EFFECTS OF NITRATE AVAILABILITY AND IRRADIANCE ON INTERNAL NITROGEN CONSTITUENTS IN CORALLINA-ELONGATA (RHODOPHYTA)

Short-term (5-h) phycobiliprotein photoacclimation was a NO3- dependent process in the red alga Corallina elongata Ellis et Soland. At low irradiance levels, phycobiliprotein synthesis (both r-phycoerythrin and r-phycocyanin) took place when N supply was sufficient but was restricted by N limitation. Exposure to saturating irradiance resulted in pigment degradation under N limitation; however, under N-sufficient conditions a partial r-phycoerythrin synthesis was observed, despite the repressing role of high photon flux densities on phycobiliprotein synthesis. Soluble protein was less affected than phycobiliprotein by N limitation at low photon flux densities indicating that N limitation stimulates the flow of internal N metabolites toward the synthesis of nonpigmented proteins rather than pigmented proteins. The addition of protein synthesis inhibitors revealed that new phycobiliprotein synthesis occurs in response to sufficient N conditions. When protein synthesis was blocked in the chloroplast and cytoplasm simultaneously (addition of chloramphenicol and cycloheximide), both pigmented and nonpigmented protein synthesis was inhibited. However, when protein systhesis was blocked in the chloroplast, only phycobiliprotein synthesis was clearly inhibited, whereas nonpigmented protein was less affected, indicating that phycobiliprotein is the main fraction of protein synthesized in the chloroplast at low photon flux densities when external N is available. This inhibition of phycobiliprotein synthesis was consistent with a maximal increase in metabolites of protein synthesis (internal NH4+ and amino acids). Our results suggest that phycobiliproteins may be an important N reservoir to meet internal N demands during N limitation in C. elongata. Moreover, r-phycoerythrin, synthesized even at saturating irradiance levels, and the major constituent of the phycobiliprotein pigments, may be more sensitive to changes in N supply than r-phycocyanin. The influence of limited irradiance levels on N assimilation and the effects of repressing protein synthesis on internal N accumulation are also discussed.