In situ nuclear magnetic resonance of N pulse labels monitors different routes for nitrogen assimilation.

Nuclear magnetic resonance offers the possibility of noninvasive in situ observation of (15)N pulse labeling in the presence of light. In vivo, exclusively the delta-nitrogen of Gln is labeled in the cyanobacterium Microcystis firma when glutamate synthase is inhibited by azaserine. In contrast, the green alga Chlorella fusca is additionally capable of incorporating nitrogen into Glu, thus providing evidence for an anabolic function of glutamate dehydrogenase in this organism.