nitro gen (TN) was assessed. In U. fenestrata, the nitrate and ammonium enrichmen ts resulted in a significant increase of TN from 2.41% dry weight (dw) to 4 .19% and 4.71% dw, respectively. All the extracted N pools increased signif icantly. In G. pacifica, TV increased more under ammonium enrichment than u nder nitrate enrichment, In both macroalgae, proteins and FAA were the most important N storage pools. Protein-N ranged from 700 to 2300 mu mol N.g dw (-1) (43%-66% of TN) and contributed the most to TN increase (41%-89%), The FAA pool was always larger in G. pacifica than in U. fenestrata. In both s pecies, the FAA pool accounted for 4%-17% of TN (70-600 mu mol N.g dw(-1)). In U. fenestrata, nitrate can represent a temporary storage pool: it accum ulated up to 200 mu mol N.g dw(-1) (7% of TN) and contributed more than FAA to overall increase in cellular nitrogen. In contrast, G. pacifica had a s mall nitrate pool. The PE pool in G. pacifica increased with TN but was nev er more than 9% of total protein-N or 6% of TN, and it was less important t han FAA as a storage pool. All TN was recovered in the extracted and insolu ble N pools at the end of the experiment in U. fenestrata. In G. pacifica, the extracted and insoluble N pools accounted on average for 83%-90% of TN.