SHORT-TERM VARIATION OF INTERNAL NITROGEN-COMPOUNDS AS A FUNCTION OF IRRADIANCE IN CORALLINA-ELONGATA

Short-term response, in the range of minutes, to a wide range of irrad iance has been studied in the red alga Corallina elongata Ellis et Sol and. Spring, N-sufficient, plants were subjected to a broad set of irr adiances (from darkness to 2000 mu mol m(-2) s(-1)) for 20 minutes in artificial seawater without N. Photosynthesis-light curves and simulta neous changes in the content of photosynthetic pigments (chlorophyll a and phycobiliproteins), soluble proteins, free amino acids, internal NH4+ and total C and N were analyzed. Irradiance affected the synthesi s of photosynthetic pigments. Phycobiliproteins showed a double bell s haped PFD-response curve, with maxima at lower and higher irradiances than pre-experimental irradiance (250 mu mol m(-2) s(-1)), whereas Chi a exhibited a complementary response to phycobiliproteins, a bell sha ped PFD-response curve with a maximum located at a intermediate irradi ance (125 mu mol m(-2) s(-1)). Soluble protein resembled the response of phycobiliproteins. When light limited photosynthesis, protein synth esis was restricted. As a consequence, there was an accumulation of am ino acids at photosynthetically light-limited irradiances (below the l ight compensation point), whereas internal NH4+ did not change signifi cantly over all the range of irradiances tested. Total C, total N and C: N ratio were affected neither by irradiance nor N limitation at thi s time scale, as both C and N will have a turnover lower than other N metabolic compounds. The increase of the content of phycobiliproteins and soluble protein at high irradiances is discussed in the contect of the oscillatory behaviour of pigments in the short-term acclimation t o irradiance in this species.