Photosynthetic Assimilation of 15N-Ammonia and 15N-Nitrate in the Marine Diatoms Bellerochea yucatanensis (von Stosch) and Skeletonema costatum

Abstract

The marine diatoms Bellerochea yucatanensis and Skeletonema costatum were grown at -I-20°C in 0.03 vol.% C 0 2 with nitrate or ammonia. The 15N -am m onia and 15N -nitrate assimilation and 15N-incorporation into various amino acids were studied of both diatoms during exponential growth phase in dependence of different nitrogen conditions. In all experiments the 15N-amm onia uptake was lower than the 15N-nitrate assimilation rate up to 2 0 -4 0 min photosynthesis. Nitrate lim itation - cells grown in nitrate followed by growth in nitrogen-free medium for 24 h – caused a strong 15N-label into aspartate after adding 15NH4C1 (1 mM). In cells grown in nitrate highest enrichment of 15N was found in glutamine. Results were discussed with reference to the operating of the GS/GOGAT system and glutamic acid dehydrogenase pathway. Photosynthetic 14CO2 fixation experiments showed a very high labelling of aspartate which was interpreted with a phosphoenolpyruvate carboxylation catalysed by phosphoenolpyruvate carboxykinase. © 1981, Walter de Gruyter. All rights reserved.