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

Short-term (5-h) phycobiliprotein photoacclimation was a NO3- dependen t process in the red alga Corallina elongata Ellis et Soland. At low i rradiance levels, phycobiliprotein synthesis (both r-phycoerythrin and r-phycocyanin) took place when N supply was sufficient but was restri cted by N limitation. Exposure to saturating irradiance resulted in pi gment degradation under N limitation; however, under N-sufficient cond itions a partial r-phycoerythrin synthesis was observed, despite the r epressing role of high photon flux densities on phycobiliprotein synth esis. Soluble protein was less affected than phycobiliprotein by N lim itation at low photon flux densities indicating that N limitation stim ulates the flow of internal N metabolites toward the synthesis of nonp igmented proteins rather than pigmented proteins. The addition of prot ein 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 clearl y inhibited, whereas nonpigmented protein was less affected, indicatin g that phycobiliprotein is the main fraction of protein synthesized in the chloroplast at low photon flux densities when external N is avail able. This inhibition of phycobiliprotein synthesis was consistent wit h 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 limitatio n in C. elongata. Moreover, r-phycoerythrin, synthesized even at satur ating irradiance levels, and the major constituent of the phycobilipro tein pigments, may be more sensitive to changes in N supply than r-phy cocyanin. The influence of limited irradiance levels on N assimilation and the effects of repressing protein synthesis on internal N accumul ation are also discussed.